Alien Annual Plants and the Desert Tortoise

Alien Annual Plants and the Desert Tortoise

 

Notes From An October 4, 1998, CALEPPC Field Trip

Prepared by

 

KRISTIN H. BERRY

 

USGS/BRD, Box Springs Field Station, Riverside, CA 92507

 

        Alien annual plants form a substantial portion of the spring biomass of ephemeral plants in the western Mojave Desert, frequently from 30 to >90%. As such, they are changing the structure and functioning of the ecosystems. We are particularly concerned about their effects on the desert tortoise, a long-lived herbivore and a State and Federal threatened species.

 

Aliens and Their Role in the Diet of the Desert Tortoise. The diet, food preferences, and nutrition of wild desert tortoises have been studied by several graduate students and research scientists during the last 25 years. In the Mojave and Colorado deserts, most tortoises eat succulent, green herbaceous perennial or annual plants, some cacti and, less commonly, grasses. The results of the research project most applicable to the western Mojave Desert can be found in the masters thesis of W. Bryan Jennings, who obtained an MS from the University of Texas at Arlington in 1993. Bryan was supported through funds from the Bureau of Land Management. Bryan is a native Californian, did his undergraduate work at UCSB, and enjoys botanizing. He conducted his research at the Desert Tortoise Research Natural Area on adult male and female tortoises, tracking individuals with radio transmitters. The following is a summary of his findings.
Desert tortoises preferred native over non-native plants. Alien annual plants formed a very small part of the diet (4.65%). Erodium cicutarium (fresh and from previous years) was 3.93%, Schismus was 0.69%, and Bromus was 0.03%. Two particular plant groups were important to the adult tortoises: the legumes (Fabaceae), which comprised over 43% of the diet; and herbaceous perennial plants, which were 30% of the diet. The latter group may play an important role during drought years.
Tortoises knew the locations of favored food plants and spent the majority of time feeding in specific areas which supported the plants, e.g., low hills and washes. The preferred plants were frequently uncommon in the environment and didn’t appear on plant transects. A summary of the top 10 preferred plants (by bite counts, 35,401 total bites observed) for spring is shown below.

 

Species % of bite counts
Lotus humistratus
Mirabilis bigelovii
Euphorbia albomarginata
Astragalus layneae
Lygodesmia exigua
Astragalus didymocarpus
Camissionia boothii
Erodium cicutarium
Chorizanthe brevicornu
Phacelia tanacetifolia
29.69
10.79
10.74
8.20
5.58
4.58
3.86
3.25
2.60
2.01

        Tortoises did not eat perennial shrubs, with the exception of the suffrutescent perennial, Mirabilis bigelovii. This plant formed 36.47% of the diet plants. Mirabilis bigelovii formed only 0.94% of the relative abundance of perennial plants in the environment.
In other parts of the Mojave, other annuals and herbaceous perennials are important, such as Plantago and Sphaeralcea ambigua. The diet of free-ranging juveniles is another critical subject that needs attention. Juvenile tortoises are very small (45-90 mm carapace length) and are likely to require more delicate plants. For example, juveniles probably don’t consume much Mirabilis bigelovii because the leaves are large, heavy and may be out of reach.

Alien Annual Plants, Heavy Metals and Other Toxicants. In 1989 a team of research scientists studying health and diseases of desert tortoise began to salvage and necropsy ill, dying, and recently dead desert tortoises to determine cause(s) of death. The team consisted of Drs. Bruce Homer, Kristin Berry, Elliott Jacobson, Mary Brown, and Mary Christopher. We found that the very ill and dying tortoises often had high concentrations of heavy metals and other toxicants, such as lead, cadmium, nickel, chromium, mercury, molybdenum, and vanadium. We also have associated the presence of elevated levels of such toxicants in the livers and kidneys of tortoises with lesions on their shells. One new shell disease, cutaneous dyskeratosis, was described in 1994. So far, we have discovered no fungal, bacterial, or viral basis for this and other shell diseases in the desert tortoise. Toxicants, nutritional deficiencies, or nutritional deficiencies induced by toxicants are the most likely causes. Of particular interest is the apparent increase in shell diseases in some populations and correlations of increases in shell disease with increasing mortality rates.
Two years ago, the disease research team was expanded to include geologists and geochemists in the USGS: Drs. Maurice Chaffee, Gordon Haxel, Joe Wooden, Roy Knight, and Brenda Houser. We are now looking at the source of the toxicants, particularly mercury and lead, in the western Mojave. We have collected soil and plant samples for analysis, including samples of Schismus and Erodium. Alien plants may be a pathway for toxicants. We hope to have the analysis completed on 50 samples of annual plants (46 elements) in the next year. As part of our interdisciplinary project, we are also looking at concentrations of elements in the bone and scute of tortoise shell. We hope to be able to use bones and scutes of long-dead tortoises for evaluating variation and change in the environment. I hasten to add that alien plants may not be involved at all and that atmospheric pollution may be a factor. We have a lot of work ahead of us on the subject!

Alien Annual Plants and Awns. One of the ill tortoises salvaged for necropsy a few years ago had a plant awn penetrating the gut. The veterinary research pathologist recorded the information but did not save the awn. At that time, he did not realize that it might be important (but he knows now, one of the benefits of an interdisciplinary team). The awns of alien grasses, e.g., “fox tails” are well known for causing medical problems to domestic pets and livestock, and we can assume that the tortoise may be similarly affected. With a tortoise ill and dying from awn-induced damage to the gut, detection would be difficult. The awns of bromes, as common names of ripgut brome and foxtail chess imply, are most likely to be culprits in tortoise habitat. We will have a difficult time planning experiments to determine whether alien grasses irritate or penetrate the gut of tortoises in sufficiently high frequencies to lower survivorship. It would be desirable to have information on whether the impact is confined to a particular size class of tortoises and the frequency of the problem.

Alien Annual Plants and the Weed-Fire Cycle. The flora and fauna of the Mojave and Colorado deserts did not evolve under a regime where fires were widespread and frequent. Fires kill tortoises and damage their habitat. Loss of the cover of shrubs is very serious: shrubs are used for cover from the sun, wind, and cold, as well as protection from predators. The coppice mounds beneath shrubs are used as sites for burrows and pallets. When the shrubs burn, the burrow opening is no longer covered and protected, and the tunnel receives no thermal protection from the shrub. As the weed-fire cycle develops in a particular habitat, the available forage for tortoises also changes to an annual “weedland.”

Livestock Grazing and Alien Annual Plants. In the Mojave and Colorado deserts, livestock consists of sheep, cattle, and feral burros. These animals can (1) disturb or damage soil crusts; (2) reduce cover of vegetation, including shrubs, perennial grasses, and annual plants; (3) alter the composition and diversity of shrubs, perennial bunch grasses, and annuals; and (4) promote the growth and spread of alien plants. They can affect the tortoises in many ways, i.e., by trampling them, by reducing cover of shrubs, by changing the supply of choice food plants, and by contributing to the growth and spread of alien plants that are less desirable food items.

Recovery of Tortoise Habitat Infested with High Biomasses of Aliens. Alien plants pose major challenges to scientists and land managers who are working toward recovery of desert tortoise populations and their habitats. We need to alter the weed-fire cycle, reduce the biomass of undesirable alien grasses and other weeds, and determine how to economically restore degraded habitats.

 

PLANT LIST

CALEPPC Field Trip, October 4, 1998

perennials | cacti | annuals | grasses | mustards | weedy natives

Species marked with an asterisk (*) are aliens

Perennial Shrubs or Herbs

 

Acamptopappus sphaerocephalus (goldenhead)
Ambrosia dumosa (burrobush, burroweed)
Asclepias subulata (milkweed)
Atriplex canescens (four-wing saltbush)
Atriplex confertifolia (shadscale)
Atriplex hymenelytra (desert holly)
Atriplex polycarpa (allscale, cattle spinach)
Atriplex spinifera (Mojave saltbush)
Bebbia juncea (sweetbush)
Chrysothamnus nauseosus (rabbit brush)
Chrysothamnus teretifolius (terete-leaved rubberbrush)
Encelia farinosa (brittle bush)
Ephedra nevadensis (Mormon tea)
Eriogonum inflatum (desert trumpet)
Grayia spinosa (spiny hopsage)
Hymenoclea salsola (cheesebush)
Krameria erecta (Pima ratany)
Krascheninnikovia lanata (winter fat)
Larrea tridentata (creosote bush)
Lepidium fremontii (sweet alyssum)
Lycium andersonii (Anderson thornbush)
Mirabilis bigelovii (wishbone bush)
Petalonyx thurberi (sandpaper plant)
Psorothamnus fremontii (indigo bush)
Salazaria mexicana (paperbag bush)
Senna armata (desert cassia, senna)
Stephanomeria pauciflora (wirelettuce)
Stillingia paucidentata (toothleaf spurge)
Xylorhiza tortifolia (Mojave aster)

Cacti

 

Echinocactus polycephalus (cotton top)
Echinocereus engelmannii (strawberry hedgehog cactus)
Opuntia basilaris (beavertail)
Opuntia echinocarpa (silver cholla)
Opuntia ramosissima (darning needle or cucumber cholla)

Annuals

 

Amsinckia tessellata (checker fiddleneck)
Astragalus sp. (locoweed)
Camissonia boothii (Booth evening primrose)
Camissonia decorticans (woody bottle-washer)
Chaenactis fremontii (Fremont pincushion)
Chaenactis carphoclinia (Pebble pincushion)
Chorizanthe brevicornu (brittle spinflower)
Chorizanthe rigida (rigid spiny-herb)
Cryptantha circumscissa (western forget-me-not)
Cryptantha sp. (forget-me-not)
Descurainia pinnata (tansy mustard)
Dithyrea californica (spectacle pod)
Eriogonum trichopes (little trumpet)
Eriophyllum pringlei (Pringle Eriophyllum)
Eriophyllum wallacei (Wallace Eriophyllum)
*Erodium cicutarium (filaree, stork’s bill)
Erodium texanum (desert heron’s bill)
Eschscholzia glyptopleura (desert gold poppy)
Eschscholzia minutiflora (little gold poppy)
Gilia sp.
Lepidium flavum (pepper grass)
Lotus humistratus (lotus)
Lupinus sp. (lupines)
Malacothrix glabrata (desert dandelion)
Mentzelia albicaulis, Mentzelia sp. (blazing stars)
Pectis papposa (chinch weed)
Pectocarya sp. (the comb-burs)
Phacelia distans, P. tanacetifolia
Rafinesquia neomexicana (desert chicory)

Grasses

 

Achnatherum [Stipa] hymenoides (Indian rice grass)
Achnatherum [Stipa] speciosum (desert needle grass)
*Bromus madritensis ssp. rubens (red brome)
*Bromus tectorum (cheat grass)
Pleuraphis rigida (galleta grass)
*Schismus arabicus, S. barbatus (split grass, Arab grass, Mediterranean grass)

Mustards (All aliens)

 

*Brassica tournefortii
*Descurainia sophia
*Hirschfeldia incana [Brassica geniculata]
*Sisymbrium irio (London rocket)

Weedy Native Species

 

Eremocarpus setigerus (turkey mullein) (Euphorbiaceae)
Ambrosia acanthicarpa (annual burweed, bursage) (Asteraceae)

 

Habitat Use and Food Preferences of the Desert Tortoise, Gopherus agassizii, in the Western Mojave Desert and Impacts of Off-Road Vehicles

Proceedings: Conservation, Restoration, and Management of Tortoises and turtles—An International Conference, pp. 42–45
© 1997 by the New York Turtle and Tortoise Society.

Habitat Use and Food Preferences of the
Desert Tortoise, Gopherus agassizii,
in the Western Mojave Desert and
Impacts of Off-Road Vehicles

W. BRYAN JENNINGS

Department of Biology, University of Texas, Arlington, TX 76019, USA
Current address: Department of Zoology, University of Texas at Austin, Austin, TX 78712, USA

        ABSTRACT: The desert tortoise, Gopherus agassizii, and its habitats in the western Mojave Desert and elsewhere are negatively affected by off-road vehicles (ORVs). Data from a study conducted at the Desert Tortoise Research Natural Area during 1992 provide insights into why ORVs are likely to affect tortoises. To determine habitat use and food preferences, 18 large immature and adult tortoises were observed. The study site contained four subhabitats or strata: washes (comprising 7.9% of the area), washlets (2.4%), hills (42.3%), and flats (47.4%). The tortoises used the four habitat strata differentially, spending significantly more time (92%) in washes, washlets, and hills throughout spring than in the flats (8%). They were observed to take bites from 2,423 individual plants of at least 43 plant species (37 annual, 6 perennial). They showed preferences for native plants (95.3% of bites) compared to non-native plants. Some of the ten most-preferred food plants were uncommon to rare in the environment. Three of the ten most-preferred food plants occurred largely in the wash strata, and an additional four species were found only in hill strata. Users of recreational vehicles also prefer washes and hills in this region, where they are more likely to encounter tortoises, increasing the possibility of direct mortality, and where they are more likely to have a greater impact upon preferred forage and habitats.

Recreational use of off-road vehicles (ORVs), popular since the late 1960s in the southwestern deserts of the United States, poses significant threats to desert tortoises in some parts of their geographic range (U. S. Fish and Wildlife Service [USFWS], 1994). The threats are both direct and indirect: direct encounters, damage to and loss of habitat, damage to or loss of burrows, and loss or changes in both the composition of the forage and the quality of shrub cover. In this paper, I report findings from research conducted in the western Mojave Desert in and adjacent to the Desert Tortoise Research Natural Area (Jennings, 1993), specifically, desert tortoise use of different habitat types, their preferred forage plants, and the possible impacts of ORVs on these two critical aspects of desert tortoise ecology.

METHODS

        The study area was typical of the western Mojave Desert, a topographic and vegetational mosaic of subhabitats or strata that includes washes, sandy flats, low hills, and rocky slopes where the most common vegetation types are saltbush (Atriplex spp.) scrub and creosote bush (Larrea tridentata) (U. S. Bureau of Land Management and California Dept. of Fish and Game, 1988; USFWS, 1994). Specifically, the 2.6 km² study area was composed of four strata or subhabitats, each with its unique composition of perennial and ephemeral plants (Jennings, 1993). The four strata were flats (comprising 47.4% of the study area), hills (42.3%), washes (7.9%), and washlets (2.4%). Wash and washlet strata were lumped for a portion of the analyses. In the flats, the dominant species were three shrubs: goldenhead (Acamptopappus sphaerocephalus), burro bush (Ambrosia dumosa), and creosote bush. In the hills the most diverse of the strata with 11 species, five species of shrubs were dominant: burrobush, California buckwheat (Eriogonum fasiculatum), goldenhead, Mojave aster (Xylorhiza tortifolia), and creosote bush. Shrubs in wash and washlet strata were burrobush, cheesebush (Hymenoclea salsola), goldenhead, bladdersage (Salazaria mexicana), creosote bush, and Anderson thornbush (Lycium andersonii). Data on absolute and relative densities of plant species were collected once for the perennial shrubs using linear transects and 2 × 5 m quadrats. Similar data were collected using the same method for herbaceous perennial and ephemeral plant species on 17–20 April, 12–15 May, and 12–13 June. Details of methodology are in Jennings (1993). Scientific names of plants are taken from Hickman (1993).
To determine how the tortoises used the four habitat strata, I observed 18 large immature and adult tortoises (8 females and 10 males), which ranged from 179 to approximately 380 mm in carapace length at the midline (Jennings, 1993). Most tortoises had been fitted with radio transmitters as part of other research programs. The tortoises were tracked from the time they emerged from hibernation through the spring (1 March–30 June), and their activities, use of habitat, and forage items were recorded. Because the ephemeral and herbaceous perennial plants on which tortoises feed have different growth, flowering, and fruiting periods during the year, I grouped the species into three phenological periods for analysis: 1 March to 30 April, 1 to 31 May, and 1 to 30 June. The use of phenological periods for data analysis also provided a better understanding of when and where tortoises were foraging, how they were using the habitats, and when the different forage plants were consumed.

