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Literature Review

Desert Tortoise

Desert tortoise

Xerobates agassizii (or Gopherus agassizii)

USFWS: Threatened
CDFG: Threatened

Background

Distribution, Abundance and Trends. The desert tortoise is widely distributed through an exceptionally broad array of habitats that span 1,100 kilometers from northern Sinaloa State, Mexico where it occupies deciduous forest, across the Sonoran (including the Colorado Desert Subdivision in California) and Mojave Deserts, to the edge of the Colorado Plateau in arid southwestern Utah (Ernst et al., 1994; Germano et al., 1994). Populations north and west of the Colorado River were listed as federally threatened in April, 1990. The species is listed by California as a threatened species, and it is the official State reptile. In California, the tortoise is naturally absent from most areas west of the Salton Sea (Luckenbach, 1982). Thus the Imperial Valley and portions of the southern Coachella Valley do not support native populations of tortoises. Tortoises, however, are found naturally along the northern, eastern and western rim of the Coachella Valley in the foothills of the Little San Bernardino Mountains, the Painted and Whitewater Hills (in the latter they are common), and the San Jacinto and northern Santa Rosa Mountains.

The Plan area represents a small, but perhaps biologically significant portion of the tortoise's overall range. Tortoises in the foothills of the southeastern San Bernardino Mountains (especially in the Whitewater Hills) represent the western-most reproductively active population of tortoises in the Colorado Desert ecosystem. The western-most records of tortoises in the San Gorgonio Pass are represented by a series of eight active burrows (with scat at four) found by Jeffrey Lovich on July 7, 1997 in T3S, R3E, NW ¼ Sec 5. Significant geographic variation in ecology, morphology, allozymes, plasma proteins markers, gene sequences and mitochondrial DNA has been noted among populations of tortoises rangewide (Weinstein and Berry, 1987; Rainboth et al., 1989; Lamb et al., 1989; Glenn et al., 1990; Lamb and Lydeard, 1994; Morafka et al., 1994), but no published comparisons have included tortoises from the Coachella Valley.

Rangewide, occupied habitats include desert alluvial fans, washes, canyon bottoms, rocky hillsides, and other steep terrain. In the Whitewater Hills and environs tortoise burrows were found on slopes averaging 17.7o and ranging from 0-45o (Lovich and Daniels, unpublished). Areas with gravelly or coarse sandy soil are preferred, but tortoises can be found in boulder piles in some areas near the Coachella Valley. Desert tortoises have been recorded at elevations of at least l,070 m in some portions of their range. Elevational records for desert tortoises in the Whitewater Hills and the Painted Hills average 735 m and range from 661-817 m (based on 150 records of 27 specimens in 1997). The particular habitat types utilized vary geographically with a preference for rocky slopes in the eastern part of the range (Schamberger and Turner, 1986; Barrett, 1990). However, it is important to emphasize that tortoises can occupy a surprising range of habitat types.

The spatial distribution of desert tortoises in relation to plant communities is not random (Baxter, 1988). High diversity plant ecotones and communities, and possibly soil characteristics, are important features in determining tortoise densities (Wilson and Stager, 1992). This may explain the relatively high density of tortoises in the Whitewater Hills as the area is situated in a transition zone between plant communities from the San Bernardino Mountains, the Mojave and Colorado Deserts, and coastal assemblages. The clustered nature of tortoise burrows in the western Coachella Valley environs is consistent with the observations of others throughout the range of the tortoise: desert tortoises frequently exhibit a contagious distribution, with clusters of individuals in some areas and large intervening areas of what appears to be suitable habitat without tortoises. Home ranges of tortoises vary from about 1-642 acres with males typically having larger home ranges than females. In southern Nevada males had an average home range of 80 acres while females used 37 acres (data summarized by Luckenbach, 1982).

In the western Coachella Valley the nesting season extends from April through at least July. Of 10 females radio-tracked and x-rayed at weekly intervals from early April through July, 1997 in the Whitewater Hills, 9 produced 72 eggs in 16 clutches. Seven produced second clutches and one tortoise produced a third clutch. Clutch sizes ranged from 1-8 (including a single female with 1 egg in the Painted Hills) with the first clutch averaging 4.33 eggs and the second clutch averaging 5.0 eggs (Lovich, unpublished). In contrast, during the same time period, only 1 of 8 females tracked and x-rayed in Joshua Tree National Park produced eggs; a single clutch of 5. The difference is attributed to the fact that winter rain produced high biomass of annuals in the Whitewater Hills, whereas tortoises in the Park are in the second year of drought conditions.

