The Desert Tortoise Preserve Committee Invites Public Comments on OHV Grant Application

For Immediate Release: March 3, 2016

The Desert Tortoise Preserve Committee (DTPC) invites public comments on its preliminary grant application to the California State Parks Off-Highway Motor Vehicle Recreation (OHMVR) Division.

THE DTPC INVITES PUBLIC COMMENTS ON OHV GRANT APPLICATION

The Desert Tortoise Preserve Committee (DTPC) invites public comments on its preliminary grant application to the California State Parks Off-Highway Motor Vehicle Recreation (OHMVR) Division. The DTPC is requesting funding to support a restoration project in the Eastern Expansion Area of the Desert Tortoise Research Natural Area, adjacent to an authorized OHV route managed by the Bureau of Land Management and California City Police Department. The purpose of the Restoration Grants Program, as defined by the OHMVR Division, is to provide Ecological Restoration or Repair to habitat damaged by authorized or unauthorized OHV use. The DTPC seeks restoration funds to prevent continued habitat degradation related to unauthorized OHV activity on approximately 2,700 acres of conservation lands and to begin the restoration process on disturbed areas by protective fencing. The DTPC will provide some matching dollars to the project. If funded, the DTPC will install wildlife-friendly fencing around areas of contiguous conservation lands, followed by trash removal and control of invasive plant infestations. Where fencing is not feasible, unauthorized routes will be camouflaged to discourage unauthorized use. Additionally, if conditions allow, the DTPC will begin collecting native seed for future revegetation projects.

The DTPC will also request funding to re-instate a responsible recreation education program providing important information to the OHV community. This program is intended to increase public knowledge about the effects of irresponsible OHV use and to help prevent the creation of illegal trails. The DTPC would appreciate your comments on these projects. You can review the grant applications, along with those from other agencies, local governments, and non-profits, on-line at www.ohv.parks.ca.gov. The public comment period is open from Tuesday, March 8, 2016 to Monday, April 4, 2016. Comments should be sent directly to the OHMVR Division at ohvinfo@parks.ca.gov and to the DTPC at dtpc@pacbell.net. Late comments will be forwarded to the division separately. Final grant applications are due May 2, 2016. For more information about the proposed restoration project, a public meeting will be held at the DTPC office in Riverside to provide an overview of both projects. Please contact the DTPC office at (951) 683 – 3872 or dtpc@pacbell.net for additional information.

For more information about the proposed restoration project, please contact Jillian Estrada at (951) 683-3872 or dtpc@pacbell.net.

_____________________________________________________________________

Public Meeting Announcement

The Desert Tortoise Preserve Committee, Inc., would like to announce a public meeting on Thursday, February 12th, at 5:00 to 6:00 PM, at its office at 4067 Mission Inn Avenue, Riverside, CA 92501. During this meeting, the Preserve Manager will present the Committee’s plans to fence its properties around the Desert Tortoise Research Natural Area in Kern County, Ca. These plans will also be on display at the DTPC’s Booth at the upcoming Annual Symposium of the Desert Tortoise Council Annual Symposium on February 20–22, 2015 at Sam’s Town Hotel and Casino, 5111 Boulder Hwy, Las Vegas, NV 89122

THE DTPC INVITES PUBLIC COMMENTS ON OHV GRANT APPLICATION

Desert Tortoise Preserve Committee, Inc.

News Release

For Immediate Release: March 4, 2014

Contact: Mary Logan (951-683-3872) or dtpc@pacbell.net

 

 THE DTPC INVITES PUBLIC COMMENTS ON OHV GRANT APPLICATION

 

The Desert Tortoise Preserve Committee (DTPC) invites public comments on its preliminary grant application to the California State Parks Off-Highway Motor Vehicle Recreation (OHMVR) Division.

The DTPC is requesting funding for a restoration project in the Expansion Areas of the Desert Tortoise Research Natural Area, which abut authorized OHV routes managed by the Bureau of Land Management and California City Police Department.  The purpose of the Restoration Grants Program, as defined by the OHMVR Division, is to provide Ecological Restoration or Repair to habitat damaged by authorized or unauthorized OHV use.  The DTPC seeks restoration funds to prevent continued habitat degradation related to unauthorized OHV activity on approximately 4,400 acres of conservation lands, and to begin restoration of the disturbed areas to their natural state.  If funded, the DTPC will install wildlife-friendly fencing around areas of contiguous conservation lands and will use vertical mulching and other techniques for camouflage restoration of unauthorized routes in areas where fencing is not feasible.  The DTPC will work with restoration crews and volunteers to remove trash from the restoration sites and to control invasive plant infestations along closed, unauthorized routes.  Additionally, if conditions allow, the DTPC will begin collecting native seeds for future revegetation projects.