RESULTS

        The tortoises made differential use of the four habitat strata (Jennings, 1993). Between 1 March and 30 April, they spent a disproportionately longer time within the hill and washlet strata (84%; X² = 1353.01, d.f. = 2, P = 0.0001) and foraged on preferred food plants located exclusively in hill areas (Mirabilis bigelovii, Astragalus didymocarpus) and washlet margins (A. layneae, Camissonia boothii). During the second phenological period, the use of hill, wash, and washlet areas continued to be important (100%; X² = 1405.8, d.f. = 2, P = 0.0001). Tortoises foraged on A. layneae and C. boothii and then moved into the hills to eat the preferred Lotus humistratus and Prenanthella exigua. (Both Lotus and Prenanthella were restricted to the hills.) During the third phenological period, tortoise activity declined markedly because of heat and dry weather, and the few tortoises that remained above ground used primarily washes and washlets (68%; X² = 753.83, d.f. = 2, P = 0.0001), drawing on plants confined to those areas (Euphorbia albomarginata and C. boothii). Overall, tortoises made little use of the more common flat stratum.
The tortoises’ diet and preferred foods were determined from observations of a total of 34,657 bites taken from 2,423 individual plants between 24 March and 21 June of 1992 (Jennings, 1993). Tortoises foraged from at least 43 species of plants (37 species of winter-spring annuals and 6 perennial species) as well as a dead leopard lizard (Gambelia wislizenii) and tortoise scat. Some important patterns emerged. These tortoises were highly selective foragers and preferred to consume native plants (33,712 bites or 95.3%) over non-native species (1,644 bites, 4.1%). The non-native species were filaree (Erodium cicutarium), Mediterranean grass (Schismus arabicus, S. barbatus), and foxtail chess (Bromus madritensis ssp. rubens), and were readily available. The tortoises also took more bites from annuals (69.2%) than from perennial plants (30.8%); with the exception of four bites from cheesebush, all bites of perennial plants were from herbaceous or suffrutescent perennial plant species. Tortoises took more bites from legumes (44%) than from any other plant family.
Some of the ten most-preferred food plants consumed during 1992 were uncommon to rare in the environment (Jennings, 1993). For example, during the first phenological period, plants of the suffrutescent perennial M. bigelovii constituted 29.7% of the bites taken by tortoises, yet M. bigelovii constituted <1% of the perennial plants in the environment and far less of the total biomass of both ephemeral and perennial plants. A. layneae was also an important forage plant (3.9% of bites) but was not found on plant transects. During the second phenological period the annual L. humistratus constituted 63.9% of bites taken, yet was not found in annual plant samples. During the third phenological period, the herbaceous perennial Euphorbia albomarginata constituted 57.4% of bites but did not appear on any plant transects. Overall, >25% of all the plants on which tortoises fed were in the washes and washlets, about twice the number as might be expected considering that washes and washlets comprised only 10.3% of the study area habitats. Three of the ten most-preferred plants, E. albomarginata, A. layneae, and C. boothii, were largely confined to washes.

DISCUSSION

        Desert vertebrates and their habitats are vulnerable to and negatively affected by ORVs (Busack and Bury, 1974; Bury et al. 1977; Luckenbach, 1982; Webb and Wilshire 1983). The desert tortoise is not exempt from these effects (Berry et al., 1986). In the western Mojave Desert where the use of ORVs is prevalent, tortoise populations have undergone steep declines, compared to relatively undisturbed desert tortoise populations and in habitat in the eastern parts of their geographic range (USFWS, 1994).
Hills and washes are favored in the western Mojave Desert for use by ORV recreationists (U.S. Bureau of Land Management, 1980). Four major ORV recreation areas with hills, washes, and canyons are adjacent to the Desert Tortoise Research Natural Area (Rand Mountains) or are within 50 km (Jawbone Canyon, Dove Springs, and Spangler Hills). The users of motorcycles, trail bikes, all-terrain vehicles, and other four-wheel vehicles prefer the washes, washlets, canyon bottoms, and hilly country for riding (see Goodlett and Goodlett, 1993 for an example of trail densities in flats, hills, and wash habitats). They gradually widen trails and create more individual tracks and trails, which damages or destroys increasing amounts of habitat. The flats are used primarily for camping, as staging areas for competitive events, and as play areas.
Desert tortoises are vulnerable to negative effects from ORVs because of their habitat preferences. The tortoises in this study spent significantly more time traveling and foraging in hills, washes, and washlets than on the flats, the same areas preferred by ORV users. In other parts of the species’ geographic range (the southern, eastern, and northeastern Mojave and the Sonoran deserts), washes are also important in the ecology and behavior (Woodbury and Hardy, 1948; Burge, 1978; Baxter, 1988). The tortoises use the washes for travel, excavation of burrows or dens, and for feeding. Because tortoises spend so much more time in washes and hills, they are also more likely to suffer direct mortality from vehicles than if they used the habitat randomly.
The food preferences and forage locations of the tortoises provide additional insights. A substantial portion of the food bites taken by tortoises were from plants that were infrequent to rare in the environment and occurred in the hill, wash, and washlet strata. Four of the ten most-preferred food plants were found exclusively in the hills, and an additional three were confined largely to washes. At least 25% of the forage plants were in or on the margins of washes or washlets. Vehicles disturb the soil and terrain in washes and other areas, which results in deterioration or denudation of vegetation (Burge, 1983; Woodman, 1983; Goodlett and Goodlett, 1993). They destroy the natural margins of washes and small washlets as the trails are widened over time (Berry et al., 1986). If the preferred forage plants are damaged or destroyed, tortoises will be forced to select other less-preferred and possibly less-nutritious species.
The 18 desert tortoises preferred native to non-native or alien plant species. The Desert Tortoise Reserve Natural Area has been protected from disturbance for almost two decades, and it has a relatively lower biomass of the alien plants than do the adjacent areas outside its protective fence (Brooks, 1995), where sheep grazing and uncontrolled ORV use occur. Most native desert plant species thrive in undisturbed habitats, in contrast to the alien species, which are common in disturbed lands. Some alien species, particularly the grasses, have invaded arid habitats, are fire prone, and have increased fire regimes globally (D’Antonio and Vitousek, 1992). The alien plant/fire cycle is prevalent throughout parts of the Mojave and Great Basin deserts, and wildfires burn thousands of hectares of desert annually (D’Antonio and Vitousek, 1992; USFWS, 1994). In areas disturbed by ORVs, these alien species are likely to constitute increasingly greater portions of the floral biomass, thus increasing the threat of fires.

Recommendations to Protect Desert Tortoises and Their Habitats

  1. Reduce or prohibit vehicle travel off existing roads. Disturbance to desert soils increases the potential for alien plants to invade and become established, causing significant and deleterious alterations to the flora. And, although washes and washlets constitute only a small portion of desert habitats, they have a disproportionate share of the forage plants favored by tortoises and are frequented by tortoises a significantly greater amount of the time. Therefore, vehicle travel off existing highways and established roads—particularly in desert washes and washlets—in desert tortoise Critical Habitat should be minimized and, where possible, prohibited (see USFWS, 1994).
  2. Investigate food habits of neonates and juveniles. The tortoises observed in this study were large immature and adult animals. Neonates and juveniles are likely to have different forage requirements and patterns of use because of their small body sizes, limited activity areas, and inability to travel great distances. The food habits of neonate and juvenile tortoises should therefore be determined also by desert region and habitat strata.

ACKNOWLEDGMENTS

        Thanks are due to Dr. Michael Klemens and Dr. Kristin Berry for inviting me to participate in this conference and to Dr. Berry and Jim Van Abbema for valuable assistance on the manuscript. The U. S. Bureau of Land Management supported research on the foraging ecology and habitat utilization of the desert tortoise under Contract No. B950-C2-0014.

LITERATURE CITED

Baxter, R. J. 1988. Spatial distribution of desert tortoises (Gopherus agassizii) at Twentynine Palms, California: Implications of relocations. In Proc. Symposium Management of Amphibians, Reptiles, and Small Mammals in North America, pp. 180–189. 19–21 July 1988, Flagstaff, Arizona.
Berry, K. H., T. Shields, A. P. Woodman, T. Campbell, J. Roberson, K. Bohuski, and A. Karl. 1986. Changes in desert tortoise populations at the Desert Tortoise Research Natural Area between 1979 and 1985. Proc. Desert Tortoise Council Symp. 1986:100–123.
Brooks, M. 1995. Benefits of protective fencing to plant and rodent communities of the western Mojave Desert, California. Environ. Manage. 19(1):65–74.
Burge, B. L. 1978. Physical characteristics and patterns of utilization of cover sites used by Gopherus agassizii in southern Nevada. Proc. Desert Tortoise Council Symp. 1978:132–140.
Burge, B. L. 1983. Impact of Frontier 500 off-road vehicle race on desert tortoise habitat. Proc. Desert Tortoise Council Symp. 1983:27–38.
Bury, R. B., R. A. Luckenbach, and S. D. Busack. 1977. Effects of off-road vehicles on vertebrates in the California desert. Wildlife Research Report 8, U.S.D.I. Fish and Wildlife Service, Washington, D.C.
Busack, S. D. and R. B. Bury. 1974. Some effects of off-road vehicles and sheep grazing on lizard populations in the Mojave Desert. Biol. Conserv. 6(3):179–183.
D’Antonio, C. M. and P. M. Vitousek. 1992. Biological invasions by exotic grasses, the grass/fire cycle, and global change. Ann. Rev. Ecol. Syst. 23:63–87.
Goodlett, G. O. and G. C. Goodlett. 1993. Studies of unauthorized off-highway vehicle activity in the Rand Mountains and Fremont Valley, Kern County. Proc. Desert Tortoise Council Symp. 1993:163–187.
Hickman, J. C. (ed.). 1993. The Jepson Manual. Higher Plants of California. University of California Press, Berkeley and Los Angeles, California. 1400 pp.
Jennings, W. B. 1993. Foraging ecology and habitat utilization of the desert tortoise (Gopherus agassizii) at the Desert Tortoise Research Natural Area, eastern Kern County, California. Report for the U. S. Bureau of Land Management, Contract No. B950-C2-0014, Riverside, California. 101 pp.
Luckenbach, R. A. 1982. Ecology and management of the desert tortoise (Gopherus agassizii) in California. In R. B. Bury (ed.), North American Tortoises: Conservation and Ecology, pp. 1–37. Wildlife Research Report 12, U.S.D.I. Fish and Wildlife Service, Washington, D.C.
U.S. Bureau of Land Management. 1980. The California Desert Conservation Area Plan. U.S. Bureau of Land Management, Riverside, California. 173 pp.
U.S. Bureau of Land Management and California Department of Fish and Game. 1988. A Sikes Act Management Plan for the Desert Tortoise Research Natural Area and Area of Critical Environmental Concern. U.S. Bureau of Land Management, Ridgecrest, California. 43 pp. + unpaginated appendices.
U.S. Fish and Wildlife Service. 1994. The desert tortoise (Mojave population) recovery plan. U. S. Fish and Wildlife Service, Region 1, Lead Region, Portland, Oregon. 73 pp. + appendices.
Webb, R. H. and H. G. Wilshire (eds.). 1983. Environmental Effects of Off-Road Vehicles. Impacts and Management in Arid Regions. Springer-Verlag New York, Inc. 534 pp.
Woodbury, A.M. and R. Hardy. 1948. Studies of the desert tortoise, Gopherus agassizi. Ecol. Monogr. 18:146–200.
Woodman, A. P. 1983. Effects of Parker 400 off-road race on desert tortoise habitat in the Chemehuevi Valley, California. Proc. Desert Tortoise Council Symp. 1983:69-79.

This article was reprinted with permission from the New York Turtle and Tortoise Society

The Desert Tortoise Recovery Plan: An Ambitious Effort to Conserve Biodiversity in the Mojave and Colorado Deserts of the United States

Proceedings: Conservation, Restoration, and Management of Tortoises and Turtles—An International Conference, pp. 430–440
© 1997 by the New York Turtle and Tortoise Society.

The Desert Tortoise Recovery Plan:
An Ambitious Effort to Conserve
Biodiversity in the Mojave and
Colorado Deserts of the United States

KRISTIN H. BERRY

U.S. Department of the Interior, Bureau of Land Management,
6221 Box Springs Blvd., Riverside, CA 92507-0714, USA
Current Agency: U.S. Geological Survey, Biological Resources Division (same address)

        ABSTRACT: In 1990 the U.S. Fish and Wildlife Service (USFWS) listed the desert tortoise, Gopherus agassizii, as “Threatened” over 30% of its geographic range and shortly thereafter selected a team to develop a plan for its recovery. The team developed a hypothesis-driven recovery plan, using population viability analyses and principles of reserve design. The Desert Tortoise (Mojave Population) Recovery Plan is designed to achieve a 50% probability of survival for the tortoise for 500 years.
Drawing from concepts outlined in the federal Endangered Species Act, the recovery team used a strategy of protecting evolutionarily significant population units and their associated ecosystems. The six population units, called “recovery units,” were identified using published and unpublished data on genetic variability, morphology, and behavior patterns of populations as well as ecosystem types. Boundaries of the six units closely approximate major ecosystem boundaries in the Mojave and Colorado deserts. The goal is to reach a target (where possible) of 50,000 breeding adult tortoises for each recovery unit.
Within the recovery units, the recovery team recommended the establishment of 14 reserves or Desert Wildlife Management Areas (DWMAs), ranging from 415 to 3,367 km² (with one exception, the Virgin River DWMA, which was very small). The USFWS followed by designating 26,087 km² as federally protected “Critical Habitat” in 1994. Additional habitat is also protected within Joshua Tree National Park (est. 2,574 km²) and within the existing boundaries of the Desert Tortoise Research Natural Area (est. 100 km²).
The recovery team attributed declines in tortoise populations to the result of human activities. To reduce and ultimately eliminate many sources of mortality that are driving the desert tortoise toward extinction, they recommended prohibition of several activities in the reserves. Within each DWMA, they also recommended that <10% of habitat be designated as “experimental management areas,” where intrusive and experimental research can occur.
Governments at the federal, state, county, and city levels have begun to implement the Recovery Plan through development of regional land-use plans (habitat conservation plans, coordinated resource plans, and multi-species plans). While tortoise recovery considerations are the driving force for land-use planning, agencies are taking a more comprehensive ecosystems approach. If implementation of the Recovery Plan and land-use plans are successful, the reserve system for the desert tortoise will not only conserve its genetic diversity, but also the biodiversity of several major ecosystems in the Mojave and Colorado deserts.

The U.S. Fish and Wildlife Service (USFWS) placed the desert tortoise, Gopherus agassizii, on the list* of “Threatened” species in 1990 (USFWS, 1990a) and shortly thereafter selected a recovery team to develop a plan for its recovery. This paper describes (1) the resulting Desert Tortoise (Mojave Population) Recovery Plan (hereafter called the Recovery Plan), (2) the system of reserves that are being established to protect the desert tortoise and the ecosystems in which it lives, (3) the threats facing desert ecosystems and measures being taken to reduce the threats, and (4) the government land-use plans that are being created to ensure long-term protection of the ecosystems.


        * In the United States, Congress has delegated the authority to determine the status of species to the USFWS, a federal agency under the Department of the Interior. Species may be placed on federal lists as “Threatened” or “Endangered” under the Endangered Species Act of 1973, as amended, and the lists are published by the government in the Federal Register. The process is known as federal listing. Each state may also develop separate lists of “Rare,” “Threatened,” or “Endangered” species, using its own criteria and standards, and the lists are known as state lists. The term listing is used to refer to the lengthy process involved in candidacy for listing, proposals for listing, and the ultimate action—formal or legal listing as “Threatened” or “Endangered.”

 

Early Efforts to Protect the Desert Tortoise
        The desert tortoise, Gopherus agassizii, is a widespread species of the arid southwestern United States and northwestern Mexico. It occupies a wide variety of habitat types in the Mojave and Sonoran deserts (including the California subsection of the Sonoran Desert known as the Colorado Desert) and occurs in four states in the U.S. and two states in Mexico (Figure 1).

Desert Tortoise Range Map
Figure 1. The geographic range of the desert tortoise, Gopherus agassizii, from Stebbins (1985). The portion of the geographic range where populations are federally listed is shaded.

The organized effort to protect significant populations and habitat of the desert tortoise from numerous human and land uses in the U.S. has spanned more than 20 years. The U.S. Department of the Interior’s Bureau of Land Management (USBLM), the agency that administers approximately 75% of the remaining high-quality desert tortoise habitat, identified the tortoise as a valued component of the deserts and as a sensitive species in the 1970s (see USBLM, 1980). At that time the USBLM and state fish and wildlife agencies selected the tortoise as one of several indicator species for long-term monitoring of environmental conditions using criteria similar to those later described by the National Research Council’s Committee on the Applications of Ecological Theory to Environmental Problems (1986). The selection was based in part on the tortoise’s longevity, low reproductive potential, and sensitivity to environmental perturbations.
In the early 1970s biologists realized that desert tortoise populations were declining in the U.S. (USFWS, 1994a). By 1980 very small segments of three populations had received substantial legal protection: the Beaver Dam Slope population in Utah (which occupied an est. 101 km²) was federally listed as “Threatened” under the Endangered Species Act (ESA) of 1973, as amended (USFWS, 1980); and parts of two populations were protected within small reserves, the Desert Tortoise Research Natural Area (est. 100 km²) and the Chuckwalla Bench Area of Critical Environmental Concern (213 km²) (USBLM, 1980). In 1984 three conservation organizations—Environmental Defense Fund, Natural Resources Defense Council, and Defenders of Wildlife—proposed federal listing for the remaining populations within the U.S. The USFWS (1985) responded by issuing a finding that federal listing was warranted but precluded by other, higher priority actions, thus briefly tabling conservation actions under the ESA.
Recognizing that tortoise populations were continuing to decline, the USBLM developed two plans to offset threats to tortoise populations and their habitats in 1988 (USBLM, 1988a, 1988b). One plan, Desert Tortoise Habitat Management on the Public Lands: A Rangewide Plan, contained a directive to “. . . manage tortoise habitats using an ecosystem management approach with emphasis on maintaining or restoring natural biological diversity” (USBLM, 1988a). The three aforementioned conservation groups also observed the continued population and habitat declines; they served notice of pending court action to the USFWS in mid-1989. Shortly thereafter (August 1989), the USFWS took emergency action to federally list approximately 30% of the tortoise populations within the geographic range (USFWS, 1989a, 1990a). Tortoise populations listed as “Threatened” occur in the Mojave and Colorado deserts; for administrative reasons, the USFWS refers to these populations as the Mojave Population (Figure 1).

