28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

Declining Trends in Desert Tortoise Populations at Long-term Study Plots in California between 1979 and 2002: Multiple Issues

Kristin Berry
U. S. Geological Survey, Western Ecological Research Center, 22835 Calle San Juan de Las Lagos, Moreno Valley, CA 92553

Between 1971 and 1980, 27 study plots were established in the Mojave and Colorado (western Sonoran) deserts in California to conduct research on demographic attributes of desert tortoise populations. Fifteen of these plots were ultimately selected for long-term monitoring of status and trends in populations and their habitats. The plots were in habitat types typical of valleys and alluvial fans on public lands throughout the Mojave and Colorado deserts. The plots had tortoise populations with densities ranging from <25 tortoises/km² (low) to >75 tortoises/km² (high) when they were initially established. The 15 plots have been surveyed periodically in spring at intervals ranging from 1 to 10 years using mark-recapture techniques. Since the Desert Tortoise (Mojave Population) Recovery Plan (Fish and Wildlife Service, 1994) was prepared in 1992-1993, 12 of the 15 plots have been re-surveyed, and recent data sets are available. Plots are in five of the desert tortoise "recovery units": Western Mojave, Northeastern Mojave, Eastern Mojave, Northern Colorado, and Eastern Colorado.

Since the plots were first established, tortoise populations have experienced declines both in numbers of tortoises registered during the censuses and in densities of live tortoises at most sites (Berry and Medica, 1995; Brown et al., 1999; Berry et al., 2002). Declines of >50% and up to 96% have occurred regardless of whether initial tortoise densities were low (<25 tortoises/km²), moderate (25-75 tortoises/km²), or high (>75 tortoises/km²). Populations at each study plot have somewhat different characteristics. For example, two populations were increasing in densities during the 1980s (Ward Valley, Chemehuevi Valley) but experienced significant declines that began in the late 1980s. The "gold standard" population at Goffs remained stable between 1977 and 1994, but declined catastrophically between 1994 and 2000. Declines in numbers and densities of live tortoises have been confirmed by corresponding increases in shell-skeletal remains, including remains of marked tortoises (except at one site, Kramer, where poaching has been documented).

In the ~25-30 years since the plots were first established, causes of mortality have varied by region, year(s) or groups of years, degree of protection of the site from anthropogenic disturbances, remoteness from human activities, and lack of access (e.g., see Berry, 1986). At many sites, multiple factors contribute to deaths of tortoises and habitat deterioration (e.g., grazing, roads, invasion and proliferation of alien plants, vandalism, off-highway vehicle use, raven predation, domestic dogs). For example, in an evaluation of shell-skeletal remains collected from the Chemehuevi plot in 1999, a substantial number of tortoises appeared to have been killed or severely injured by vehicles. Likewise, the Goffs plot also had shell-skeletal remains of road-killed or vehicle-injured tortoises.

Diseases appear to be playing increasingly important roles in mortality, especially since the mid-1980s. Shell diseases, present at low levels in many populations when the plots were first established, increased during the 1980s and 1990s (Jacobson et al., 1994; Christopher et al., 2003). Shell diseases appear to be associated with high mortality rates at several sites, particularly in the northeastern and eastern Mojave and Colorado deserts (e.g., Jacobson et al., 1994; Berry, 1997). Necropsies provide important information on causes of ill health and death (Homer et al., 1998). Necropsies of tortoises with shell disease from the Goffs and Chemehuevi Valley plots have confirmed the presence of shell diseases, thyroid degeneration and dysplasia, possible toxic changes, degeneration and/or atrophy of pancreas, liver, testes, atrophy, and/or degeneration of skeletal muscle, and elevated levels of one or more elements (Homer and Berry, unpublished data). Upper respiratory tract disease (URTD) has been documented at Mojave plots but not, at this time, on Colorado Desert plots. URTD undoubtedly has contributed to population declines within the Desert Tortoise Research Natural Area (Berry, 1997; Brown et al., 1999) and other sites in the western Mojave Desert.