The DTPC would appreciate your comments on this project.  You can review the grant application, along with those from other agencies, local governments, and non-profits, on-line at www.ohv.parks.ca.gov .  The public comment period is open from Tuesday, March 4, 2014 until Monday, April 7, 2014. Comments should be sent directly to the OHMVR Division at ohvinfo@parks.ca.gov and to the DTPC at dtpc@pacbell.net.  Late comments will be forwarded to the division separately.  Final grant applications are due May 5, 2014.

For more information about the proposed restoration project, please contact Mary Logan at (951) 683-3872 or dtpc@pacbell.net.

Mohave Ground Squirrel Observations, Spring 2011

Mohave Ground Squirrel Observations, Spring 2011

Article and photos by Freya Reder

The Desert Tortoise Research Natural Area (DTRNA) provides protected habi tat not only for the desert tortoise, but for all wildlife and plant species that exist within its boundaries.  The Mohave ground squirrel (Xerospermophilus mohavensis) is one such species. The Mohave ground squirrel is a small herbivorous rodent found only in the western Mojave Desert in desert-scrub habitats.  Because of habitat loss and fragmentation, the Mohave ground squirrel has long received state protection and has been listed as Threatened under the California Endangered Species Act since 1985.  The species is currently under review for federal listing.  Like the desert tortoise, the Mohave ground squirrel has a limited period of activity.  For adults, this active season usually extends from February through July, with the rest of the year spent in dormancy.  For juveniles, this period of activity is extended through August, as additional time is needed for their growth and dispersal from their natal burrows.  Past studies have shown that the amount and timing of winter rains affect Mohave ground squirrel reproduction and that in years with significantly low winter rainfall, Mohave ground squirrels will not reproduce in the spring.  One of the most exciting discoveries for us at the DTRNA this spring was that it proved to be a reproductive year for the Mohave ground squirrel.

I began the season as the Interpretive Naturalist with equal interests in the desert tortoise and the Mojave ground squirrel, due to their threatened status.   Documentation of all species encountered is part of the job of the Naturalist, and so it was with the Mohave ground squirrel.  Having had a few fleeting glimpses, I was interested to see and point out a Mohave ground squirrel to a visiting friend.  We stopped to observe what turned out to be a lactating female, evident by her dark and swollen nipples.  She stood watching us while feeding on unidentifiable seeds in the nearby wash.  Making a mental note, I began to look for this female daily when out walking on the trails, and more often than not was rewarded with a sighting of her.  This lactating female had several dark patches of skin on her back where hair was missing, making her easily identifiable and leading me to refer to her as “Patches” from then on.

A week later, just after opening the gate to the DTRNA, a couple from Ohio arrived. True wildlife enthusiasts, this couple described themselves as primarily birders who also had a “life list” of mammals throughout the world they intended to see.  Today, they had come in search of the Mohave ground squirrel.  I pointed our visitors in the direction of Patches’ burrow, telling them I would catch up with them shortly and we would look for ground squirrels and tortoises together.  When I joined them a short time later on the Animal Loop, I asked if they had luck and they said yes, they had in fact seen 3 juvenile Mohave ground squirrels!  Excited by this news I asked them to show me where they had seen the juveniles.   Thirty meters downstream from Patches’ burrow were three juveniles of undetermined sex basking in the morning sun.  Later the same day, I was rewarded with a sighting of Patches with the juveniles nearby.

Within a short period of time, I began to observe several adult Mohave ground squirrels.  Soon, I spotted another lactating female near the latrine in the Interpretive Center (IC). This female also had dark, swollen nipples but lacked the dark patches of skin on her back. A few weeks later, another separate litter of four juveniles emerged, this time on the entrance road into the DTRNA.  Simultaneou sly, a litter of antelope squirrels emerged in the same area, on the same day, often appearing to use the same burrows.  I took advantage of their proximity to the road to capture some brilliant footage on my camera of the juveniles, two while they were being bitten by red ants. One of these juveniles I captured on film encountering this tiny but formidable foe for the first time face to face. Additionally I observed them feeding on the seeds of checker fiddleneck (Amsinckia tessellata) seeds and red-stemmed filaree (Erodium cicutarium), and the forbs of rose and white wild buckwheat (Eriogonum gracillimum); one individual sampled the dried flower of a goldfield (Lasthenia californica).