Preparing the Desert Tortoise Recovery Plan

The Desert Tortoise Recovery Team
The USFWS has prepared several recovery plans for chelonians, such as the St. Croix population of the leatherback turtle (Dermochelys coriacea), the ringed sawback turtle (Graptemys oculifera), the Alabama red-bellied turtle (Pseudemys alabamensis), and the flattened musk turtle (Sternotherus depressus) (USFWS, 1981, 1988, 1989b, 1990b). In each of these cases, a single person prepared a short plan using traditional USFWS guidelines. In 1990 the USFWS took a different approach to draft a recovery plan for the desert tortoise, selecting a recovery team composed of nationally recognized scientists with expertise in genetics, plant and animal ecology, physiology, biogeography, veterinary medicine, and conservation biology. The recovery team, which was chaired by Peter F. Brussard, included Kristin H. Berry, Michael E. Gilpin, Elliott R. Jacobson, David J. Morafka, Cecil R. Schwalbe, C. Richard Tracy, and Frank C. Vasek. Judy Hohman of the USFWS was Executive Secretary. Six of the eight team members were academicians and two were government research scientists. The team met 17 times over a period of four years to develop the Recovery Plan.
The review process for the Recovery Plan was extensive. Comments received during the review process not only improved the Recovery Plan and associated documents, but also ultimately contributed to the acceptance of the concepts contained in the documents. Government agencies, the public, and the scientific community played important roles. Prior to release to the public, two drafts of the plan were prepared for government review, including review by a four-state, multi-government agency committee, the Desert Tortoise Management Oversight Group (MOG). The MOG, formed in 1989 after publication of the USBLM’s Desert Tortoise Habitat Management on the Public Lands: A Rangewide Plan (USBLM, 1988a), coordinates research, management, conservation, and recovery efforts for the desert tortoise in the U.S. Government review of the Recovery Plan was followed by an official draft, published for a 90-day public comment period in 1993. Public hearings were held, and the USFWS received a total of 143 letters. The draft Recovery Plan was modified to reflect the additional information and criticisms, and the final Recovery Plan was distributed in 1994. During the same year, the government determined the boundaries of Critical Habitat and published the decision in the Federal Register (see first footnote, above). The four-year time span* from the initial federal listing of the Mojave population as “Threatened” and selection of the recovery team to publication of the final Recovery Plan and determination of Critical Habitat is in large part a reflection of the complexity of the task, the disparate nature of the available data bases, and the large amounts of land involved. Much of what follows is taken directly or paraphrased from the Recovery Plan (USFWS, 1994a) and is, in part, an enlarged abstract of the plan.


        * However, four years is considerably less than the average 9.4 years reported by Tear et al. (1995) for completion of recovery plans for threatened and endangered vertebrates in general.

 

The Approach: Using the Principles of Conservation Biology
The recovery team recognized that the tortoise is a widespread species and exhibits substantial variation in genetic, morphological, ecological, physiological, and behavioral characteristics throughout its geographic range (USFWS, 1994a). Drawing from the ESA and the works of Ryder (1986) and Waples (1991), the recovery team decided to use evolutionarily significant units, which they termed “population segments” or “recovery units,” to encompass the genetic and environmental variation present in the species. Six recovery units were identified: Western Mojave, Eastern Mojave, and Northeastern Mojave; Northern Colorado and Eastern Colorado; and Upper Virgin River (Figure 2 and Table 1).

 

Desert tortoise recovery units
Figure 2. The portion of the desert tortoise population (Mojave population) that is federally listed as “Threatened.” The six recovery units and 14 Desert Wildlife Management Areas (DWMAs) described in the Desert Tortoise (Mojave Population) Recovery Plan (USFWS, 1994a; Brussard et al., 1994) are shown.

 

        The six recovery units vary considerably in climate and vegetation (USFWS, 1994a). The mean number of freezing days annually (which affects length of tortoise burrows and amount of seasonal activity above ground) varies from as low as 2–16 days in the two Colorado Desert recovery units to 46–127 days in the Northeastern Mojave Desert recovery unit. The mean annual precipitation and distribution of precipitation within the year differ considerably from the western to the eastern portions of the geographic range and are important factors that affect amount and timing of vegetation available to tortoises for forage. The Western Mojave recovery unit, for example, is in a region where annual precipitation primarily occurs in winter and produces ephemeral vegetation in late winter and spring, but little precipitation (6–10%) and forage occur in summer. In contrast, the other five recovery units are in eastern or southern regions that receive two periods of precipitation per year, which in turn can result in two distinct seasonal floras that may be utilized for food.
Within each recovery unit, from one to four reserves or Desert Wildlife Management Areas (DWMAs) were identified as locations where desert tortoise populations could be managed to achieve recovery (USFWS, 1994a; Brussard et al., 1994). A total of 14 DWMAs were identified (Figure 2 and Table 1).
Genetic factors, minimum viable population size, sizes of reserves (DWMAs), and the probability of long-term persistence are critical elements in the strategy to recover the “Mojave Population” of desert tortoises (USFWS, 1994a). From a genetic standpoint, the recovery team concluded that a minimally viable population should probably contain at least 2,000–5,000 adult animals (USFWS, 1994a). Three population viability analyses were prepared, and predictions were developed based on the probabilities that tortoise populations would persist for 500 years. Using these analyses, the recovery team concluded that (1) tortoise populations at minimum densities (3.9 adults/km²) require reserves of at least 518–1,295 km² to be genetically viable; (2) where the discrete population growth rate (lambda) is slightly below 1.0 but varies over a range of approximately 25%, extremely large reserves (12,950 km² to support 50,000 adults at minimal density) are necessary to support populations that would be relatively resistant to extinction within the next half-century; and (3) if lambdas fall below 0.975 on average, no population size is sufficient to persist for 500 years.
The recovery team recommended a target size of >2,590 km² for DWMAs because reserves of this size would be likely to provide sufficient buffering from demographic stochasticity and genetic problems and would be sufficiently large to support recovered populations with a reasonable probability of persistence.
The shape and arrangement of DWMAs are essential to their success (USFWS, 1994a). The recovery team recommended the use of current theory and practice of reserve design (e.g., Thomas et al., 1990; Noss, 1991). Seven guidelines were followed in recommending DWMA boundaries (USFWS, 1994a):

  1. Reserves that are well distributed across a species’ native range will be more successful in preventing extinction than reserves confined to small portions of a species’ range.
  2. Large reserves (>2,590 km²) containing large populations of the target species are superior to small blocks of habitat containing small populations.
  3. Blocks of habitat that are close together are better than blocks far apart.
  4. Habitat that occurs in less fragmented, contiguous blocks is preferable to habitat that is fragmented.
  5. Habitat patches that minimize edge-to-area ratios are superior to those that do not.
  6. Interconnected blocks of habitat are better than isolated blocks, and linkages function better when the habitat within them is represented by protected preferred habitat for the target species.
  7. Blocks of habitat that are roadless or otherwise inaccessible to humans are better than blocks containing roads and blocks easily accessible to humans.

The recovery team emphasized three related points: the desirability of redundancy, or more than one reserve per recovery unit, a strategy likely to increase the probability of recovery of populations within the recovery units; the importance of connecting small reserves with corridors containing functional habitat; and intensive management into perpetuity where small reserves are the only option. The recovery team also recognized the role of small, isolated populations, in the event that epidemic disease (such as upper respiratory tract disease) contributes to near extirpation in DWMAs.
The USFWS (1994b) used the guidelines to establish Critical Habitat in February 1994 (Table 2). Of the original 22,616–27,407 km² recommended for protection in 14 DWMAs (Brussard et al., 1994), the USFWS designated 26,087 km² as Critical Habitat (USFWS, 1994b). The USFWS (1994b) recognized that additional habitat was already adequately protected within Joshua Tree National Park (est. at 2,574 km², C. Collins, pers. comm.) and the Desert Tortoise Research Natural Area (est. 100 km²) and did not require designation as Critical Habitat. Thus, the overall total of protected habitats for the desert tortoise is 28,761 km².

Causes of Tortoise Population Declines and
Recommended Regulations for DWMAs to Reduce Threats

Government agencies and the recovery team recognized that declines in desert tortoise populations as well as losses to their habitats were primarily due to human activities (USFWS, 1994a). The list of threats and factors contributing to declines is lengthy and is similar to the list of threats to tortoises worldwide (Swingland and Klemens, 1989). To reduce the factors contributing to tortoise mortalities and reverse population declines, the recovery team identified the human activities considered to be incompatible with recovery of the tortoise and recommended that the following activities be prohibited:

  • all vehicle activity off of designated roads; all competitive and organized commercial and recreation events (associated with vehicles) on designated roads;
  • habitat-destructive military maneuvers, clearing for agriculture, landfills, and other surface disturbances that diminish the capacity of the land to support desert tortoises, other wildlife, and native vegetation;
  • domestic livestock grazing and grazing by feral burros and horses;
  • vegetation harvest, except by permit (issued by the county for private land, by the USBLM for public land);
  • collection of biological specimens, except by permit;
  • dumping and littering;
  • deposition of captive or displaced desert tortoises or other animals, except under authorized translocation research projects (guidelines established within the Recovery Plan);
  • uncontrolled dogs out of vehicles; and
  • discharge of firearms, except for hunting of big game or upland game birds from September through February.

The above recommendations will be a challenge to implement quickly and effectively because much of the habitat (Table 2) for the Mojave Population of desert tortoises is on federal land administered by the USBLM (74.3%), where there is a long history of multiple-use activities (USFWS, 1994b). Tortoise habitats on the Department of Defense facilities (3.8%) and National Park Service properties (2.9%) also receive intensive use in some areas and are likely to require adjustments to land-use practices in the immediate future.
The recovery team expected that people would visit the DWMAS (USFWS, 1994a). They identified some limited human activities that are compatible with desert tortoise recovery efforts, including:

  • non-intrusive monitoring of desert tortoise population dynamics and habitat;
  • limited-speed travel on designated, signed roads and maintenance of these roads;
  • non-consumptive recreation (e.g., hiking, bird watching, casual horseback riding, and photography);
  • parking and camping in designated areas;
  • fire suppression that minimizes surface disturbance;
  • permitted or otherwise controlled maintenance of existing utilities;
  • surface disturbances that enhance the quality of habitat for wildlife, enhance watershed protection, or improve opportunities for non-motorized recreation (includes construction of visitor centers, wildlife guzzlers or drinkers, camping facilities, etc. where appropriate);
  • population enhancement of native wildlife species, such as desert bighorn, Gambel’s quail, etc.;
  • mining on a case-by-case basis, provided that the cumulative impacts of these activities do not significantly impact desert tortoise habitats or populations, that any potential effects on desert tortoise populations are carefully mitigated during the operation, and that the land is restored to its pre-disturbance condition; and
  • non-manipulative and non-intrusive biological or geological research, by permit.

An important element in the recovery strategy was the division of DWMAs into core areas where human activities would be restricted, and experimental management zones (EMZs) where certain prohibited activities may be permitted on an experimental basis during the recovery period (USFWS, 1994a). As envisioned by the recovery team, the EMZs would be composed of no more than 10% of tortoise habitat within a DWMA and would be located at the DWMA periphery. The types of research recommended for the EMZs include research on effects of cattle grazing on tortoises and their habitats and intrusive research on the tortoises themselves (e.g., affixing radio transmitters to shells, monitoring health profiles by drawing blood, etc.). The recovery team recommended that experimental translocations occur outside of DWMAs and that no desert tortoises be introduced into DWMAs, at least until relocation is much better understood (Appendix B in USFWS, 1994a).

Hypothesis Testing and Long-Term Monitoring
Recovery of desert tortoise populations is likely to require decades, if not centuries (USFWS, 1994a). The recovery team based the Recovery Plan on a series of hypotheses and models that can be tested as new data are acquired. The effectiveness of the recovery strategies (e.g., establishing recovery units, 14 DWMAs, and removing or reducing perceived threats from DWMAs) can be most appropriately tested by comparing changes in desert tortoise population densities inside and outside of DWMAs. The key to such comparisons is a reliable and economical method for estimating population densities of large immature and adult tortoises (>140 mm in carapace length) on a regional scale. No single method has yet to be embraced by government and the scientific community as “scientifically credible,” an essential part of delisting criterion 1 (see following).
Hypothesis testing should also be a part of long-term research programs to evaluate threats to desert tortoise populations and habitat using the EMZs. Several subjects requiring attention are described in the Recovery Plan, e.g., research on the effects of cattle grazing and road density, the effectiveness of tortoise-proof barriers along freeways and highways, and feasibility of restoration of habitat.

Criteria that Must be Met for Tortoise Populations to be Considered “Recovered”
An essential part of the Recovery Plan is a description of recovery objectives and “delisting criteria,” the threshold at which populations can be considered “recovered” and can be removed from the list of federally “Threatened” species. The USFWS (1994a) determined that desert tortoise populations could be delisted by recovery unit and that the Mojave Population could be delisted when populations in all six recovery units were considered to be recovered. Five criteria must be met for recovery to occur within a unit:*

  1. The population must exhibit a statistically significant upward trend or remain stationary for at least 25 years (one tortoise generation); trends must be measured using a scientifically credible monitoring plan, with population estimates taken at five-year intervals.
  2. Sufficient habitat must be protected within a recovery unit (at least one DWMA of >2,590 km²) or, in unusual circumstances, the tortoise populations must be managed intensively enough to ensure long-term population viability.
  3. At each DWMA, population lambdas must be maintained at or above 1.0 into the future.
  4. Regulatory mechanisms or land management commitments must be implemented to ensure long-term protection of tortoises and their habitats.
  5. The population in the recovery unit should be unlikely to need protection under the ESA in the foreseeable future (as determined by detailed genetic, demographic, physiological, behavioral, and environmental analyses).

        * The Recovery Plan states: “These recovery criteria were designed to provide a basis for consideration of delisting, but not for automatic delisting. Before delisting may occur, the Fish and Wildlife Service must determine that the following five listing factors are no longer present or continue to adversely affect the listed species: (1) the present or threatened destruction, modification, or curtailment of the species’ habitat or range; (2) overutilization for commercial, recreational, scientific, or educational purposes; (3) disease and predation; (4) inadequacy of existing regulatory mechanisms [e.g., laws, existing land use]; and (5) other human-made or natural factors affecting the continued existence of the species . . .”

 

Implementing the Recovery Plan: The Use of Multi-Species,
Ecosystem, and Bioregional Plans

The Recovery Plan (USFWS, 1994a) is being implemented through preparation of up to six bioregional, multi-species, or ecosystem plans (Table 3). The plans, most of which are regional in nature, are delimited in part by state boundaries. Three of the plans—the Western Mojave Coordinated Management Plan, the Clark County Desert Conservation Plan, and the Proposed Habitat Conservation Plan, Washington County, Utah—will probably be completed between 1995 and 1997, at least as draft plans, whereas the others are still in early stages. With one exception, the Beaver Dam Slope of Arizona, plans are underway or proposed for all DWMAs and recovery units.
One group of plans belongs to a special subset, “habitat conservation plans” (HCPs). Habitat conservation plans are an option described in the ESA, as amended, for protection and management of “Threatened” and “Endangered” species, while at the same time allowing for “incidental take” of individual animals and losses to their habitats. The best-known of the desert tortoise HCPs is the three-year or short-term HCP developed for Clark County, Nevada (Regional Environmental Consultants, 1991; Beatley, 1994), which has been followed by a long-term HCP (Clark County, 1994; USFWS, 1995).
These management plans, whether developed by federal, state, or county governments, are “desert tortoise driven”: they would not have been identified and scheduled for preparation and implementation if the desert tortoise had not been federally listed and the Recovery Plan had not been prepared. The desert tortoise, because of its widespread distribution, public interest and support, scientific value, and charisma, is being used as an umbrella or “flagship” species to represent many different plants and animals and their ecosystems.
The management protections required to recover the desert tortoise necessitate major changes in existing land-use plans, some of which are 16 years old (e.g., USBLM, 1980), thereby stimulating new land-use planning efforts on a large scale. In all cases, preparers of the new plans are fully aware of the importance of using the multi-species and ecosystems approaches. They are following mandates in the ESA, which provides for protecting the ecosystems on which “Threatened” and “Endangered” species depend; agency directives in the USBLM’s 1988 Desert Tortoise Habitat Management on the Public Lands: A Rangewide Plan (USBLM, 1988a); and current scientific thinking in conservation biology.
The draft Western Mojave Coordinated Management Plan (USBLM, 1995) provides an example of the scope and the numbers of at-risk species that will benefit. Twenty species of plants and animals within the planning region are already federally listed as “Threatened” or “Endangered” or are proposed for listing, and another 46 species are candidates for listing. The area covered by the plan is 37,969 km², of which 18% is designated Critical Habitat for the tortoise. When the protected habitats at the Desert Tortoise Research Natural Area and Joshua Tree National Park are added to Critical Habitat, 25.3% of the planning area would be managed for long-term recovery and survival of desert tortoise populations. Additional areas will be protected for at least some of the other species, potentially raising the percentage even higher.
A key point about the desert tortoise is its federal status under the ESA as a “Threatened” species. Because the tortoise is classified as “Threatened” and is not considered endangered, the government may still allow some multiple use of the land, and some time is still permitted to allow ecosystems to recover naturally. With many endangered species, such opportunities have been lost. Because the remaining ecosystem remnants have reached such severe states of perturbation, draconian measures are necessary. Some endangered species, such as the California condor, remain extant primarily through breeding programs. It is hoped that the recovery measures for the tortoise can be quickly implemented, thereby reversing declining population trends. Recovery is expected to require centuries for some desert tortoise populations.
In summary, the Recovery Planfor the desert tortoise follows a recent trend of recovery plans, e.g., the grizzly bear (USFWS, 1993) and the spotted owl (Thomas et al., 1990), which are regional in scope, are designed to improve management of troubled ecosystems, and are potentially controversial. Recovery plans for single species—especially when the species are widespread, large, showy, charismatic or well-known to the public—may serve to stimulate public support for large-scale, bioregional or ecosystem land use plans. In the case of the tortoise, over 26,000 km² in the Mojave and Colorado deserts may receive new and significant management and conservation efforts. Single umbrella species such as the desert tortoise also can assist immeasurably in educating the government and the public about conservation biology, biodiversity, and the need for reserves. 

