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26th Annual Meeting and Symposium of the
Desert Tortoise Council, March 16-18, 2001
Abstracts
Recent Declines in Desert Tortoise Populations in Eastern California:
The Fenner and Chemehuevi Valleys
Kristin H. Berry1, Tracy Goodlett2, and Kemp Anderson2
1USGS, Western Ecological Research Center, 6221 Box Springs Blvd., Riverside, CA 92507
2On Track Consulting and Research, 435 Rebel Road, Ridgecrest, CA 93555
Since 1988, surveys of long-term study plots for desert tortoises (Gopherus agassizii) at study plots in the eastern part of California have provided evidence for declines in densities. In 1999, the Chemehuevi Valley and Wash study plot was surveyed after a seven-year hiatus. Previous surveys were in 1979, 1982, 1988, and 1992. The population gradually increased between 1979 and 1988 from a density of 56 to 86 tortoises/km2. In 1992, the density figure was almost the same as for 1988 (88 tortoises/km2), but decreased to 6 tortoises/km2. The increases between 1979 and 1988 were thought to be associated with recovery from General Patton's military maneuvers in the early 1940's. The loss of live tortoises between 1992 and 1999 is supported by the recovery of >300 shell-skeletal remains. The high mortality rate may be due to a combination of factors, including disease and canid predation. The invasive alien, Brassica tournefortii, may also play a role.
In 2000, the Goffs plot in Fenner Valley was surveyed after a six-year hiatus. Previous surveys were in 1980, annually between 1983 and 1986, as well as in 1990 and 1994. The Goffs study plot was considered the Gold Standard for stable populations, drawing on the 1980-1994 data sets. Between 1994 and 2000, densities declined from 173 tortoises/km2 to 34 tortoises/km2. Remains of many marked and unmarked tortoises were located. Contributors to the high mortality rates are likely to include disease and possibly periodic drought, although tortoises were dying in years of both high and low rainfall.
The population declines observed at these two study plots may be widespread in eastern California. Government employees and environmental contractors with expertise in tortoise biology have observed large numbers of shells from the Fenner Valley south into Ward Valley. Similar declines were observed at the Chuckwalla Bench and Chocolate Mountains Aerial Gunnery Range in the 1980's, continuing to the present. Drs. Mary Christopher and Bruce Homer will present papers supporting hypotheses associated with disease (Christopher et al., 2001; Homer et al. 2001).
References
Christopher, M. M., K. H. Berry, B. T. Henen, and K. A. Nagy. 2001. Clinical disease and laboratory abnormalities in free-ranging desert tortoises (Gopherus agassizii) in California (1990-1995). Abstract of paper presented at 26th Annual Meeting and Symposium of the Desert Tortoise Council, InnSuites, Tucson. March 16-19, 2001.
Homer, B. L., and K. H. Berry. 2001. Results of Necropsy Findings of Desert Tortoises from the Chemehuevi Valley Study Site in 1999 and from the Goffs Study Site in 2000. Abstract of paper presented at 26th Annual Meeting and Symposium of the Desert Tortoise Council, InnSuites, Tucson. March 16-19, 2001.