A third lactating female was observed while on “morning rounds” with visitors.  I wanted to inspect a tortoise burrow in a nearby mineral assessment mound.  When first approaching the mound I had seen and pointed out a Mohave ground squirrel to the visitors.  Through binoculars I observed while it stood in alarm and then disappeared into the mouth of tortoise burrow.  Upon closer inspection of the tortoise burrow and its fresh tortoise tracks, the ground squirrel’s head appeared a few meters away in the mound.  She glanced at me, chirped twice in alarm, and disappeared into the same hole.  Only in the photographs did I later see that the squirrel was a lactating female.

In preparation for the long period of dormancy during fall and winter, the Mohave ground squirrel must acquire bulk mass in the form of fat reserves in order to survive.  It is common and necessary for them to triple in body weight and mass during this time.   Adult females take longer to acquire bulk mass due to their reproduction; therefore, the male’s state of enormity becomes evident well before that of the females or juveniles.  One male in particular appeared to put on bulk mass well before the others and I began to seek him out daily.

This adult male’s burrow was situated in the same mineral assessment mound previously mentioned, the female now presumably displaced or had simply moved house. He was what I refer to as “user friendly” in his tolerance and seeming disinterest in my presence on foot, providing I approached cautiously.  It was not unusual to spend 15 or 20 minutes, sometimes longer observing and photographing him as he would forage and sometimes cache the seeds of thistle sage (Salvia carduacea) and dried fiddleneck.  Of equal interest to him were Fremont pincushion (Chaenactis fremontii) and creosote (Larrea tridentata) flowers. This male began acquiring bulk mass earlier than any of the other adult males I observed and by this time seemed quite stationary; I observed him foraging no further than a meter or so from one of his burrow entrances.

Excited further by these field observations, I reported my findings to Dr. Kristin Berry who then shared this information with the rest of the Desert Tortoise Preserve Committee (DTPC).  Given that it was clearly a good year for Mohave ground squirrel reproduction in the DTRNA, the DTPC determined that it would be advantageous to survey the surrounding expansion areas for the presence of Mohave ground squirrels. I undertook their request with relish and over the course of the next three weeks undertook the task of surveying these expansion areas by vehicle and on foot.  Recording the geographic locations, date, times, approximate temperature, behavior, forage species, and  the presence or absence of white-tailed antelope squirrels, and using photographic documentation whenever possible, I covered areas of frequent sightings regularly by vehicle and expanded into new, unexplored areas daily.

During the hours of field observation, the Mohave ground squirrels foraged heavily on dry fiddleneck seeds and filaree seeds, both dry and green.  Additionally they were seen foraging on flowers of creosote bush, Anderson’s thorn bush (Lycium andersonii), desert calico (Loeseliastrum sp.), and what appeared to be miniature woolly star (Eriastrum diffusum), and the seeds of little gold poppy (Eschscholzia minutiflora), thistle sage, and Fremont pincushion.   At the end of the three weeks and approximately 75 hours of surveying, we had recorded 69 observations of Mohave ground squirrels in and around the DTRNA.  Based on geographic locations and timing of observations and the physical features of the animals, we estimate at least 32 individuals were sighted, including 19 adults (7 females, 3 males, and 9 of undetermined sex) and 14 juveniles (3 females, 1 male, and 9 of undetermined sex).  The DTRNA is indeed an important area not only for the preservation of the desert tortoise, but also that of the Mohave ground squirrel.  The DTPC hopes to continue studying this species and its habitat needs in and around the Natural Area.

I would like to give special thanks to Denise LaBerteaux from her assistance in sex determination and identification.   Dr. Berry and the other DTPC board members deserve special thanks as well for their prompt response and for launching this field survey.  Lastly, I would like to thank Mary Kotschwar for her unyielding support, encouragement and assistance throughout this study and the season.

 

References

Bartholomew, G.A. and J.W. Hudson. 1960. Aestivation in the Mohave ground squirrel Citellus mohavensis. Bulletin of the Museum of Comparative Zoology 124:193–208.

Best, T. L. 1995. Spermophilus mohavensis. American Society of Mammalogists, Mammalian Species Number 509:1–7.