Threats and Limiting Factors. Coyotes (Canis latrans), bobcats (Lynx rufus), ravens (Corvus corax), golden eagles (Aquila chrysaetos), and Gila monsters (Heloderma suspectum) [which do not occur in the Plan area] are known predators of either eggs, juveniles or adults (Hensley, 1950; Barrow, 1979; Luckenbach, 1982; Barrett and Humphrey, 1986), and ring-tailed cats (Bassariscus), badgers (Taxidea), skunks (Mephitis, Spilogale), kit foxes (Vulpes), domestic dogs (Canis familiaris), large hawks (Buteo), owls (Athene), roadrunners (Geococcyx), bullsnakes (Pituophis), and coachwhip snakes (Masticophis) are suspected predators (Ernst and Barbour, 1972; Luckenbach, 1982; H. Avery, pers. comm.). The presence of a high density of local ravens (Corvus corax) has a detrimental affect on populations of G. agassizii through predation on young tortoises (Boarman (1993).

Desert tortoise habitat can be lost to urbanization and other human-related activities, including off-highway-vehicle use, overgrazing of domestic livestock, and construction of roads and utility corridors. Secondary contributions to degradation include the proliferation of exotic plant species and a higher frequency of anthropogenic fire. Effects of these impacts include alteration or destruction of macro- and microvegetation elements, establishment of disclimax plant communities, destruction of soil stabilizers, soil compaction, erosion, and pollution (Lovich, 1992). Off-road vehicle (ORV) use may contribute to declines of tortoise populations directly by crushing individuals (above or below ground), or by collapsing burrows. Vehicle activity may also destroy vegetation used by tortoises for food or cover, making habitat unsuitable for sustaining their populations.

Certain key tortoise food plants may comprise over 40% of the cattle diet, and, since cattle are larger and more mobile than tortoises, these plants may be severely depleted with heavy grazing (Berry, 1978; Coombs, 1979). The Whitewater Grazing Allotment managed by the BLM overlaps significant tortoise habitat in the Whitewater and Painted Hills. Cattle have been observed to step on burrows and cause their collapse in the area, including burrows occupied by tortoises or used as nest sites. Recent research by Hal Avery of the U.S. Geological Survey demonstrates conclusively, for the first time, that cattle can out-compete tortoises for key forage species. Cattle grazing in the Whitewater Hills tortoise habitat has also lead to visible increases in soil destruction and increased erosion in some areas.

Disease has contributed to declines of some desert tortoise populations. Wild and captive desert tortoises are afflicted with Upper Respiratory Tract Disease (URTD) in many areas within the geographic range. Jacobson et al. (1991; 1995) isolated a species of Mycoplasma, a small bacterium lacking a cell wall, as a potential pathogen causing URTD. Introductions of infected captive tortoises into the desert may have caused the spread of this potentially lethal disease in wild tortoise populations. No evidence of URTD has been observed in tortoises in the Whitewater Hills or the Painted Hills (Lovich). A new disease, called shell disease, has recently been reported in tortoises. In extreme cases, the scutes overlying the bony shell flake off, exposing skeletal tissue to desiccation and invasion by pathogens (Jacobson et al., 1994). Evidence of incipient shell disease on the lower shell of tortoises in the Whitewater Hills has been observed (Lovich), but no cases of scute exfoliation or mortality have been observed.

Jeff Lovich believes that fire is the biggest threat to the continued survival of tortoises in the western Coachella Valley. He reports that the proliferation of exotic annual grasses and forbs in the region has dramatically increased the frequency and extent of wildland fires in an ecosystem poorly adapted to perturbations of such periodicity or magnitude. Other than direct mortality, habitat conversion of desert scrub and semi-desert chaparral to exotic grasslands will diminish the prospects for long-term survival of viable tortoise populations.

Special Considerations. Except for the northeastern most portion of the Plan area, the Coachella Valley was not included in the recovery plan for the desert tortoise. Under the Endangered Species Act, the species is still protected in the Coachella Valley and no incidental take can occur without a permit. The CVMSHCP is intended to provide conservation of the species in the Plan area, resulting in the issuance of a Section 10(a) incidental take permit.