ACKNOWLEDGMENTS

        I thank Frank Vasek, David J. Morafka, Peter Brussard, Larry Foreman, Peter C. H. Pritchard, Jim Van Abbema, Richard Crowe, Betty L. Burge, and E. Karen Spangenberg for useful and constructive comments on the manuscript. I am grateful to Keith Mann for providing GIS data, Marilet Zablan and Todd Esque for details on land-use plans for public and private land in Utah, and Tim Duck for information on the USBLM’s management plans in the Arizona Strip District.

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U.S. Fish and Wildlife Service. 1994b. Endangered and threatened wildlife and plants; Determination of critical habitat for the Mojave population of the desert tortoise. Federal Register 59(26):5820–5866.
U.S. Fish and Wildlife Service. 1995. Final environmental impact statement: Issuance of a permit to allow incidental take of desert tortoises, Clark County, Nevada. Fish and Wildlife Service, Region 1, Portland, Oregon. 139 pp. + appendices.
Waples, R. S. 1991. Pacific salmon, Oncorhynchus spp., and the definition of “species” under the Endangered Species Act. Marine Fisheries Review 53(2):11–22.
Washington County Habitat Conservation Plan Steering Committee and SWCA, Inc., Environmental Consultants. 1995. Habitat conservation plan, Washington County, Utah. Submitted by the Washington County Commission, Utah to the U.S. Fish and Wildlife Service.

 

TABLE 1A comparison of sizes of Desert Wildlife Management Areas (DWMAs) recommended for protecting desert tortoises (Brussard et al., 1994) and the names and sizes of Critical Habitats ultimately designated by the federal government in February 1994 (USFWS, 1994b).
Recovery units
DWMAs 
Size
(km²)
Names of corresponding
Critical Habitat(s)
Size
(km²)

Northern Colorado 
     Chemehuevi  2,590.0–3,367.0 Chemehuevi 3,793.54
Eastern Colorado
     Chuckwalla 1,942.5–2,460.5 Chuckwalla a 4,130.24
     Joshua Tree a see Western Mojave Pinto Mountains a –––
Western Mojave
     Fremont-Kramer  1,942.5–2,460.5 Fremont-Kramer 2,096.28
     Ord-Rodman  1,165.5–1,424.5 Ord-Rodman 1,024.67
     Superior-Cronese  2,331.0–2,849.0 Superior-Cronese 3,103.55
     Joshua Tree a 2,136.75–2,913.75 Pinto Mountains 694.85
Eastern Mojave
     Fenner, California b 1,372.7–1,631.7 Piute-Eldorado, California 1,836.47
     Piute-Eldorado, Nevada 1,740.16 c Piute-Eldorado, Nevada 2,091.43
Northeastern Mojave
     Ivanpah Valley d 2,201.5–2,719.5 Ivanpah Valley 2,559.24
     Coyote Spring  2,460.5–2,719.5 see Mormon Mesa
     Mormon Mesa  2,072.0–2,590.0 Mormon Mesa 1,731.66
     Gold Butte-Pakoon   699.3–802.9  Gold Butte-Pakoon, Nevada
Gold Butte-Pakoon, Arizona
778.21
1,197.88
     Beaver Dam Slope in 
Utah, Nevada, Arizona
 414.4–440.3  See below, by state:
Beaver Dam Slope, Nevada
Beaver Dam Slope, Arizona
Beaver Dam Slope, Utah
353.70
172.80
301.49
Upper Virgin River 
     Upper Virgin River  No number given Upper Virgin River 220.96
Totals 22,615.56–27,407.06 26,086.97
     a The Joshua Tree DWMA was located primarily in Joshua Tree National Park, with the vast majority of habitat in the Western Mojave recovery unit; only the southeastern part of the DWMA was in the Eastern Colorado recovery unit.  When Critical Habitat was formally designated, the portions of the DWMA within the park were excluded.  The northern part of the DWMA (in the Western Mojave recovery unit) became the Pinto Mountain Critical Habitat unit, and the southeastern portion of the DWMA outside the park was designated as part of the Chuckwalla unit of Critical Habitat.
     b Located primarily in the Eastern Mojave recovery unit, with a small portion in the Northern Colorado recovery unit.  When the habitat within the Fenner DWMA was designated as Critical Habitat, the name was changed to Piute-Eldorado, California.
     c An estimate of the size, using 430,000 acres described in a proposed management plan (see Brussard et al., 1994).
     d Located in both the Eastern Mojave and Northeastern Mojave recovery units.

 

TABLE 2Critical Habitat for the Mojave Population of the desert tortoise and ownership of land, as of February 1994 (USFWS, 1994b).
Land owner or administrator Size (km²) Percent

U.S. Dept. of the Interior, Bureau of Land Management 19,386.96 a 74.32 a
Dept. of Defense 980.15   3.76  
U.S. Dept. of the Interior, National Park Service 595.70 a 2.28 a
State lands 672.59   2.58  
Tribal lands 6.48   0.02  
Private lands 4,445.09 b 17.04 b
Total 26,086.97   100.00  
     a On 31 October 1994 (eight months after designation of Critical Habitat), a substantial amount of public land under the jurisdiction of the Bureau of Land Management was transferred to the National Park Service for the Mojave National Preserve (California) and for additions to Joshua Tree National Park and Death Valley National Park (California). The transfer of land was part of the California Desert Protection Act of 1994, which was created by the 103rd Congress (Public Law 103-433, 108 STAT. 4471). Therefore, the figures shown in the above table are no longer accurate.
b A significant portion (no figures available) of private lands are owned by Catellus Corporation, formerly Southern Pacific Railroad lands.

 

TABLE 3The relationships between desert tortoise recovery units and the existing and proposed multi-species, ecosystem, and bioregional plans.
Recovery units Size of area
(km²)
Proposed year
of completion
Name of management plan

Western Mojave
Draft West Mojave Coordinated Management Plan (draft) (1) 37,969 est. 1997
Eastern Mojave
  California:
Northern and Eastern Mojave Desert Ecosystem Coordinated Management Plan (proposed*) (2) 31,239 est. 1999
  Nevada:
Short-term Habitat Conservation Plan for the Desert Tortoise in Las Vegas Valley, Clark County, Nevada (3) 89,121 1991
Clark County Desert Conservation Plan (4) and Final Environmental Impact Statement: Issuance of a Permit to Allow Incidental Take of Desert Tortoises, Clark County, Nevada 56 1995
Stateline Resource Management Plan (amend. & rev.) (6) 14,973 1992, 1994
Northeastern Mojave
  California:
(same as Eastern Mojave recovery unit, combined into one plan*) (2) no data available est. 1999
  Nevada:
(same as Eastern Mojave recovery unit, may be combined into one plan*) (2) will also include parts of Stateline Resource Management Plan and Caliente Resource Management Plan (6) no data available est. 1997
  Utah:
Dixie Resource Management Plan (7) no data available no date
  Arizona:
Arizona Strip Resource Management Plan (8) 12,140 1992
Upper Virgin River
Proposed Habitat Conservation Plan, Washington County, Utah (draft) (9) 226 June 1995
Northern Colorado
Northern and Eastern Colorado Desert Coordinated Management Plan (10) 22,391 est. 1999
Eastern Colorado
Planning effort combined with Northern Colorado recovery unit (10) no data available est. 1999
* May combine California and Nevada regions, crossing state jurisdictions.

References:

  1. USBLM, 1995
  2. U.S. National Park Service and USBLM (in prep.)
  3. Regional Environmental Consultants, 1991
  4. Clark County, Nevada, 1995
  5. USFWS, 1995
  6. USBLM, 1992a; supplement in 1994 (must be amended)
  7. USBLM (in prep., 1985–1995; incomplete, may be amended)
  8. USBLM, 1992b (must be amended)
  9. Washington County Habitat Conservation Plan Steering Committee and SWCA, Inc., Environmental Consultants, 1995
  10. USBLM and others (in prep.)

This article was reprinted with permission from the New York Turtle and Tortoise Society

Demographic Consequences of Disease in Two Desert Tortoise Populations in California, USA

Proceedings: Conservation, Restoration, and Management of Tortoises and Turtles—An International Conference, pp. 91–99
© 1997 by the New York Turtle and Tortoise Society.

Demographic Consequences of Disease
in Two Desert Tortoise Populations in California, USA

 

KRISTIN H. BERRY

 

U.S. Department of the Interior, Bureau of Land Management,
6221 Box Springs Blvd., Riverside, CA 92507-0714, USA
Current Agency: U.S. Geological Survey, Biological Resources Division (same address)

 

        ABSTRACT: Disease is a causal factor in declines of desert tortoise, Gopherus agassizii, populations at two locations in California. In the interior of the Desert Tortoise Research Natural Area (DTNA), population densities of all sizes of tortoises declined 76% from 75/km² in 1979 to 18/km² in 1992. Densities of adults followed the same pattern and declined 90% from 61/km² in 1979 to 6/km² in 1992. Declines of adult tortoises are attributed primarily to an upper respiratory tract disease (URTD) caused by the pathogen Mycoplasma agassizii. Additional disease-related mortalities are expected to occur, because 25–38% of four samples of adult tortoises from an adjacent site within the DTNA tested positive for antibodies to M. agassizii in 1992. This disease may have been introduced to the DTNA through release of ill captive tortoises.
A second disease, cutaneous dyskeratosis, is present at another reserve, the Chuckwalla Bench Area of Critical Environmental Concern, and is linked to population declines. Between 1982 and 1988 the incidence of cutaneous dyskeratosis increased and lesions became more severe. Between 1982 and 1992 the total tortoise population (all size classes) declined 54% from 153 tortoises/km² to 70 tortoises/km². The adult population declined 61% from 87 tortoises/km² to 34 tortoises/km². The cause(s) of cutaneous dyskeratosis remain uncertain, but possibilities include deficiency diseases or environmental toxicosis.
Diseases, especially if introduced to a native population or if environmentally caused, can have serious consequences for threatened chelonians and will complicate conservation and recovery efforts. Of the two diseases, URTD is of the greatest immediate concern because of its potential to harm tortoise populations on a global scale.

 

Desert tortoise, Gopherus agassizii, populations have declined substantially in the past two decades due to a wide variety of causes, including collecting, vandalism, predation, and habitat loss and deterioration (U.S. Fish and Wildlife Service [USFWS], 1994). Declines became so precipitous that in August 1989, the USFWS listed the species as “Endangered” under the emergency provisions of the Endangered Species Act of 1973, as amended. Disease was a factor in the emergency listing. Subsequently, in April 1990 the USFWS permanently listed the desert tortoise as “Threatened.”
Disease was first implicated as having a significant role in mortality of wild desert tortoises in the 1980s, when remains of tortoises from the Beaver Dam Slope of Arizona and Utah were found to have osteopenia, indicative of malnutrition (Jacobson, 1994). Disease was documented as the major cause of mortality in a wild desert tortoise population in 1988, when ill tortoises with upper respiratory disease were observed by field-workers at a long-term permanent study plot in the interior of the Desert Tortoise Research Natural Area (DTNA) in the western Mojave Desert of California (Berry, 1990; Jacobson et al., 1991). Further research in 1989 and 1990 (e.g., Knowles, 1989; Berry, 1990) confirmed that many tortoises at the DTNA were ill, dying, or had recently died. The signs of disease (nasal discharge, lassitude, cachexia) were similar to signs observed in an often fatal upper respiratory disease of captive desert tortoises throughout southern California (W. Rosskopf, DVM, pers. comm.). Prior to 1988 upper respiratory disease had not been observed in wild, free-living tortoises at the DTNA (Berry and Nicholson, 1984; Berry et al., 1986).
Upper respiratory disease was considered a threat to wild populations of the desert tortoise in the 1970s, long before the disease appeared. When the DTNA was established in 1972, the author and the U.S. Bureau of Land Management (USBLM) expressed concerns about the potentially infectious nature of the disease. We suspected wild populations may become infected through release of ill captive tortoises, which were numerous throughout southern and central California (St. Amant, 1977; Berry and Nicholson, 1984). Because of the potential threat, the USBLM provided funds to Dr. Murray Fowler and his students to conduct research on the disease (Fowler, 1977; Snipes and Biberstein, 1982). No specific bacterial organism was identified as causing respiratory disease. Fowler (1977) concluded that the disease was not caused by a single organism and was not infectious. Stress, especially from malnutrition, was considered the prime predisposing factor.
By the mid-1970s the USBLM, the California Department of Fish and Game (CDFG), the Desert Tortoise Preserve Committee, Inc., and the California Turtle & Tortoise Club distributed literature and issued public statements warning that captive tortoises should not be released to wild lands because of the potential for spreading disease, contaminating genetic stock, and adversely affecting the behavior of wild resident populations (e.g., St. Amant, 1979, 1980; Berry and Nicholson, 1984).
When several wild tortoises with respiratory disease were discovered at a long-term permanent study plot in the DTNA interior in 1988, the CDFG and USBLM provided financial support to initiate research on the pathogenesis of the disease. Within two years research scientists at the University of Florida described the disease as an upper respiratory tract disease (URTD) associated with a new and undescribed mycoplasma (Jacobson et al., 1991). Shortly thereafter a transmission study demonstrated that a new mycoplasma, M. agassizii, is a highly infectious pathogen and causes URTD (Brown et al., 1994). An enzyme-linked immunosorbent assay (ELISA) test for M. agassizii was developed (Schumacher et al., 1993) and was used to determine whether wild desert tortoises carried antibodies to the pathogen (Brown et al., 1994b; Jacobson et al., 1995).
Within weeks of the time that ill tortoises were discovered at the DTNA in 1988, tortoises at another long-term study site in the Chuckwalla Bench Area of Critical Environmental Concern (ACEC), in the eastern Colorado Desert of Riverside County, were discovered to have shell lesions. An abnormally high number of tortoises, many of which were previously marked, were discovered dead (Berry, 1990). Research on the pathogenesis of shell lesions was initiated by Jacobson et al. (1994).
In this paper, I describe the demographic consequences of disease on desert tortoises at the long-term permanent study sites in the interior of the DTNA and on the Chuckwalla Bench, and the evidence linking disease to deaths. I also review the status of the populations as of 1992, discuss the implications of the two diseases for chelonian populations, and outline actions taken to cope with diseases within the two preserves and elsewhere.

 

METHODS

Desert tortoise study site localities
Figure 1. Locations of two protected areas for the desert tortoise: the Desert Tortoise Research Natural Area in the western Mojave Desert, eastern Kern County, California, and the Chuckwalla Bench Area of Critical Environmental Concern in the eastern Colorado Desert, Riverside County, California.