The effects of diseases on tortoises are likely exacerbated by altered habitats and changes in available food items. For example, in desert tortoise critical habitat in the western Mojave Desert, alien annual plants now compose >60% of the annual biomass (Brooks, 1998), and aliens effectively invade tortoise habitats and compete with native annuals for nutrients (Brooks, 1999, 2000). In this region, tortoises prefer native to non-native plants (Jennings, 1993). Protection has a beneficial effect on tortoise populations and habitat. At the Desert Tortoise Research Natural Area, tortoise numbers and densities are higher in the protected area inside the fence compared with the unprotected area outside the fence. Surveys conducted in 2002 indicated that densities of tortoises inside the fence were about three times higher than outside the fence.

Acknowledgements: All surveys prior to 1994 were funded by the Bureau of Land Management (BLM). For 1995 and 1996, the National Biological Survey and Department of the Army (Ft. Irwin), provided funds. In 1997, the Desert Tortoise Preserve Committee, California Department of Fish and Game, and California Department of Fish and Game and BLM provided support for the interpretive center plot at the Desert Tortoise Research Natural Area. Between 1999 and 2002, the U. S. Geological Survey provided support for one plot per year. In 2002, the California Department of Fish and Game funded four plots, necropsies, and health surveys; the Desert Tortoise Preserve Committee and National Fish and Wildlife Foundation also contributed to work at the Desert Tortoise Research Natural Area. Numerous biologists have made major contributions to field work since 1977, including but not limited to: K. Anderson, T. Bailey, B. L. Burge, P. Frank, L. Nicholson, T. Shields, and P. Woodman.

Literature Cited

Berry, K. H. 1986. Incidence of gunshot deaths in desert tortoises in California. Wildl. Soc. Bull. 14:127-132.

Berry, K. H. 1997. Demographic consequences of disease in two desert tortoise populations in California, USA. Pp. 91-97 in J. Van Abbema (ed.), Proceedings: Conservation, Restoration, and Management of Tortoises and Turtles--An International Conference. WCS Turtle Recovery Program and the New York Turtle and Tortoise Society, NY. 

Berry, K. H., and P. Medica. 1995. Desert tortoises in the Mojave and Colorado deserts. Pp. 135-137 in E. L. LaRue, et al. (eds.), Our Living Resources: A report to the nation on the distribution, abundance, and health of U.S. plants, animals, and ecosystems. USDI, National Biological Service, Washington, D.C. 530pp. 

Berry, K. H., E. K. Spangenberg, B. L. Homer, and E. R. Jacobson. 2002. Deaths of desert tortoises following periods of drought and research manipulation. Chelonian Conservation and Biology 4(2): 

Brooks, M. L. 1998. Alien annual grass distribution, abundance, and impact on Desert Tortoise Habitat in the Western Mojave Desert. Ph.D. Dissertation. University of California at Riverside. 

Brooks, M. L. 1999. Habitat invisibility and dominance of alien annual plants in the western Mojave Desert. Biol. Inv. 1:325-327. 
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Brown, M. B., K. H. Berry, I. M. Schumacher, K. A. Nagy, M. M. Christopher, and P. A. Klein. 1999. Seroepidemiology of upper respiratory tract disease in the desert tortoise of California. J. Wildlife Diseases 35(4):716-727.

Christopher, M. M., K. H. Berry, I. R. Wallis, K. A. Nagy, B. T. Henen, and C. C. Peterson. 1999. Reference intervals and physiologic alterations in hematologic and biochemical values of free-ranging desert tortoises in the Mojave Desert. J. Wildl. Dis. 35(2):212-238.

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Fish and Wildlife Service. 1994. The Desert Tortoise (Mojave Population) Recovery Plan. U.S. Fish and Wildlife Service, Portland, Oregon. 73 pp plus Appendices 

Homer, B. L., K. H. Berry M. B. Brown, G. Ellis, and E. R. Jacobson. 1998. Pathology of diseases in wild desert tortoises from California. J. Wildlife Diseases 34(3):508-523. 

Jacobson, E. R., T. J. Wronski, J. Schumacher, C. Reggiardo, and K. H. Berry. 1994. Cutaneous dyskeratosis in free-ranging desert tortoises, Gopherus agassizii, in the Colorado Desert of Southern California. J. Zoo and Wildlife Medicine 25(1):68-81.

Jennings, W. B. 1993. Foraging ecology and habitat utilization of the desert tortoise (Gopherus agassizii) in the western Mojave Desert. M.S. Thesis. University of Texas at Arlington.