California Department of Fish and Game. 2011. State & Federally Listed Endangered & Threatened Animals of California.  < http://www.dfg.ca.gov/biogeodata/cnddb/plants_and_animals.asp> Accessed September 9, 2011.

Harris, J.H. and P. Leitner. 2004. Home range and use of space in Mohave ground squirrels (Spermophilus mohavensis). Journal of Mammalogy 85: 517–523.

Harris, J. H., and P. Leitner. 2005 Long-distance movements of juvenile Mohave ground squirrels, Spermophilus mohavensis. Southwestern Naturalist 50: 188-196

 

Source: Tortoise Tracks 31:3 Fall 2011

The Mysterious Mohave Ground Squirrel

THE MYSTERIOUS MOHAVE GROUND SQUIRREL

By Phil Leitner

One of the more remarkable denizens of the California desert is a small brown ground squirrel. About 9 inches from nose to tip of tail, the Mohave ground squirrel (Spermophilus mohavensis) is found only in the western Mojave Desert. Their sophisticated desert survival skills allow them to avoid the extremes of the hostile climate. Hard to find and even more difficult to observe and study, these rare and elusive little rodents have baffled biologists over the years. Now, new efforts are underway to discover their habitat requirements and determine their conservation status.

We do know that Mohave ground squirrels are active only in the spring and summer, when they feed eagerly on the leaves and seeds of native shrubs and annual plants. As the desert dries out in June and July, they fatten in preparation for a long period of dormancy. By midsummer they curl up in their underground nests and allow body temperature, heart rate, and metabolism to fall drastically. In this physiological state, they can survive on stored body fat until the winter rains bring a new flush of green vegetation.

Mohave ground squirrels begin to emerge from their burrows in February, when the males may travel up to a mile per day in search of mates. The success of these amorous excursions becomes evident by the end of March, when litters of 6-9 young are born. The babies grow quickly and are weaned by early May. In just a few weeks, they are ready to set off in search of their own patch of desert. Young Mohave ground squirrels disperse in late May and early June. Often they move in next door to their mother’s home range, but some, especially the young males, can move up to four miles before settling down.

In the Mojave Desert, it is not unusual for the winter rains to fail, creating hard times for all desert wildlife. Mohave ground squirrels have their own approach to coping with drought. If the total winter rainfall is under three inches, they simply don’t reproduce. All available forage is converted to body fat and they can enter dormancy as early as April. Better to try again next year than to give birth to young who probably won’t survive and jeopardize your own chances of putting on enough fat to make it through dormancy. As a result, Mohave ground squirrel numbers decline precipitously after a low rainfall year and two successive years of drought can lead to the extinction of local populations. After a couple of good years, dispersing young may recolonize these areas.

The Mohave ground squirrel has long been listed as Threatened under the California Endangered Species Act. In spite of its protected status, little is known of its habitat needs or even where it still occurs. In many areas within its historic range, there are no recent records. This information is essential to the development of a conservation strategy for the species. The Desert Tortoise Preserve Committee is currently taking the lead in a new research effort, with funding from the California Energy Commission. Field studies began this spring in the Desert Tortoise Research Natural Area, the Pilot Knob Grazing Allotment, and the Kramer Hills to locate populations for long-term ecological study. Much more work will be needed to clear up the mysteries surrounding the Mohave ground squirrel and to assure it a secure future in the Mojave Desert ecosystem.

Source: Tortoise Tracks 19:2 Summer 1999

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.