Description of the Two Long-Term Permanent Study Sites for Desert Tortoise Populations

        Desert Tortoise Research Natural Area (DTNA). The DTNA is a 100 km² reserve established in 1972 in the western Mojave Desert to protect high-density tortoise populations and their habitats (Figure 1) (USBLM and CDFG, 1988). Formally designated as a Research Natural Area and Area of Critical Environmental Concern in 1980, the DTNA is protected from recreational vehicle use and sheep grazing by a hog wire fence of which the lower edge is approximately 25 cm above the ground (USBLM, 1980; USBLM and CDFG, 1988). The raised fence permits wild animals to move into and out of the DTNA unimpeded. Since 1972 federal and state governments and the Desert Tortoise Preserve Committee, Inc. have invested considerable effort in establishing and securing habitat in the DTNA to create a viable reserve. To date, more than 8,000 km² of private inholdings have been acquired. The USBLM took action to eliminate all livestock grazing and hardrock mineral mining and also established long-term stewardship and education programs.
Two research study sites on the DTNA were sources of information: (1) a long-term study plot (2.8 km²), established in 1973 in the central interior to monitor changes in populations and habitat (Berry, 1984, 1990); and (2) a short-term, adjacent study plot, established in 1988 for research on health profiles (Christopher et al., 1993, this volume), physiology (water balance and energy flow), and epidemiology of diseases (Brown, 1994b). Habitats within the study plots are typical of diverse creosote bush (Larrea tridentata) scrub plant communities in the western Mojave Desert. Joshua trees (Yucca brevifolia) are widely scattered; creosote bushes are the predominant shrub; and more than three dozen perennial species of shrubs and grasses are present in the understory, e.g., burrobush (Ambrosia dumosa), goldenhead (Acamptopappus sphaerocephalus), cheesebush (Hymenoclea salsola), Anderson thornbush (Lycium andersonii), Nevada joint fir (Ephedra nevadensis), desert needle grass (Achnatherum speciosum), Indian rice grass (A. hymenoides), and one-sided blue grass (Poa secunda ssp. secunda). Elevations range from 853 to 914 m.
Chuckwalla Bench ACEC. In 1980 the USBLM established the Chuckwalla Bench ACEC in the eastern Colorado Desert in recognition of unique wildlife and vegetation values, including high densities of desert tortoises (Figure 1) (USBLM, 1980, 1986). The 371 km² protected area is long (approx. 60 km) and narrow (2.4–12 km) and is a raised bench between the Orocopia, Chocolate, and the Chuckwalla mountains. The bench drains into Milpitas Wash, which has the largest known populations of ironwood, Olneya tesota. In 1980, 35% of the ACEC was in numerous scattered privately owned parcels. Since that time, the USBLM has acquired most of these inholdings (S. Eubanks, pers. comm.) through land acquisition and mitigation programs.
The 2.8 km² long-term study plot (elev. 640 m) contains areas of desert pavement cut by rocky gullies and microphyll woodland washes. The predominant vegetation on the pavements and flat areas is diverse creosote bush scrub typical of the eastern Colorado Desert. It contains burrobush, white rhatany (Krameria grayi), California joint fir (Ephedra californica), cheesebush, ocotillo (Fouquieria splendens ssp. splendens), silver cholla (Opuntia echinocarpa), and Mojave yucca (Yucca schidigera). Washes are dominated by blue palo verde (Cercidium floridum), smoke tree (Psorothamnus spinosus), desert willow (Chilopsis linearis ssp. arcuata), ironwood, and catclaw (Acacia greggii).Collecting Data on Population Densities of Desert Tortoises
Data for estimates of densities were collected during 60 day spring surveys conducted in 1979, 1982, 1988, and 1992 (Berry, 1984, 1990), according to the following procedure established for these ongoing studies: Each study site was divided into a grid of quadrats, each of which was 0.0259 km², and was permanently marked by rebar (iron reinforcing rods). Between late March and early June, one or two field-workers walked transects on each quadrat in search of tortoises. The plot was thoroughly and evenly covered twice, with each coverage requiring about 30 person days of effort. Each coverage constituted a census, with a total of two censuses conducted in each survey year. Surveys were conducted over a minimum of 45 and a maximum of 90 calendar days to minimize the effects of immigration and emigration of tortoises. For each tortoise located, the following data were collected for density estimates: date, unique identification number assigned to each tortoise, size (carapace length at the midline or MCL), sex, capture type (i.e., first capture of a previously unmarked tortoise, first capture in a given year of a previously marked tortoise, second or subsequent capture of a previously marked tortoise in a given year), location, and survey type (first or second census).

Analyzing Data on Population Densities of Desert Tortoises
The Stratified Lincoln Index (Overton, 1971) was selected as the density estimator because it satisfies two critical criteria for estimating densities of long-lived species such as the tortoise: (1) the population data can be stratified into size classes based on capturability, and (2) the equations allow for growth of individual tortoises and their shift from a smaller to a larger size-age class between the first and second censuses (Berry, 1990). The stratification is of particular importance, because larger tortoises are much easier to find than smaller tortoises.
Samples were sorted for analysis by study site, year, and census period (Berry, 1990). For each census within a year, each tortoise was assigned to one of five strata based on MCL: juveniles 1 and 2 (<100 mm MCL), immature 1 (100–139 mm MCL), immature 2 (140–179 mm MCL), small or young adults (180–208 mm MCL), and adult (>208 mm MCL). Tortoises were placed in three groups based on time of capture: (1) tortoises captured during the first census, (2) tortoises captured during the second census, and (3) tortoises captured during both the first and second censuses. Capture-recapture data were arranged in matrices to allow for growth of individual tortoises from one size group to another between censuses. The 95% Confidence Interval (CI) was used to establish level of significance for changes in population densities. An additional source of data was the number of individual tortoises encountered and marked during the 60-day survey for each sample year. If CIs did not overlap from one survey year to the next, the changes in the density estimates were considered statistically significant.

Evaluating Effects of Diseases on the Populations
Data on health status was gathered in detail on the two plots in 1992, including information on presence or absence and type of nasal discharge, patency of the nares, appearance of eyelids and eyes, condition of chin glands, presence or absence of ocular discharge, presence of active or healed injuries, presence and extent of lesions on the shell and limbs, and other signs of disease. In all survey years, 35 mm slide transparencies were taken of the plastron and carapace of most tortoises (Berry, 1984, 1990). These slides document the presence and extent of shell lesions.
Data were also available from necropsies of ill and dying tortoises salvaged from or adjacent to the plots (Jacobson et al., 1991, 1994; J. Klaassen, pers. comm.) from the ELISA tests of tortoises in the research program on epidemiology of URTD at the DTNA (Brown et al., 1994b) and from mortality rates of tortoises in the health profile research program at the DTNA (Christopher et al., 1993). The latter program began in May 1989 with ten adult males and ten adult female tortoises, each fitted with radio transmitters. The tortoises were monitored four times annually (Christopher et al., 1993). Over the next 35 months—through March 1992—as tortoises died, new individuals were added to the program.

 

RESULTS

Desert tortoise density at the DTRNA
Figure 2. Estimates of population densities of desert tortoises at the Desert Tortoise Research Natural Area in Kern County, California in 1979, 1982, 1988, and 1992. Density estimates are presented for all sizes of tortoises and for the adult tortoises (greater than or equal to 180 mm carapace length at midline). The histogram shows the midpoint of the density estimates, and the brackets enclose the 95% Confidence Intervals for the population estimates.
The DTNA
In the 13 years between 1979 and 1992, both the total population and the adult component of the tortoise population experienced statistically significant declines in densities (Figure 2). The total population density, which included all sizes of tortoises, steadily decreased 76%, from a high of 149 tortoises/km² (95% CI = 115–195) in 1979 to a low of 18 tortoises/km² (95% CI = 8–44) in 1992. The pattern of decline of adults differed. First, the adult population component rose in density from 59 tortoises/km² (95% CI = 43–78) in 1979 to 92 tortoises/km² (95% CI = 71–119) in 1982 as a result of recruitment of immature tortoises into the adult size class (Berry, 1990). However, the adult component then steadily declined to 61 tortoises/km² (95% CI = 47–79) in 1988 and thence to 6 adults/km² (95% CI = 2–15) in 1992. The 13-year decline resulted in a 90% loss of adults, during which the total of number of tortoises registered per survey year declined from 189 to 25 individuals.
The deaths of tortoises and the population declines are attributable to several causes (Berry, 1990), including predation by common ravens on the juvenile and small immature size classes. However, between 1988 and 1992 the declines of adults are clearly attributable to URTD caused by M. agassizii. The evidence is from several sources. Prior to 1988 wild, free-ranging tortoises at the DTNA and other long-term tortoise study plots in California were not observed with signs of URTD, and very few were observed with signs of illness or in a dying state (Berry and Nicholson, 1984; Berry et al., 1986). In contrast, many ill, dying, and dead tortoises (remains of animals dead <2 years), most of which were previously marked during censuses, were found on the DTNA plot in 1988, 1989, and 1990, and many dead animals were discovered in 1992 (Berry, unpubl. data). Ill tortoises showed classic signs of URTD (Jacobson et al., 1991; Brown et al., 1994a). In May 1989, 12 tortoises (11 males and 1 female), all of which showed signs of advanced and chronic URTD, were removed from the DTNA for observation and necropsies during the initial research on URTD (Jacobson et al., 1991). The tortoises had chronic inflammatory changes in the upper respiratory tract. A mycoplasma-like organism was seen on the surfaces of the macroepithelial cells (Jacobson et al., 1991). Subsequently, a transmission research program demonstrated that M. agassizii was the causative pathogen (Brown et al., 1994a).
Tortoises with signs of URTD were not confined to a single part of the DTNA but were present throughout the DTNA and in adjacent areas of the Fremont Valley and Rand Mountains (Knowles, 1989; Berry, unpubl. data). In spring 1989, 13 sample plots (1.3–2.6 km² each) were established in the 100 km² DTNA and in adjacent lands within the Fremont Valley and Rand Mountains to determine distribution and frequency of ill tortoises (Knowles, 1989). The sample plots totaled 31.2 km². Four hundred sixty-eight live tortoises were found, of which 202 (43%) showed signs of URTD. From 9.4% to 66.7% of tortoises on 12 of the 13 sample plots showed signs of URTD.
The DTNA experienced a catastrophic epidemic that is not yet over. In 1992 from 12 to 14 adult tortoises in the research program for epidemiology of URTD were tested for antibodies to M. agassizii (Brown et al., 1994b) using the ELISA test (Schumacher et al., 1993). The tortoises were tested during late winter, spring, summer, and fall and were in a study area immediately adjacent to the long-term plot. From 25 to 38 percent of the tortoises in the four seasonal samples tested positive for M. agassizii, indicating that the tortoises had been exposed to the pathogen, or were currently ill, or had been ill and were recovered. Between 1993 and 1995, from 7 to 62% of the tortoises in the four samples taken each year have produced positive ELISA tests, indicating that the population is in a chronic disease state (M. B. Brown et al., pers. comm.)
Of the 27 tortoises in the health profile research program between May 1989 and March 1992, six died between late 1989 and mid-1991. An additional 11 tortoises disappeared and have not been found as of October 1995. The death rate for adults is abnormally high, compared to approximately 2% per year in stable populations (Turner et al., 1987). The disappearance rate is also high and unusual for tortoises fitted with radio transmitters. Many of the missing animals should be considered dead.
Desert tortoise density at Chuckwalla Bench
Figure 3. Estimates of population densities of desert tortoises at the Chuckwalla Bench Area of Critical Environmental Concern in Riverside County, California in 1979, 1982, 1988, and 1992. Density estimates are presented for all sizes of tortoises and for the adult tortoises (greater than or equal to 180 mm carapace length at midline). The histogram shows the midpoint of the density estimates, and the brackets enclose the 95% Confidence Intervals for the population estimates.
Chuckwalla Bench ACEC
Between 1979 and 1992 both the total population and the adult component of the tortoise population exhibited statistically significant declines (Figure 3). The total population density, which included all sizes of tortoises, steadily decreased from a high of 223 tortoises/km² (95% CI = 177–283) in 1979 to a low of 64 tortoises/km² (95% CI = 46–92) in 1988 (Berry, 1990). The 1992 census figures were slightly higher but not significantly different from the figures recorded for 1988: 70 tortoises/km² (95% CI = 48–102). The pattern of decline of adults was similar. There were no statistically significant differences in the adult population component in the 1979 and 1982 censuses; in 1979 density estimates were 75 tortoises/km² (95% CI = 56–98), whereas in 1982 estimates were 87 tortoises/km² (95% CI = 68–112). By 1988 census figures showed statistically significant declines with 42 tortoises/km² (95% CI = 29–62). The 1992 estimates were lower still, 33 adults/km² (95% CI = 22–49), but not significantly different from the 1988 figures.
During the 13-year time frame, the total number of tortoises registered per survey year on the study site also declined, from 265 individuals in 1979 and 262 in 1982 (Berry, 1990) to 107 individuals in 1992 (Berry, unpubl. data). Of considerable concern is the differential loss of adult females. In 1979, 74 adult females and 79 adult males were registered. The numbers of registered tortoises increased to 98 adult females and 80 adult males in 1982 and then declined to 42 adult females and 44 adult males in 1988. By 1992 female numbers had further declined to 26 individuals, while male numbers remained similar at 42. As of mid-1992 (the end of the 1992 field survey season), deaths of adults continued to occur at a rate higher than the 2% annualized death rate estimated for stable populations in the Mojave Desert (Turner et al., 1987). The remains of 20 adults, all of which had been captured in previous years, were found during the 1992 survey. Of the 20, eight had died within the last year and ten had died within the last one to two years. Of the 20 adults, 15 were females.
The population decline appears to be linked to the appearance of shell lesions on the tortoises. In a retrospective analysis of the 35 mm slides taken during surveys conducted between 1979 and 1990, the disease was evident in 1979 but affected only 56% of the tortoises and was generally of limited extent (mild) on the shell (Jacobson et al., 1994). Between 1982 and 1988 the percentage of tortoises affected increased to 90%, and the severity of the lesions on the shells likewise increased. The lesions, which were on the scutes of the plastron and carapace and on the scales of the forelimbs, consisted of white-gray or sometimes orange flaky areas. They appeared at the seams, spreading outward in irregular patterns onto the scutes. In severe cases, bone was exposed. The lesions were described as cutaneous dyskeratosis, but the exact cause could not be determined. The locations on the shell and body and histology of the lesions were suggestive of either a deficiency disease or toxicosis. Both deficiency diseases and environmental toxicants are known to affect keratin in other vertebrates. A wide variety of toxicants are responsible for lesions in the epidermal hard parts of domestic hoofstock, for example (Blood et al., 1989).
Subsequent to the study of the shell and limb lesions in tortoises at the Chuckwalla Bench, 32 ill or damaged tortoises (including some with similar shell lesions) were salvaged for necropsy from the Mojave and Colorado deserts of California and the Sonoran Desert of Arizona (Homer et al., 1994, 1996a, 1996b). The salvaged tortoises with cutaneous dyskeratosis had elevated concentrations of toxicants in the liver, kidney, or plasma (e.g., barium, calcium, cadmium, chromium, magnesium, molybdenum, nickel, phthalates, and selenium in plasma), and/or nutritional deficiencies (e.g., low copper, zinc, selenium, plasma vitamin A). The toxicants and/or nutritional deficiencies may be the cause of the shell disease. Tortoises with cutaneous dyskeratosis may be more vulnerable to bacterial and fungal infections, other diseases, and predation because of the thin scutes and loss of laminae.

