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




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

        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.



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)



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



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)



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)


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