136 



Reptiles and Amphibians — Onr Living Resaiirces 



300- 



®lvanpah Valley: (^ Desert Tortoise Natural Area: 



northeastern Mojave Desert, ^ western Mojave Desert, CA 

 CA (1979-90) (1979-92) 



©Chuckwalla Bencti: 

 eastern Colorado Desert, 

 CA (1979-92) 



(K\ Ward Valley: 



northern Colorado Desert, 

 CA (1980-91) 



©Goffs: 

 eastern Mojave Desert, CA 

 (1980, 1990) 



©Piule Valley: 

 eastern Moiave Desert, NV 



250- 



(1983, 1983; 



©Sheep Mountain: 

 eastern Mojave Desert, NV 

 (1984, 1992) 



/TT\ Gold Butte: 



northeastern Mojave Desert, 

 NV (1986, 1990) 



Trout Canyon: 



eastern Mojave Desert, NV 



(1987,1992) 



200- 



150- 



100- 



50- 



k 



i=l 



80 



Year 



90 



80 



Year 



90 



Year 



90 



Fig. 2. Examples of changes in 

 desert tortoise population densities 

 at nine study sites in California 

 and Nevada. Tfie midpoint for 

 density estimates of all sizes of 

 tortoises (orange line) is shown by 

 a dot on a bar representing the 

 959c confidence interval (CD; the 

 midpoint for density estimates for 

 adult tortoises only (red lines) is 

 depicted by a square on a bar rep- 

 resenting the 95% CI. Causes of 

 declines vary by site. 



Causes of population declines differed 

 somewhat within and between population seg- 

 ments, but were primarily related to human 

 activities. Higher than normal losses or mortal- 

 ity rates were attributed to many causes, such as 

 illegal collecting, vandalism, upper respiratory 

 tract disease or shell disease, predation by com- 

 mon ravens, crushing by vehicles both on and 

 off roads, and trampling by livestock (BLM 

 1988; USFWS 1994). For example. 14.6'7r- 

 28.9% of desert tortoise carcasses collected 

 from western Mojave plots in the 1970"s and 

 early 1980's showed signs of gunshots (tortois- 

 es were shot while still alive), but only 0%- 

 3.1% of carcasses from the less-visited eastern 

 Mojave and northern Colorado deserts showed 

 such signs (Berry 1986). Deaths from vehicles 

 on paved roads were also highest in the western 

 Mojave. where densities of dirt roads and vehi- 

 cle trails are higher than elsewhere. 



Of particular concern is the recent appear- 

 ance of a highly infectious and usually fatal 

 upper respiratory tract disease caused by the 

 bacterium Mycoplasma agassizii. The disease, 

 apparently introduced through the release of 

 captive tortoises (Jacobson 1993), has caused 



80 Year 90 



the deaths of thousands of wild tortoises in the 

 Mojave Desert during the last few years (K.H. 

 Berry, unpublished data). 



Fragmented and deteriorated habitats also 

 affect population vitality. Populations in areas 

 with high levels of exotic annual plants are 

 declining at substantially higher rates than those 

 in less disturbed areas. 



In summary, tortoise populations occurring 

 in relatively undisturbed and remote areas with 

 little vehicular access and low human visitation 

 generally were stable, or exhibited lower rates 

 of decline than tortoise populations in areas 

 with high levels of disturbance, high vehicular 

 access, and high human visitation. 



References 



Berry. K.H. 1986. Incidence of gunshot deaths in desert tor- 

 toises in California. Wildlife Society Bull. 14:127-132. 



Berry. K.H. 1990. The status of the desert tortoise in 

 California in 1989 (with amendments to include 1990- 

 1992 data sets). Draft report from U.S. Bureau of Land 

 Management to U.S. Fish and Wildlife Service, Region 

 I.Portland. OR. 



BLM. 1988. Desert tortoise habitat management on the pub- 

 lic lands: a rangewide plan. U.S. Bureau of Land 

 Management. Washington, DC. 23 pp. 