DISCUSSION

        Desert tortoises are slow-maturing animals that require 15–20 years to reach reproductive maturity (Woodbury and Hardy, 1948). Once reproductive maturity is reached, wild females generally produce relatively small and few clutches of eggs (Turner et al., 1986, 1987; Henen, 1994). During a six-year study of egg production in the eastern Mojave Desert, females produced from 0 to 3 clutches per year. Mean clutch sizes ranged from 3.53 ±0.26 to 5.15 ±0.34 eggs and clutch frequency from 1.06 ±0.06 to 1.89 ±0.11 per year. Eggs and hatchlings are vulnerable to many predators and pre-adult mortality is generally high (Turner et al., 1987). However, under normal conditions, adult survivorship is about 98% per year. Populations cannot rapidly recover from catastrophic losses of adults, such as the 90% decline experienced at the DTNA or the 50–60% losses at the Chuckwalla Bench because recruitment of young adults requires so many years. These populations face serious threat of extinction (USFWS, 1994). According to analyses prepared by the Desert Tortoise Recovery Team, desert tortoise populations that have declined to 4 adults/km² would require three doublings, or 210 years, to reach a density of 31 adults/km²—if the population is able to grow at an average rate of 1% per year (USFWS, 1994). Using these projections, the adult population at the DTNA would require 280 years to reach the 1979 level of 61 adults/km². One of the more serious aspects of the population declines for the DTNA and the Chuckwalla Bench is that the declines may not have reached the lowest point and recovery may not be underway. Furthermore, the long-term effects of the two diseases on reproduction, viability of eggs, and general health of young are not yet known.
One of the diseases, URTD, is highly infectious (Brown et al., 1994a) and has demonstrated its potential for producing catastrophic impacts on populations at the DTNA. Captive tortoises are implicated in the spread of this disease to wild populations of both desert and gopher tortoises, G. polyphemus (Jacobson, 1993a; Jacobson et al., 1995). In the last five years the number of desert tortoise populations with clinical signs of URTD has increased and the disease appears to be spreading. Wild tortoises that have positive ELISA tests and show clinical signs of disease have been identified throughout the Mojave Desert in California (Brown et al., 1994b; Homer et al., 1994), Nevada (Jacobson et al., 1995), northern Arizona (Dickinson et al., 1995), and Utah (Dickinson et al, 1995; Jacobson et al., 1991).
In California, government agencies have taken measures to prevent release of captive turtles and tortoises since the late 1970s (e.g., St. Amant, 1979, 1980). In 1993 more than 7,500 copies of a booklet with information about wild and captive desert tortoises, URTD, and adoption programs for captives (Berry, 1993) were distributed to people perceived to be authorities on management of wild and captive tortoises (government agencies, librarians, humane societies, police departments and county sheriffs, and veterinarians). The booklet focuses on why captive tortoises should not be released, what one should do when encountering desert tortoises in wild settings, and whom to contact when a tortoise is found in a city or town. A four-page brochure that addresses scientific and medical aspects of URTD, including treatment and guidelines for care and husbandry of tortoises, (Jacobson, 1993b) was also distributed with the booklet. These materials were prepared and distributed not only to educate the public but also to protect the considerable investment by both government and the Desert Tortoise Preserve Committee in existing reserves and legally designated Critical Habitat (USFWS, 1994) for the desert tortoise.
Will education reduce the threat of captive releases and better protect the tortoise? Education is likely to reduce but not entirely eliminate the threat. Data are available on behavior of the general public from the monitoring reports of naturalists at the DTNA (Howland, 1989; Ginn, 1990; Jennings, 1992; Ogg and Gallant, 1992; Kidd, 1993; Boland, 1994, 1995). Since spring 1989 the Desert Tortoise Preserve Committee and the USBLM have sponsored naturalists at the DTNA. Naturalists, who are generally present five to seven days per week for three months in spring at the Interpretive Center, a single site on the 57.5 km fenced boundary of the reserve, have reported that visitors arrive from throughout the state with the intention of releasing captives for various reasons. These reasons include the belief that the DTNA is the appropriate place to release tortoises or that public officials have told them to do so. Some of these captives showed signs of URTD. The naturalists also reported that visitors bring wild tortoises from nearby areas to the DTNA, believing it to be a place that is safe from the hazards of vehicles and other land uses. The 31 illegal incidents summarized in Table 1 occurred at a rate of 2.4 incidents/1000 visitors over a seven-year period. The documented incidents are a minimum number; they represent only incidents of which the naturalists were aware and only at a single point along the DTNA boundary. Certainly, such illegal activities occur more widely in the Southwest (USFWS, 1994).
The second disease, cutaneous dyskeratosis, is also widespread in desert tortoise populations (Berry, unpubl. data; Homer et al., 1994, 1996a, 1996b), with some populations more affected than others. Cutaneous dyskeratosis may have multiple causes and be associated with the presence of environmental toxicants and/or nutritional deficiencies. Field-workers and research scientists need to be taught to identify and describe the lesions. The sources of the environmental toxicants need to be identified, and the link of toxicants to cutaneous dyskeratosis and to the high mortality rates needs to be established. Ultimately, the cause(s) of the disease must be identified and actions taken to reduce the deaths in wild populations.

 

Recommendations

  1. Because our knowledge of infectious diseases in tortoises is limited (Jacobson, 1993a), research scientists and government biologists should take precautions to prevent transmission of known and unidentified diseases from one tortoise to another in the laboratory and field. The field-worker should use a separate pair of disposable gloves for each tortoise; sterilize equipment (e.g., calipers, scales) after use with each animal; sterilize probes used to measure or view burrows after use at each burrow; and ensure that the tortoise does not touch or contaminate the field worker’s clothing, day pack, or other equipment. Equipment should be sterilized or properly disinfected between uses at different sites. If the field-worker is visiting more than one study site, consideration should be given to sterilizing clothing and shoes before traveling to the second site. Where an infectious disease is known to occur in a population, protocols must be established to prevent spread to unexposed populations (including cleaning field vehicles and camping equipment). Sites where infectious diseases are known to occur should be visited last on field trips.
  2. Field-workers should keep detailed records on the health of each wild tortoise in research programs, including 35 mm slides of both healthy and ill individuals.
  3. Ill, dying, and recently dead (but not autolyzed) wild tortoises should be salvaged for necropsies (especially if the tortoise is ill or is a victim of trauma from a vehicle or predator). Where possible, necropsies should be performed by a licensed veterinary pathologist, or a professional pathologist with expertise in reptile pathology. A complete necropsy can provide invaluable information on diseases of tortoises.
  4. Legally designated “Natural Areas” and reserves can appear as attractive, natural, and safe places for release of unwanted captive or illegally collected tortoises. Government employees, research scientists, and the general public must be educated about the hazards of releasing captive tortoises or translocating wild tortoises to a reserve. As a minimum precautionary measure, Natural Areas and reserves should be posted with signs to reduce the likelihood of release. Costs for patrols by recreation specialists and law enforcement personnel should be included in management plans.
  5. Persons with unwanted captive tortoises must have ready access to facilities (e.g., adoption centers) or groups that will accept and properly care for the unwanted animals. Such facilities and organizations serve as deterrents to illegal releases.

 

ACKNOWLEDGMENTS

        C. Knowles, P. Knowles, and P. Gould contributed to the fieldwork at the DTNA study site, and A. P. Woodman, J. Howland, and T. Shields made significant contributions at the Chuckwalla Bench study site. The following people provided constructive comments on the manuscript: B. Homer, M. Brown, V. Dickinson, E. R. Jacobson, and J. Oldemeyer. The USBLM at Riverside, California supported all long-term research efforts on desert tortoise study plots. Research on pathogenesis and epidemiology of URTD was supported by USBLM contracts to E. R. Jacobson at the University of Florida (Contract No. CA-950-CT9-28) and M. B. Brown (Contract No. B950-C2-0046), respectively. Research on cutaneous dyskeratosis was supported through USBLM contracts to E. R. Jacobson and T. J. Wronski (No. CA951-CT0-046) and to B. Homer and E. R. Jacobson (No. CA 950-C1-0062), as well as a National Biological Service Research Work Order to B. Homer and E. R. Jacobson.

 

LITERATURE CITED

Berry, K. H. 1984. A description and comparison of field methods used in studying and censusing desert tortoises. Appendix 2. In K. H. Berry (ed.), The Status of the Desert Tortoise (Gopherus agassizii) in the United States. Desert Tortoise Council report to U.S. Fish and Wildlife Service, Order No. 11310-0083-81, Sacramento, California.
Berry, K. H. 1990. The status of the desert tortoise (Gopherus agassizii) in California in 1989. Draft report to the U. S. Fish and Wildlife Service, Portland, Oregon.
Berry, K. H. 1993. Answering questions about desert tortoises: A guide for people who work with the public in California. Report No. BLM-CA-PT-93-003-6840, U.S. Bureau of Land Management and California Department of Parks and Recreation. 43 pp.
Berry, K. H. and L. L. Nicholson. 1984. A summary of human activities and their impacts on desert tortoise populations and habitat in California. Chapter 3. In K. H. Berry (ed.), The Status of the Desert Tortoise (Gopherus agassizii) in the United States. Desert Tortoise Council report to U.S. Fish and Wildlife Service, Order No. 11310-0083-81, Sacramento, California.
Berry, K. H., T. Shields, A. P. Woodman, T. Campbell, J. Roberson, K. Bohuski, and A. Karl. 1986. Changes in desert tortoise populations at the Desert Tortoise Research Natural Area between 1979 and 1985. Proc. Desert Tortoise Council Symp. 1986:100–123.
Blood, D. C., O. M. Radostits, J. H. Arundel, and C. C. Gay. 1989. Veterinary Medicine. A Textbook of the Diseases of Cattle, Sheep, Pigs, Goats, and Horses. 7th ed. Bailliere Tindall, London. 1502 pp.
Boland, C. 1994. Observations and activities of the naturalists at the Desert Tortoise Research Natural Area, Kern County, California: 1 March through 31 May 1994. Prepared for the Desert Tortoise Preserve Committee, Inc., in cooperation with the U.S. Bureau of Land Management, Riverside, California. 39 pp.
Boland, C. 1995. Observations and activities of the naturalists at the Desert Tortoise Research Natural Area, Kern County, California: 1 March through 31 May 1995. Prepared for the Desert Tortoise Preserve Committee, Inc., in cooperation with the U.S. Bureau of Land Management, Riverside, California. 45 pp.
Brown, M. B., I. M. Schumacher, P. A. Klein, K. Harris, T. Correll, and E. R. Jacobson. 1994a. Mycoplasma agassizii causes upper respiratory tract disease in the desert tortoise. Infection and Immunity 62(10):4580–4586.
Brown, M. B., P. A. Klein, I. M. Schumacher, and K. H. Berry. 1994b. Health profiles of free-ranging desert tortoises in California: Results of a two-year study of serological testing for antibody to Mycoplasma agassizii. Final Report. U. S. Bureau of Land Management, Contract. No. B950-C3-0046, Riverside, California. 54 pp.
Christopher, M. M., I. Wallis, K. A. Nagy, B. T. Henen, C. C. Peterson, B. Wilson, C. Meienberger, and I. Girard. 1993. Laboratory health profiles of free-ranging desert tortoises in California: Interpretation of physiologic and pathologic alterations (March 1992–October 1992). Report to the U.S. Dept. of the Interior, Bureau of Land Management, Contract No. B950-C1-0060, Riverside, California.
Christopher, M., K. A. Nagy, I. Wallis, and K. H. Berry. 1997. Laboratory health profiles of desert tortoises in the Mojave Desert: A model for health status evaluation of chelonian populations. In J. Van Abbema (ed.), Proceedings: Conservation, Restoration, and Management of Tortoises and Turtles—An International Conference, pp. 76-82. July 1993, State University of New York, Purchase. New York Turtle and Tortoise Society, New York.
Dickinson, V. M., T. Duck, C. R. Schwalbe, and J. L. Jarchow. 1995. Health status of free-ranging Mojave desert tortoises in Utah and Arizona. Arizona Game and Fish Department Tech. Rept. 21, Phoenix. 70 pp.
Fowler, M. E. 1977. Respiratory disease in desert tortoises. In Annual Proceedings of the American Association of Zoo Veterinarians, pp. 79–99.
Ginn, S. 1990. Observations and activities of the naturalist for the Desert Tortoise Natural Area, Kern County, California: March 18–June 2, 1990. Report. to the Desert Tortoise Preserve Committee, Ridgecrest, California and the U. S. Bureau of Land Management, Riverside, California. 37 pp.
Henen, B. T. 1994. Seasonal and annual energy and water budgets of female desert tortoises (Xerobates agassizii) at Goffs, California. Ph.D. dissertation, University of California, Los Angeles.
Homer, B. L., K. H. Berry, M. M. Christopher, M. B. Brown, and E. R. Jacobson. 1994. Necropsies of desert tortoises from the Mojave and Colorado deserts of California and the Sonoran Desert of Arizona. Final report to U.S. Dept. of the Interior, Bureau of Land Management, Contract No. B950-C1-0062, Riverside, California.
Homer, B. L., K. H. Berry, and E. R. Jacobson. 1996a. Necropsies of eighteen desert tortoises from the Mojave and Colorado deserts of California: 1994–1995. Final Report for U.S. Dept. of Interior, National Biological Service Work Order No. 131. 120 pp.
Homer, B. L., K. H. Berry, F. Ross, C. Reggiardo, and E. R. Jacobson. 1996b. Potentially toxic metals and minerals in liver and kidney of desert tortoises in California. In Abstracts from the Twenty-first Annual Meeting and Symposium of the Desert Tortoise Council, pp 19–20. Held 29 March–1 April 1996, Las Vegas, Nevada.
Howland, J. 1989. Observations and activities of the naturalist for the Desert Tortoise Natural Area, Kern County, California: March 12–July 12, 1989. Report from the Desert Tortoise Preserve Committee, Inc., Ridgecrest, California to U.S. Bureau of Land Management, Contract No. CA950-CT9-44, Riverside, California. 59 pp.
Jacobson, E. R. 1993a. Implications of infectious diseases for captive propagation and introduction programs of threatened/endangered reptiles. J. Zoo Wildl. Med. 24(3):245–255.
Jacobson, E. R. 1993b. The desert tortoise and upper respiratory tract disease. Prepared for the Desert Tortoise Preserve Committee, Inc., and U.S. Bureau of Land Management, Report No. BLM-CA-PT-93-004-6840, Riverside, California. 4 pp.
Jacobson, E. R. 1994. Causes of mortality and disease in tortoises: A review. J. Zoo Wildl. Med. 25(1):2–17.
Jacobson, E. R., J. M. Gaskin, M. B. Brown, R. K. Harris, C. H. Gardiner, J. L. LaPointe, H. P. Adams, and C. Reggiardo. 1991. Chronic upper respiratory tract disease of free-ranging desert tortoises (Xerobates agassizii). J. Wildl. Dis. 27(2):296–316.
Jacobson, E. R., T. J. Wronski, J. Schumacher, C. Reggiardo, and K. H. Berry. 1994. Cutaneous dyskeratosis in free-ranging desert tortoises, Gopherus agassizii, in the Colorado Desert of Southern California. J. Zoo Wildl. Med. 25(1):68–81.
Jacobson, E. R., M. B. Brown, I. M. Schumacher, B. R. Collins, R. K. Harris, and P. A. Klein. 1995. Mycoplasmosis and the desert tortoise (Gopherus agassizii) in Las Vegas Valley, Nevada. Chelon. Conserv. Biol. 1(4):279–284.
Jennings, W. B. 1992. Observations and activities of the naturalists for the Desert Tortoise Natural Area, Kern County, California: March 2–May 27, 1991. Report to the Desert Tortoise Preserve Committee, Inc., Ridgecrest, California, in cooperation with the U.S. Bureau of Land Management, Grant No. B950-A1-0034, Riverside, California. 40 pp.
Kidd, J. 1993. Observations and activities of the naturalists at the Desert Tortoise Research Natural Area, Kern County, California: 1 March through 31 May 1993. Prepared for the Desert Tortoise Preserve Committee, Inc., San Bernardino, California, in cooperation with the U.S. Bureau of Land Management, Riverside, California.
Knowles, C. 1989. A survey for diseased desert tortoises in and near the Desert Tortoise Natural Area. Spring 1989. Report for the U.S. Bureau of Land Management, Contract No. CA950-CT9-23, Riverside, California. 26 pp.
Ogg, S. and R. Gallant. 1992. Observations and activities of the naturalists for the Desert Tortoise Natural Area, Kern County, California: 3 March–31 May, 1992. Report to the Desert Tortoise Preserve Committee, Inc., San Bernardino County, California, in cooperation with the U.S. Bureau of Land Management, Grant No. B950-A1-0034, Riverside, California. 53 pp.
Overton, W. C. 1971. Estimating the numbers of animals in wildlife populations. In R. G. Giles (ed.), Wildlife Management Techniques, pp. 403–456. The Wildlife Society, Washington, D.C.
Schumacher, I. M., M. B. Brown, E. R. Jacobson, B. R. Collins, and P. A. Klein. 1993. Detection of antibodies to a pathogenic mycoplasma in desert tortoises (Gopherus agassizii) with upper respiratory tract disease. J. Clin. Microbiol. 31:1454–1460.
Snipes, K. P. and E. L. Biberstein. 1982. Pasteurella testudinis sp. nov.: A parasite of desert tortoises (Gopherus agassizi). International Journal of Systematic Bacteriology 32(2):201–210.
St. Amant, J. 1977. State Report—California. In M. Trotter (ed.), Proc. 1977 Symposium of the Desert Tortoise Council, pp. 21–22. Desert Tortoise Council, San Diego, California.
St. Amant, J. 1979. State Report—California. In E. St. Amant (ed.), Proc. 1979 Symposium of the Desert Tortoise Council, pp. 75–77. Desert Tortoise Council, Long Beach, California.
St. Amant, J. 1980. State Report—California. In K. A. Hashagen (ed.), Proc. 1980 Symposium of the Desert Tortoise Council, pp. 68–69. Desert Tortoise Council, Long Beach, California.
Swingland, I. R. and M. W. Klemens (eds.). 1989. The Conservation Biology of Tortoises. Occasional Papers of the IUCN Species Survival Commission (SSC) No. 5. IUCN, Gland, Switzerland. iv + 202 pp.
Turner, F. B., K. H. Berry, D. C. Randall, and G. C. White. 1987. Population ecology of the desert tortoise at Goffs, California, 1983–1986. Report to Southern California Edison Co., Rosemead, California. 101 pp.
Turner, F. B., P. Hayden, B. L. Burge, and J. B. Roberson. 1986. Egg production by the desert tortoise (Gopherus agassizii) in California. Herpetologica 42:93–104.
U.S. Bureau of Land Management. 1980. The California Desert Conservation Area Plan. U.S. Dept. of the Interior, Bureau of Land Management, Desert District, Riverside, California. 173 pp.
U.S. Bureau of Land Management. 1986. Chuckwalla Bench Area of Critical Environmental Concern: Management Plan and Environmental Assessment. U.S. Dept. of the Interior, Bureau of Land Management, California Desert District, Indio Resource Area, California. 40 pp. + appendices.
U.S. Bureau of Land Management and California Dept. of Fish and Game. 1988. A Sikes Act management plan for the Desert Tortoise Research Natural Area and Area of Critical Environmental Concern. U. S. Dept. of the Interior, Bureau of Land Management, California Desert District, Ridgecrest Resource Area, California. 43 pp. + unpaginated appendices.
U.S. Fish and Wildlife Service. 1994. Desert Tortoise (Mojave Population) Recovery Plan. U.S. Dept. of the Interior, Fish and Wildlife Service, Portland, Oregon. 73 pp. + appendices.
Woodbury, A. M. and R. Hardy. 1948. Studies of the desert tortoise, Gopherus agassizi. Ecol. Monogr. 81:146–200.