LITERATURE CITED

Beatley, T. 1994. Preserving the desert tortoise: The Clark County habitat conservation plan. Chapter 10 in Habitat Conservation Planning: Endangered Species and Urban Growth, pp. 146–172. University of Texas Press, Austin.
Brussard, P. F., K. H. Berry, M. E. Gilpin, E. R. Jacobson, D. J. Morafka, C. R. Schwalbe, C. R. Tracy, F. C. Vasek, and J. Hohman. 1994. Proposed desert wildlife management areas for recovery of the Mojave population of the desert tortoise. U.S. Fish and Wildlife Service, Region 1-Lead Region, Portland, Oregon.
Clark County, Nevada. 1995. 1995 Clark County desert conservation plan. Clark County, Nevada.
Committee on the Applications of Ecological Theory to Environmental Problems, Commission on Life Sciences and National Research Council. 1986. Indicator species and biological monitoring. Chapter 7 in Ecological Knowledge and Environmental Problem-Solving: Concepts and Case Studies, pp. 81–87. National Academy Press, Washington, D. C.
Noss, R. F. 1991. Protecting habitat and biological diversity. Part I: Guidelines for regional reserve systems. Report to the National Audubon Society.
Regional Environmental Consultants (RECON). 1991. Short-term habitat conservation plan for the desert tortoise in Las Vegas Valley, Clark County, Nevada. Prepared for Clark County, Nevada, by RECON, San Diego, California.
Ryder, O. A. 1986. Species conservation and systematics: The dilemma of subspecies. Trends in Ecol. Evol. 1:9–10.
Stebbins, R. C. 1985. A Field Guide to Reptiles and Amphibians in the Western United States. Houghton-Mifflin Company, Boston.
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.
Tear, T. H., J. M. Scott, P. H. Hayward, and B. Griffith. 1995. Recovery plans and the Endangered Species Act: Are criticisms supported by data? Conserv. Biol. 9(1):182–195.
Thomas, J. W., (Chairman), E. D. Forsman, J. B. Lint, E. C. Meslow, B. R. Noon, and J. Verner. 1990. A conservation strategy for the northern spotted owl. Interagency Scientific Committee to Address the Conservation of the Northern Spotted Owl. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior: Bureau of Land Management, Fish and Wildlife Service, and National Park Service, Portland, Oregon. 427 pp.
U.S. Bureau of Land Management. 1980. The California Desert Conservation Area Plan, 1980. U.S. Department of the Interior, Bureau of Land Management, Riverside, California. 173 pp. + appendices.
U.S. Bureau of Land Management. 1988a. Desert tortoise habitat management on the public lands: A range-wide plan. U.S. Department of the Interior, Bureau of Land Management, Washington, D.C. 23 pp.
U.S. Bureau of Land Management. 1988b. Recommendations for management of the desert tortoise in the California Desert. U.S. Department of the Interior, Bureau of Land Management, Riverside, California. 54 pp. + appendices.
U.S. Bureau of Land Management. 1992a. Draft Stateline Resource Management Plan and Environmental Impact Statement. U.S. Department of the Interior, Bureau of Land Management, Las Vegas District Office, Las Vegas, Nevada. (Supplement to the Draft issued in May 1994. No final plan has yet been issued.)
U.S. Bureau of Land Management. 1992b. Arizona Strip Resource Management Plan. U.S. Department of the Interior, Bureau of Land Management, St. George, Utah.
U.S. Bureau of Land Management. 1995. Draft West Mojave coordinated management plan and environmental impact statement. May 1995 administrative review draft. U.S. Department of the Interior, Bureau of Land Management, Riverside, California.
U.S. Fish and Wildlife Service. 1980. Endangered and threatened wildlife and plants: Finding on desert tortoise petition. Federal Register 50:49868–49870.
U.S. Fish and Wildlife Service. 1981. Recovery plan for St. Croix population of the leatherback turtle (Dermochelys coriacea). Region 4, U.S. Fish and Wildlife Service, Fish and Wildlife Reference Service, Denver, Colorado. 20 pp.
U.S. Fish and Wildlife Service. 1988. A recovery plan for the ringed sawback turtle, Graptemys oculifera. Southeast Region, U.S. Fish and Wildlife Service, Atlanta, Georgia. 28 pp.
U.S. Fish and Wildlife Service. 1989a. Endangered and threatened wildlife and plants; Emergency determination of endangered status for the Mojave population of the desert tortoise. Federal Register 54(149):32326.
U.S. Fish and Wildlife Service. 1989b. Alabama red-bellied turtle recovery plan. U.S. Fish and Wildlife Service, Jackson, Mississippi. 17 pp.
U.S. Fish and Wildlife Service. 1990a. Endangered and threatened wildlife and plants; Determination of threatened status for the Mojave population of the desert tortoise. Federal Register 55(63):12178–12191.
U.S. Fish and Wildlife Service. 1990b. Flattened musk turtle (Sternotherus depressus) recovery plan. U.S. Fish and Wildlife Service, Southeast Region, Jackson, Mississippi. 15 pp.
U.S. Fish and Wildlife Service. 1993. Grizzly bear recovery plan. Missoula, Montana. 181 pp.
U.S. Fish and Wildlife Service. 1994a. The desert tortoise (Mojave population) recovery plan. U.S. Fish and Wildlife Service, Region 1-Lead Region, Portland, Oregon. 73 pp. + appendices.
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

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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.
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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