The Mojave desert is located in the southwestern United States, covering more than 20 million hectares spread across four states: California, Nevada, Arizona, and Utah. It is home to more than sixty cities and towns, including Las Vegas, Barstow, Kingman, and Victorville. There are numerous military bases and installations in the Mojave, including the China Lake Naval Air Weapons Testing Center, the Marines Air-Ground Combat Center, Fort Irwin, and the Nevada Test Site. The region is used by many different interest groups, ranging from off-road vehicle enthusiasts to backpackers, ranchers, new-age disciples, and scientists. There are a variety of land management entities at work in the Mojave, with the vast majority of land falling under the control of the U.S. Bureau of Land Management.
In the image on the upper right, the larger Mojave ecoregion is depicted in green. Our study area is the California portion of the Mojave as determined by The Jepson Manual (Hickman 1993), shown in white. Although our project is focusing on a subset of the Mojave, it still encompasses approximately 7,400,000 hectares.
The overall plan is to characterize the desert tortoise (Gopherus agassizii) very well, selected focal species well, and all terrestrial vertebrate species adequately. Thus we are working at three points on a continuum between a single species and all species, trading off much knowledge for one species against a little knowledge for many species. Ideally we would create models of population viability for each species that could be used to evaluate alternative future scenarios. However, such analyses are expensive and time consuming. We will be able to construct such an analysis only for the desert tortoise. For the other species we focus on overall diversity, and not on the viability of any single species, relying on the power of statistical sample size. Each group will be studied on different spatial scales as well. Three scales will be applied to the tortoise and the focal species and two scales to total diversity. The three scales are:
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This is the flagship species of the ecosystem. It is a Federally Threatened Species. It will
be studied in detail and its relationship to focal species and then total vertebrate biodiversity
carefully studied. A great deal of information is available on the tortoise so our approach
will be one of integration and synthesis of existing data and information followed by two
Few animals now are as well studied as the desert tortoise: Grover and DeFalco (1995) present a bibliography for the biology of this species with over 700 citations prior to 1992. Among the most important works are Woodbury and Hardy (1948) who produced the first major population study on this species (and one which is still extremely valuable as their population is still being studied), Desert Tortoise Recovery Team (1993) who present the recovery plan for the listed Mojave populations, Bury and Corn (1995) whose volume contains several articles, Herpetological Monographs Number 8, 1994, contains 12 articles summarizing work done in the eastern Mojave, and Moratfka (1995) who attempts to synthesize all management relevant knowledge of the species. The ongoing investigation of the desert tortoise at Fort Irwin is described at http://curly.tec.army.mil/mojave/mojave.html.
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Habitat models for all focal species will be constructed from a combination of literature and field work. The first part of this effort will be to determine a list of habitat variables (substrate, landform, vegetation etc) that can be measured for each species. Once the environmental variability can be characterized, a quantitative study will be undertaken to associate each species with its measured habitat. Two types of field survey methodology have been utilized to associate species with habitat. Walking transect surveys where intiated in 1998 and continued through 1999. Additional transects are planned for 2000 as well. These surveys include the Marine Corps Air/Ground Combat Center (MCAGCC) and Joshua Tree National Park (JOTR). Similar transect surveys have been done for some of the lizard species in the Mojave Desert (Morafka and Adest, 1997) and appear to work well. One of the great advantages of working in an environment such as the Mojave Desert is the great visibility afforded by the lack of dense vegetation. The second survey methodology uses pitfall trap arrays. The design follows that developed by Case and Fisher (Department of Biology, University of California at San Diego) and applied with great success in the California Coast Range. Pitfall arrays have been placed on MCAGCC, JOTR and surrounding Bureau of Land Management (BLM) lands.
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This group includes all native species of amphibians, reptiles, birds, and mammals that breed in the Mojave Desert. These number approximately 300 species. This number is estimated from the California Wildlife Habitat Relations documents and includes peripheral species. As the research progresses some peripheral species may be dropped from the analysis as not really belonging in the Mojave ecoregion. We will determine to drop species from the Mojave list on a case by case basis for those species whose ranges only barely enter the Mojave (e.g., species only occurring along the Mojave portion of the Colorado River). Current species list consists of 266 species. This total diversity is one of the major endpoints of the entire study.
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