 

TABLE 1Number of attempts to release captive and illegally translocated desert tortoises at or adjacent to the Desert Tortoise Research Natural Area, eastern Kern County, California, between 1989 and 1995.
Number of incidents regarding tortoises: Number of illegal incidents
Year Captive releases Translocations per 1000 visitors Reference

1989 5 0 2 Howland, 1989
1990 1 4 5 Ginn, 1990
1991 2 2 2 Jennings, 1992
1992 3 4 4 Ogg and Gallant, 1992
1993 0 3 1 Kidd, 1993
1994 2 2 2 Boland, 1994
1995 1 2 1 Boland, 1995
Totals 14 17

This article was reprinted with permission from the New York Turtle and Tortoise Society

THE DESERT TORTOISE AND EARLY PEOPLES OF THE WESTERN DESERTS

THE DESERT TORTOISE
AND EARLY PEOPLES OF THE WESTERN DESERTS
 

by
 

Joan S. Schneider, Ph.D.
 

Department of Anthropology, University of California, Riverside
Riverside, California 92521, USA
 

A Special Report
prepared for the Desert Tortoise Preserve Committee, Inc.
 

March 1996
 

On the cover: Cahuilla Basketry Bowl made with natural and dyed juncus on a deergrass foundation with a tortoise or turtle motif, circa 1927. Collected at the Torres-Martinez Reservation, near Indio, California by Ira Caswell. In the collection of the Palm Springs Desert Museum. The basket is 15.7 centimeters (cm) or 6.2 inches (in) in diameter and 7.5 cm (3 in) high.

 

 

 

THE DESERT TORTOISE
AND EARLY PEOPLES OF THE WESTERN DESERTS

Desert tortoises (Gopherus [=Xerobates] agassizii) have been inhabitants of the Mojave and Colorado deserts of North America since Ice Age times.’ When people arrived on the scene, they interacted with tortoises in several ways: they noted their way of life, they found household and ritual uses for them, and they ate them. The past and present importance of desert tortoises to native peoples is reflected in the many archaeological sites that contain the physical remains of tortoises (bones and shell fragments), in native languages and oral traditions, and in media of artistic and symbolic expression.

THE IMPORTANCE OF DESERT TORTOISES TO EARLY PEOPLES

Archaeological, ethnographic, and historical data, gathered from many sources, have allowed a reconstruction of the ways that desert tortoises were important to the early peoples of the deserts. The archaeological record indicates that tortoises were used as early as 9,500 years ago2 and that their importance increased over time.3

As Food

The remains of desert tortoises that have been cooked and eaten have been identified at many archaeological sites in the desert West (Fig. 1); those sites include campsites on open landscapes, large roasting pits, inhabited caves and rockshelters, and residential structures.

 

Map of US distribution of Gopherus agassizii

Fig. 1. The current limit of the distribution of the desert tortoise in the United States (1920 to present) is designated by the screened area (adapted from Stebbins 1966; Kristin Berry, personal communication 1994). Each small dot marks one archaeological site where the cultural remains of desert tortoises have been found; each large dot represents a group of three or more sites located close together. The dates assigned to the sites range from approximately 9,500 to about 150 years ago. The numbers and distribution of archaeological desert tortoise locations shown on the map are biased because large areas of the deserts have not been studied. Archaeological studies, for the most part, have been carried out only in areas where they have been mandated by federal, state, or local government regulations.

According to a number of ethnographers and from information gathered from various historical documents, many desert-dwelling groups ate desert tortoises (Fig. 2). Some groups, however, especially those that lived along the Lower Colorado and Gila rivers, e.g., the Mohave, reportedly had an aversion to tortoise meat (see citation 3 below for specific information). Tortoise meat has been described as delicious and delicate in flavor, similar to chicken, but somewhat coarser in texture and with slightly fewer calories than chicken.4 Early Euroamerican miners and traders also reportedly enjoyed tortoise meat.5, 6 Tortoises were prepared in a variety of ways including roasting over fire or within roasting pits and boiled in stews (see citation 3, page 1, for more information).

 

Native American tribal territories within the geographic range of the desert tortoise

Fig. 2. A map of tribal territories of Native American Indian groups in the United States in the general area of the geographic range of the desert tortoise. Symbols within each territory indicate the way(s) that desert tortoises were important to each group: ceremony/ritual, symbolism/myth, household utensils, medicine, and food. The map was compiled and adapted from Keys to Tribal Territories included in Volumes 8, 10, and 11 of the Handbook of North American Indians published by the Smithsonian Institution, Washington, D.C. Group boundaries were fluid in the past and the boundary lines here are only approximations. Information on the uses of tortoises was gathered from ethnographies of the various groups (i.e., information gathered in the early 20th century) except for the Serrano information which was derived from archaeological data. Ethnographic information is usually limited by many factors, including the types of questions asked, the knowledge and/or cooperation of the person questioned, and selectivity in groups represented.

Exactly how desert tortoises were found and captured during prehistoric times is not known. Our best historic account comes from a description of tortoise-hunting practices of the Seri Indians of northern mainland Mexico. Among the Seri, tortoises were sometimes lured out of burrows with water placed at the entrance to the burrows or were sometimes dragged out of their burrows with long hooks.7 Dogs were used to locate ranging tortoises and tortoise burrows.

It is likely that tortoises were taken at all seasons of the year. Tortoise burrows have a distinctive shape; when tortoises are within burrows, they can easily be seen and removed. Although tortoises are more active during the spring and summer than at other times, they occasionally leave their winter burrows when the weather is warm and moisture is available and could be taken then.

As Medicine

Tortoise shell was sometimes powdered and used to relieve stomach and urinary tract afflictions among the Yavapai.8

As Household Utensils

Tortoise shells were used for bowls, ladles, seed-parching containers, spoons for children, scoops for digging or removing soil, and pottery-making tools (see citation 3, page 1 for specific details). The bowl shape of the upper shell (carapace) made it an ideal container. The slight curvature and smoothness of carapace fragments made them useful for spoons, scoops, and smoothing tools.

As Ceremonial Items

Rattles made from tortoise or turtle shells were often used on ceremonial occasions. Although most commonly used in areas within the range of desert tortoises, the rattles were of great value and were traded to groups far beyond the range. The upper and lower shells usually were laced or otherwise fastened together, either stones or hard seeds were placed within the hollow interior, and openings were sealed, often with asphaltum. A number of fragmental specimens of rattles have been recovered from archaeological sites. One ethnographic Cahuilla specimen, probably made in the early part of the 20th century, is in the collections of the Palm Springs Desert Museum (Fig. 3).

 

 

Fig. 3. Cahuilla Tortoise or Turtle Shell Rattle: shells (carapace and plastron) held together with copper wire; cotton twine-wrapped wooden handle; shell attached to handle with rawhide strips. Probably made in the 1940’s and purchased for the collections of the Palm Springs Desert Museum in 1961 or 1962. The rattle is about 29 cm (11.4 in) long, including the handle; the shell is 13 x 9 cm (5 x 3.5 in).

A collection of animal bones from an archaeological site at Joshua Tree National Park, interpreted as that of the cremation of an important person sometime in the last 1,000 years, contained 36 burned tortoise scapulae (shoulder blades). The unusual occurrence suggests that the tortoise bones were strung as a necklace, used as gaming or divining pieces or used for other ornamental or ritual purposes.9

Ancient Mesoamerican texts (codices) and architectural decorative motifs show representations of tortoise or turtle shells used as ceremonial vessels, rattles, drums, and as ceremonial garb.

 

TORTOISES IN SYMBOLISM AND MYTH

The unique characteristics of tortoises, sharply contrasting with those of other animals–their physical form, long life, relatively slow pace of travel, and other behaviors–contributed to their symbolic importance. Tortoise or turtle motifs and themes in Native American design and oral tradition suggest that spiritual values and symbolic significance were attached to the animals.

Tortoise/turtle figures in Indian rock art are not common, but they do exist. A number of tortoise/turtle petroglyphs (pecked designs on rock) are present in the Valley of Fire, Nevada (Fig. 4), and at a number of other Mojave Desert archaeological sites in California and Nevada. The interpretations of these depictions is uncertain; they may represent the importance of capturing tortoises for food or may be mythological, clan, or personal symbols.10 Tortoise or turtle designs are also incorporated in basketry (front cover) and sometimes used as decorative motifs on pottery. One group of Southern Paiute fed tortoise, chuckwalla, and rabbit meat to young eagles, captured as hatchlings and raised specifically for ceremonial purposes.

 

 

Fig. 4. Rock art motifs representing desert tortoises. Both of these petroglyphs are pecked into red sandstone outcrops in the Valley of Fire, Nevada. They are near archaeological sites where abundant fragments of desert tortoises indicate that the reptile was an important food source in earlier times. The tortoise petroglyph on the left is approximately 11 x 9 cm (4.3 x 3.5 in); note the characteristic posture of the legs. The tortoise petroglyph on the right is somewhat weathered so that the outlines are indistinct, but it is particularly interesting because the large dots probably represent the scutes covering the shell of the desert tortoise; this figure is about 22 x 19 cm (9 x 7.5 in).

Among the Chemehuevi, the “turtle” was a symbol of the spirit of the people and had an aura of sacredness. In one Chemehuevi myth, reported by ethnographer Carobeth Laird, “Turtle” accepted inevitable doom and died with great dignity. The animal expressed the Chemehuevi ideal: enduring patience, stamina for survival, and courage in hopeless situations.12 In Cahita Indian myth, the tortoise/turtle is portrayed as a semi-villain13; in Yavapai myth as a stranger (see citation 8, page 4, for details). A Mohave “song” tells the tale of an ancestral westward journey toward the Chemehuevi, a group that ate turtle.14 One Paiute coyote tale known as “Iron-Clothes” tells how the “land turtle” came to be used as food and how it was prepared and eaten.15

Tortoise/turtle symbolism in a wide range of cultures often incorporates themes such as long or eternal life, revered old age, and the tortoise/turtle shell as a foundation for or form of the earth in creation stories. For example, the Mayan calendrical system was sometimes represented as a segmented wheel, and tortoises or turtles, with their circular form and marginal scutes, were an obvious symbol of the passage of time in the world. Ritual self-inflicted bloodletting occurred at the end of certain Mayan time periods and turtle or tortoise, motif vessels are often depicted as being associated with this ritual.16, 17 Perhaps the importance of the tortoise/turtle as a symbol of the passage of time is the reason why the Mayan cosmology includes a constellation called the “turtle” (the same as we know as Orion, the hunter).18, 19

 

SUMMARY

Desert tortoises were important, both economically and ideologically, to the early peoples of the western deserts. Tortoises were available over a wide area and on a year-round basis. Although eating tortoise was “taboo” in a few groups, those same groups incorporated tortoise/turtle symbols in their art and mythology. Desert tortoises, along with rabbits, hares, and bighorn sheep, contributed to the protein portions of the diets of the majority of the prehistoric peoples of the area. The shells of desert tortoises were suitable for many utilitarian purposes, both in the household and for special ceremonial occasions.

 

ACKNOWLEDGEMENTS
G. Dicken Everson gathered a large proportion of the information on mythology and symbolism. Kathy Clewell, Museum Registrar at the Palm Springs Desert Museum, loaned photographs of the artifacts in the museum’s collections. Many archaeologists and faunal analysts in California, Nevada, Arizona, and Utah shared their own data. The Desert Tortoise Preserve Committee provided funds for the art work by Spring Warren. Kristin Berry assisted with editing and Carolyn Kameen assisted with word processing and format.

 

RECOMMENDED READINGS
Connolly, C., and N. Eckert. 1969. The archaeological significance of desert tortoise. Nevada State Museum Anthropological Paper No. 14: 80-92.

D’Azevedo, W. (ed.). 1986. Handbook of North American Indians.- Great Basin. Volume 11.Smithsonian Institution, Washington, D.C.

Douglas, C. L., D. L. Jenkins, and C. N. Warren. 1988. Spatial and temporal variability in faunal remains from four Lake Mojave-Pinto Period sites in the Mojave Desert. In: J. A- Willig, C. M. Aikens, and J. L. Fagan (eds.), Early Human Occupation in Far Western North America: the Clovis-Archaic Interface. Nevada State Museum Anthropological Papers No. 21: 131-144.

Ebling, W. 1986. Handbook of Indian Foods and Fibers of Arid America. University of California Press, Berkeley.

Felger, R., M. Moser, and E. W. Moser. 1981. The desert tortoise in Seri Indian culture. Pp. 113-120 in: K A- Hashagen (ed.), Proceedings of the 1981 Desert Tortoise Council Symposium. Long Beach, California.

Heizer, R. F. (ed.). 1978. Handbook of North American Indians: California. Volume 8. Smithsonian Institution, Washington, D.C.

Morafka, D. J., and C. J. McCoy (eds.). 1988. The ecogeography of the Mexican bolson tortoise (Gopherus flavomarginatus): Derivation of its endangered status and recommendations for its conservation, Annals of the Carnegie Museum 57 (1). Carnegie Museum of Natural History, Pittsburgh, Pennsylvania.

Ortiz, A. (ed.). 1983. Handbook of North American Indians.- Southwest. Volume 10. Smithsonian Institution, Washington, D.C.

Schneider, J. S., and G. D. Everson. 1989. The desert tortoise (Xerobates agassizii) in the prehistory of the southwestern Great Basin. Journal of California and Great Basin Anthropology 11(2):175-202.

Woodbury, A- M., and R. Hardy. 1948. Studies of the desert tortoise, Gopherus agassizii. Ecological Monographs 18:145-200.
Footnotes

1 Stebbins, R. C. 1966. A Field Guide to Western Reptiles and Amphibians. Boston, Massachusetts: Houghton Mifflin.

2 Douglas, C. L., D. L. Jenkins, and C. N. Warren. 1988. Spatial and temporal variability in faunal remains from four Lake Mojave-Pinto Period sites in the Mojave Desert. Pp. 131-144 in: J. A. Willig, C. M. Aikens, and J. L. Fagan (eds.), Early Human Occupation in Far Western North America: the Clovis-Archaic Interface. Nevada State Museum Anthropological Papers No. 21.

3 Schneider, J. S. and G. D. Everson. 1989. The desert tortoise (Xerobates agassizii) in the prehistory of the southwestern Great Basin and adjacent areas. Journal of California and Great Basin Anthropology 11(2):175-202.

4 Connolly, C. and N. Eckert. 1969. The archaeological significance of the desert tortoise. Nevada State Museum Anthropological Papers No. 14:80-92.

5 Fairchild, M. D. 1933. A trip to the Colorado mines in 1862. California Historical Society Quarterly 12:11-17.

6 Pepper, C. 1963. The truth about the tortoise. Desert Magazine 26(10):10-11.

7 Felger, R., M. Moser, and E. W. Moser. 1981. The desert tortoise in Seri Indian culture. Pp. 113-120 in: K. A. Hashagen (ed.), Proceedings of the 1981 Desert Tortoise Council Symposium. Long Beach, California.

8 Gifford, E. W. 1936. The Northeastern and Western Yavapai. University of California Publications in American Archaeology and Ethnology 34(4).

9 Goodman, J. D. II. 1992. Vertebrate faunal remains from the Campbell Collection. P. 9-4 in: A- B. Schroth (ed.), Cremations and Associated Artifacts from the Campbell Collection; Joshua Tree National Monument. Report on file at the National Park Service, Western Region.

10 Green, E. M. 1987. A Cultural Ecological Approach to the Rock Art of Southern Nevada. Master’s Thesis, University of Nevada, Las Vegas.

11 Kelly, I. T. Unpublished manuscript. Notebook of the Las Vegas Band, Southern Paige Field Notes. Berkeley: University of California Archives No. 138 2m. Anthropology Document 18 (UCARC microfilm CU 23.1, frames 18-24, 18-93).

12 Laird, C. 1976. The Chemehuevis. Banning, California: Malki Museum Press.

13 Beals, R. 1945. The Contemporary Culture of the Cahita Indians. Bureau of American Ethnology Bulletin No. 142.

14 Kroeber, A- L. 1925. Handbook of the Indians of California. Bureau of Ethnology Bulletin No. 78.

15 Sapir, E. 1930. Texts of the Kaibab Paiutes and Uintah Utes. Proceedings of the American Academy of Arts and Sciences 65(2).

