Oiti Livini^ Rt'sourci's — Reptiles ami Aniphibiuns 



IJ5 



The desert tortoise [Gopherus uiiussizii) is a 

 widespread species of the southwestern 

 United States and Mexico. Within the United 

 States, desert tortoises Uve in the Mojave. 

 Colorado, and Sonoran deserts of southeastern 

 California, southern Nevada, southwestern 

 Utah, and western Arizona (Fig. 1 ). A substan- 

 tial portion of the habitat is on lands adminis- 

 tered by the U.S. Department of the Interior. 



The U.S. government treats the desert tor- 

 toise as an indicator or umbrella species to mea- 

 sure the health and well-being of the ecosys- 

 tems it inhabits. The tortoise functions well as 

 an indicator because it is long-lived, takes 12-20 

 years to reach reproductive maturity, and is sen- 

 sitive to changes in the environment. In 1990 

 the U.S. Fish and Wildlife Service listed the 

 species as threatened in the northern and west- 

 ern parts of its geographic range (Fig. 1) 

 because of widespread population declines and 

 overall habitat loss, deterioration, and fragmen- 

 tation. 



Because some populations exhibit signifi- 

 cant genetic, moiphologic (see glossary), and 

 behavioral differences, the Desert Tortoise 

 Recovery Team identified six distinctive popu- 

 lation segments (Fig. I ) for critical habitat pro- 

 tection and long-term conservation within the 

 Mojave and Colorado deserts (e.g.. Lamb et al. 

 1989: USFWS 1994). The population segments 

 are representative of distinctive climatic, floris- 

 tic, and geographic regions. 



Surveys 



The primary sources of information on sta- 

 tus and trends of desert tortoise populations are 

 from study plots established by the U.S. Bureau 

 of Land Management and state fish and game 

 agencies. More than 30 permanent study plots, 

 each of which is 2.6 km- or larger ( I mi- or 

 more), are surveyed at intervals ranging from 2 

 to 10 years. Study plots provide data on popula- 

 tion characteristics, including density, size-age 

 class structure, sex ratios and numbers of breed- 

 ing females, recruitment of juveniles into the 

 adult population, causes of death, and mortality 

 rates (Berry 1990). Researchers use mark- 

 recapture techniques to conduct 60-day surveys 

 in spring for live and dead tortoises. 



Trends for habitat condition on study plots 

 are measured by using quantitative data on 

 native and exotic annual and perennial vegeta- 

 tion (Berry 1990). Associated data on past and 

 recent human activities or influences include 

 numbers of visitors per season; density of dirt 

 roads, trails, and vehicle tracks; levels and types 

 of livestock grazing; and acreage disturbed by 

 mining and mineral development and utility 

 corridors. 



The data base for the six population seg- 

 ments varies considerably; some segments con- 

 tain several plots that have been sampled for 1 1 - 

 17 years, whereas others have few plots that 

 have been sampled only I or 2 years (Berry 

 1990; USFWS 1994). 



Trends 



Condition and trends in tortoise populations 

 vary within and between population segments. 

 One measure of population condition is change 

 in density. Examples of changes in density for 

 nine study plots in California and Nevada are 

 shown in Fig. 2 (Berry 1990; D.B. 

 Hardenbrook, Nevada Division of Wildlife, and 

 S. Slone, Bureau of Land Management, person- 

 al communication). The greatest declines in 



Desert 

 Tortoises in 

 the Mojave 

 and Colorado 

 Deserts 



by 



Kristin H. Berry 



Philip Medica 



National Biological Service 



densities, for all size classes and for breeding 

 females (up to 90%), occurred in the western 

 Mojave segment between the 1970"s and 

 I990"s. Similar declines (30%-60%) also 

 occurred in the eastern Colorado Desert seg- 

 ment between 1979 and 1992, with the greatest 

 declines registered at the Chuckwalla Bench 

 plot (Fig. 2). Moderate declines of 20%-25% 

 were reported from some sites in the eastern 

 Mojave Desert segment (Piute Valley and 

 Goffs). The northeastern Mojave also exhibited 

 declines on some plots (e.g.. Ivanpah Valley and 

 Gold Butte). In contrast, the northern Colorado 

 Desert population segment showed indications 

 of growth in the breeding adults at one plot 

 (Ward Valley), and the upper Virgin River seg- 

 ment appears stable (USFWS 1994). 



Fig. 1. Li.S. range of tfie desert 

 tortoise (Gapherus agassizii). Tfie 

 six population segments for desert 

 tortoises federally listed as tfireat- 

 ened occur in parts of the Mojave 

 and Colorado deserts that lie north 

 and west of the Colorado River. 



r^cscit toiloisc iOopheriis ui;as' 

 sizii). 