16 Seler, E. 1939. Gesammelte Abhandlungen zur Amerikanischen Sprach – und Alterhumskunde, Vol. 4. Unpublished English translation under the direction of Charles P. Bowditch. Cambridge, Massachusetts: Carnegie Institution of Washington.

17 Taube, K A, 1988. A prehistoric Maya Katun Wheel. Journal of Anthropological Research 44(2):183-203.

18 Lounsbury, F. 1982. Astronomical knowledge and its uses at Bonompak, Mexico. Pp. 143-168 in: A.F. Aveni (ed.), Archaeoastronomy in the New World. Cambridge, England: Cambridge University Press.

19 Freidel, D., L. Schele, and J. Parker. Maya Cosmos. Three Thousand Years on the Shaman’s Path. New York: William Morrow Co., Inc.

THE DESERT TORTOISE AND UPPER RESPIRATORY TRACT DISEASE

THE DESERT TORTOISE
AND UPPER RESPIRATORY TRACT DISEASE

by

Elliot Jacobson, D.V.M., Ph.D.

University of Florida, Gainesville
Florida 32510, USA

A Special Report
prepared for the Desert Tortoise Preserve Committee, Inc.

rev. August 1992

THE DESERT TORTOISE AND UPPER RESPIRATORY TRACT DISEASE

BACKGROUND — UPPER RESPIRATORY TRACT DISEASE IN CAPTIVE TORTOISES

A disease characterized by a mild to severe nasal discharge has been seen for many years in captive tortoises in Europe, England, and the United States. Although a complete list of the number of species of tortoises known to develop this disease is unavailable, it would be fair to say that until proven otherwise, all species of tortoises should be considered susceptible. In England, this disease is commonly seen in Greek (Testudo graeca) and Hermann’s (T. hermanni) tortoises.1 The disease has also been seen in free-ranging gopher tortoises (Gopherus polyphemus) in Florida (Jacobson, pers. comm.). At the Veterinary Medical Teaching Hospital, University of Florida, species of tortoises presented with nasal discharge include Greek tortoises, leopard tortoises Geochelone pardalis), radiated tortoises (Geochelone radiata), Indian star tortoises (Geochelone elegans) and gopher tortoises (Gopherus polyphemus). The disease has also been commonly seen in captive desert tortoises (Gopherus [=Xerobates] agassizii).2

Until 1990-1991, attempts at demonstrating or incriminating a casual agent were unsuccessful. Because of negative findings and the failure to incriminate a specific bacteria, a virus was considered as a possible cause.3 In studies conducted on captive desert tortoises, a bacterial organism, Pasteurella testudinis, was isolated and incriminated as a possible cause.4 However, P. testudinis, has also been isolated from healthy tortoises and the significance of this organism remains unknown.

THE APPEARANCE OF UPPER RESPIRATORY TRACT DISEASE IN WILD TORTOISE POPULATIONS

In the 1970’s desert tortoises with signs of the disease were observed on the Beaver Dam Slope of Utah, a site where many captive tortoises were being released. In 1988, desert tortoises at the Desert Tortoise Natural Area (DTNA), Kern County, California were seen with clinical signs of illness similar to that of captive desert tortoises. Signs included a mucopurulent discharge from the nares, puffy eyelids, eyes recessed into the orbits, and dullness to the skin and scutes. Based upon these clinical signs, Upper Respiratory Disease Syndrome (URDS) was used to characterize this syndrome.

Surveys of the DTNA in 1989 and 1990 revealed that many tortoises were ill with the disease, and shells of many tortoises indicated a major die-off was underway. Research on long-term study plots with marked tortoises showed that more than 70% of adult tortoises died between 1988 and 1992 (Kristin Berry, pers. comm.). Other surveys indicated that free-ranging desert tortoises with URDS also widespread in the western Mojave Desert of California, around Las Vegas Valley in Nevada, on the Beaver Dam Slope of Utah/Arizona, and sporadically in low numbers in the Sonoran Desert of Arizona.

RESEARCH ON THE CAUSES OF UPPER RESPIRATORY TRACT DISEASE

In May 1989, with a contract from the U.S. Bureau of Land Management, we initiated studies on desert tortoises ill with URDS in an attempt to elucidate the responsible pathogens. During the course of these studies, the pathology of the disease was better understood and findings indicated that the upper respiratory tract was the major site of involvement.5 Based on these findings the disease was determined to be a chronic upper respiratory tract disease and the acronym URTD was used. Today, URTD more appropriately designates this illness and should replace URDS.

Microbiologic investigations with URTD failed to incriminate a virus as a potential causal agent. Pasteurella testudinis was isolated from most of the ill tortoises examined and a previously unidentified Mycoplasma was also isolated from ill tortoises.5 Electron microscopic studies confirmed the presence of Mycoplasma on the surface membranes of the upper respiratory tract of desert tortoises ill with URTD.

In 1992, research was conducted on transmission of the disease. The findings support the contention that Mycoplasma is the most likely cause of URTD. Koch’s postulates have been fulfilled and a causal relationship between Mycoplasma and URTD has been established. Still, Pasteurella and other bacteria may affect the severity of the disease.

A serologic (blood) test has been developed at the University of Florida to determine exposure status of tortoises to Mycoplasma. Preliminary studies are very promising in that this test may ultimately be useful in assessing condition of tortoises.

Predisposing factors such as poor nutrition (resulting from habitat degradation), drought, and release of captive desert tortoises ill with URTD into the wild are also more than likely involved. The whole issue of release of ill pet desert tortoises needs to be publicized, because this practice should not continue. Transmission studies have clearly demonstrated the infectious nature of URTD. Thus, it is safe to assume that captive ill tortoises can transmit this disease to both captive and free-ranging clinically healthy tortoises.

TREATMENT OF UPPER RESPIRATORY TRACT DISEASE

Until recently, no antibiotics or combination of antibiotics have been efficacious for treating tortoises ill with URTD. With evidence that Mycoplasma is the etiologic agent of URTD and that Pasteurella testudinis and other gram negative bacteria may contribute to the severity of the disease, antibiotic therapy with enrofloxacin (Baytril, Mobay Corp., Shawnee, Kansas) at 5 mg/kg of body weight every other day for IO treatments, is considered the therapy of choice. Additionally, injectable enrofloxacin should be diluted 1:10 in sterile saline and a small quantity (up to 0.5 cc) should be flushed up both nares of the affected tortoise utilizing a syringe and attached catheter of appropriate diameter. Flushing should be continued daily for 1 month (at least until the rhinitis has abated). Since enrofloxacin is very irritating to the mucous membranes surrounding the eyes, it is important to avoid contact of enrofloxacin with those tissues. It is important to maintain tortoises at an optimum environmental temperature during the course of treatment. While antibiotic therapy may result in clinical improvement and complete regression of clinical signs, this does not mean that this tortoise will be free of disease thereafter. Turtles may remain carriers of Mycoplasma for life with recurrence of the disease at some point in time in the future.

Results of clinical trials with these new drugs and drug combinations for treating tortoises ill with URTD are extremely promising for captive tortoises. Unfortunately the situation for ill free-ranging tortoises in not as promising. Since this disease more than likely is multifactorial, schemes for managing URTD in free-ranging populations are going to be difficult to develop and implement. Minimally tortoise hobbyists and veterinarians can make a major contribution by getting the word out that captive tortoises should not be released to the wild. More than likely this practice has contributed to the spread of URTD in wild populations of desert tortoises.

SUMMARY

The following points should be remembered with regard to the desert tortoise and URTD:

  1. URTD is a chronic infectious disease affecting not only the desert tortoise, but other tortoises as well.
  2. Scientific evidence supports the belief that Mycoplasma is the infectious agent responsible for URTD.
  3. Once infected with Mycoplasma, a tortoise may remain a carrier for life.
  4. URTD is a transmissible disease. Because of this, tortoises showing clinical signs of illness should be isolated from healthy tortoises.
  5. Different species of tortoises should not be kept together in captivity since foreign pathogens may be introduced into new hosts.
  6. Although antibiotic treatment may result in complete remission of clinical signs, tortoises may still develop the disease at a future date.
  7. III or formerly ill desert tortoises should never be released to the wild. Releases of captive tortoises may be responsible for disease outbreaks in the Mojave Desert.

Footnotes

1 Lawrence, K. and J. R. Needham. 1985. Rhinitis in long term Mediterranean tortoises (Testudo graeca and T. hermanni). Veterinary Record. 117:622-664.

2 Jackson, O. F., and J. R. Needham. 1983. Rhinitis and virus antibody titers in chelonians. Journal of Small Animal Practice. 24:31-36.

3 Snipes K. P., E. L. Biberstein, and M. E. Fowler. 1980. A Pasteurella sp. associated with respiratory disease in captive desert tortoises. Journal of the American Veterinary Medical Association. 177:804-807.

4 Snipes, K. P., and E. L. Biberstein. 1982. Pasteurella testudinis sp. nov.: a parasite of desert tortoises. International Journal of Systematic Bacteriology. 32:201-210.

5 Jacobson, E. R., J. M. Gaskin, M. B. Brown, R. K Harris, C. H. Gardiner, J. L. LaPointe, H. P. Adams, and C. Reggiardo. 1991. Chronic upper respiratory tract disease of free-ranging desert tortoises (Xerobates agassizii). Journal of Wildlife Diseases 27(2):296-316.

 

Prepared for the Desert Tortoise Preserve Committee, Inc.,
and U.S. Bureau of Land Management

EVIDENCE OF UNAUTHORIZED OFF-HIGHWAY VEHICLE ACTIVITY IN THE RAND MOUNTAINS AND FREMONT VALLEY, KERN COUNTY CALIFORNIA

EVIDENCE OF UNAUTHORIZED OFF-HIGHWAY VEHICLE ACTIVITY IN THE RAND MOUNTAINS AND FREMONT VALLEY, KERN COUNTY CALIFORNIA
The following Special Report is an excerpt of a study commissioned by the Desert Tortoise Preserve Committee, Inc. and prepared
by
Gilbert 0. Goodlett and Glenn C. Goodlett
The study was completed in spring, 1991.
 

EVIDENCE OF UNAUTHORIZED OFF-HIGHWAY VEHICLE ACTIVITY IN THE RAND MOUNTAINS AND FREMONT VALLEY, KERN COUNTY CALIFORNIA

 

PROJECT DESCRIPTION

At the request of the Desert Tortoise Preserve Committee, EnviroPlus Consulting undertook a project to analyze unauthorized off-highway vehicle (OHV) activity in the Rand Mountains and Fremont Valley of eastern Kern County, California. The area is adjacent to the northeastern part of the Desert Tortoise Natural Area (DTNA) and Area of Critical Environmental Concern (ACEC), contains the western Rand Mountain Area of Critical Environmental Concern (ACEC), and has significant habitat for the desert tortoise (U.S. Bureau of Land Management 1980; Sievers et al. 1988).

 Specific objectives of the study included the following:

  1. Investigate the degree of OHV impact on desert tortoise habitat in the study area with specific emphasis on those impacts that have occurred since the area was reopened to public use on November 21, 1990.
  2. Determine the degree to which public use of the land, most of which is OHV activity, conforms with the publicly announced BLM policies.
  3. If significant vehicle activity is occurring and if the vehicle use does not conform to BLM policies, identify the area of use.
  4. Determine if relationships exist between open routes and unauthorized activity.

BACKGROUND

The desert tortoise (Gopherus [Xerobates] agassizii) was listed by the State of California in June 1989 as a threatened species. A few months later, the U.S. Fish and Wildlife Service listed the species as endangered under an emergency rule, then followed with a permanent listing as threatened on April 4, 1990. The tortoise was listed because of rapidly declining populations, habitat loss and fragmentation. The sources for population losses include vandalism, vehicle kills, collections, disease, and excessive raven predation. For habitat damage and loss, the causes are multifold.

Declines in tortoise populations are well-documented for the western Mojave desert (Berry, 1990). Vandalism, damage to habitat from sheep grazing and off-highway vehicles (OHV), upper respiratory tract disease (URTD), and ravens are particularly critical issues in the Rand Mountains and Fremont Valley.

In 1989 the U.S. Bureau of Land Management (BLM) placed a significant portion of public land in the Rand Mountains and Fremont Valley under a temporary emergency quarantine and road closure to provide increased protection for the desert tortoise and its habitat (U.S. Bureau of Land Management, 1989). The area under quarantine included the Desert Tortoise Research Natural Area (DTNA) and Area of Critical Environmental Concern (ACEC) and West Rand Mountains ACEC. All human activities, except those administratively authorized, were excluded from the DTNA and West Rand Mountains ACEC.

The protective action was lifted on November 21, 1990 (U.S. Bureau of Land Management, 1990). According to a BLM media release, “Approximately 150 miles of roads will be opened in the area to provide access. Open routes will be signed with a brown post indicating their open status. Unmarked routes and trails and those marked with a red ‘closed’ post may not be used by motorized vehicles.” This is a “…75 percent reduction in the existing routes.” Further, “… camping will be allowed within 100 feet of a road in previously disturbed areas only.”

SUMMARY OF FINDINGS

Field surveys were conducted December 13-15, 1990 and January 20, 1991. Methods used to evaluate OHV impacts included: driving 46.2 miles of open routes and recording the unauthorized tracks and trails which crossed the open routes; walking 37 transacts, each of which was 500-feet long, perpendicular to open routes and recording OHV impacts; raking closed routes and rechecking them 34 days later for unauthorized vehicle use; and incidental observations.

A total of 287 unauthorized (unmarked or closed) trails with at least five tracks per trail were observed to cross 46.2 miles of surveyed open routes. Of these, 93% of trails were unmarked, and the remaining 7% were marked closed. The signed, closed routes represent a small fraction of the total number of trails being used by OHV enthusiasts.

On each of the 37 transacts, a mean of 27 unauthorized tracks were found, an average of one track every 20 feet. Impacts were found to vary in an inverse proportion to its distance from an open route. Near the edge of an open route (0-20 feet), an average of 2.70 OHV impacts (tracks and trails) per 20 linear feet were found. Further from the trail impacts tapered off to an average of 0.87 per 20 linear feet.

Twenty-one signed, closed routes were raked on December 15, 1991. Five of the signs marking these trails had been vandalized. When 16 of the trails were rechecked 34 days later, 206 new OHV tracks were found with a mean of 13 tracks per closed route.

Unauthorized OHV activity was observed during both survey periods. In one instance, group of about eight OHV riders were observed riding on unmarked trails. In another instance truck and motorcycle were observed riding on a signed, closed route.

Bureau of Land Management (BLM) policies limiting vehicle use to signed, open routes are ineffective. Intensive, negative impacts to desert tortoise habitat are occurring as a result. Only a small fraction of unauthorized trails (7%) are marked as closed. Even on the trails marked as closed, unauthorized use is continuing. Results from 37 transacts suggest that unauthorized OHV impacts are related to open routes with these impacts decreasing as the distance from the open route increased.

LEVELS OF UNAUTHORIZED VEHICLE USE

BLM instructions governing OHV activities are not being heeded. As a result, significant degradation of tortoise habitat is occurring. Unauthorized use is astoundingly high. From trail and track mapping, an average of 47 unauthorized tracks per linear mile was found. This estimate is low since single tracks not associated with a trail and trails with fewer than 5 tracks were not recorded. A more accurate estimate of unauthorized tracks is derived from the data set of 37 transacts. On the average, one unauthorized track was encountered every 20 linear feet. This represents an intensive, negative impact to the habitat of a federally listed species.

Transect data also reveal a relationship between open routes and unauthorized OHV impacts. Impacts are highest close to the open route, suggesting that the presence of an open route may induce negative impacts for substantial distances from the route edge. Even at 500 feet from an open route, unauthorized tracks were observed at a rate of almost one per 20 linear feet. These impacts are apparently difficult to control.

Marking routes as “closed” is an ineffective measure against trespassing. The contrary seems to be the case. Five of twenty-one signs on closed routes were vandalized. The degree of trespassing is intensive. An average of 11 tracks were found per closed route.

REFERENCES

Berry, K. H. 1990. The Status of the Desert Tortoise in California. Draft Report from the U.S. Bureau of Land Management, Riverside, California, to the U.S. Fish and Wildlife Service, Portland, Oregon.

Sievers, A., J. B. Aardahl, K. H. Berry, B. L. Burge, L. D. Foreman, G. E. Moncsko, and J. St. Amant. 1988. Recommendations for Management of the Desert Tortoise in the California Desert. California Desert District, Riverside, California, 55 pp. with Appendices.

U.S. Bureau of Land Management. 1980. The California Desert Conservation Area Plan. U.S. Bureau of Land Management, Riverside, California. 173 pp.

U.S. Bureau of Land Management. 1989. U.S. Bureau of Land Management. Temporary emergency quarantine in the Desert Tortoise Natural Area and western Rand Mountain Area of Critical Environmental Concern. Federal Register 54(181).

U.S. Bureau of Land Management. 1990. Temporary emergency quarantine in the Desert Tortoise Natural Area and West Rand Mountains Area of the Critical Environmental Concern (ACEC); Ridgecrest Resource Area, Kern County, California. Federal Register 55(197): 41392-41393.