West Mojave Plan FEIR/S - Desert Managers Group

of the Lucerne and Victor valleys and from the entire western portion of Antelope Valley; whatis the evidence that they ever occurred there? Lucerne Valley is relatively straightforwardbecause a few tortoises can still be found along the base of the San Bernardino Mountains, southof the developed portion of the community. They are documented to the north, east, and west, soit is clear that Lucerne Valley once supported suitable habitat, if not fully occupied by tortoisesprior to recent development.The Victor Valley’s southern demarcation of natural tortoise absence (from westernLucerne Valley through southern Hesperia, Oak Hills, Baldy Mesa, to Phelan) cannot be welldefined with existing data, which have mostly been collected since the 1990 listing. With theexception of two questionable data points near the southwestern corner of Hesperia, no tortoisesign has been found south of Highway 18 from Apple Valley to Highway 395 since 1990 (seeMap 3-6). Highways 18 and 138 generally separate areas to the south where tortoises areapparently absent from areas to the north, where habitats are substantially degraded and only afew residual aggregations occur 10 .In 1990, it was judged that only about 90 square miles within the 225-square mile City ofLancaster and its sphere of influence still supported potential occupied tortoise habitat (TierraMadre Associates, Inc. 1991). Only three carcasses were found during surveys along 330 linearmiles of transects (LaRue, pers. obs.). No evidence of living tortoises was found, nor has anybeen found over the last 12 years during surveys required by the City of Lancaster (BrianLudicke, pers. comm.). Even so, these carcasses, and numerous accounts documented by Berryand Nicholson (1984) show that the Antelope Valley, west of Highway 14 was historicallyoccupied. Data were found for only four or five surveys in the southern half of the AntelopeValley, west of Lancaster. No 1975-1982 BLM data were collected there.Carrying Capacity: Carrying capacity is the inherent ability of the land to support agiven number of tortoises per unit area. The tortoise carrying capacity of any area cannot bestated with precision, except for certain lands (such as playas and lands above 5,000 feetelevation) where the carrying capacity is zero. Based on Dr. Berry’s study plot data, there wereas many as 238 adult tortoises per square mile at the DTNA in 1982 and as many as 70 atFremont Peak in 1979. These are more accurately described as 1979-1980 baseline populationnumbers for two square miles and an unknown contiguous area containing similar vegetation,and are not reflective of the carrying capacity of the two regions in which they occurred. Nor dothey reflect trends in the population that preceded the 1970’s, when the baseline studies werefirst conducted. The declines observed on these two plots between 1980 and 1996, however, doapparently reflect declines that were occurring on a regional scale during that time.These studies do not reflect how many tortoises may have occurred in the region in the1950s, for example, or in 1900. Had high tortoise populations existed and suffered catastrophicdie-offs, sufficient time would have elapsed for all carcasses to disintegrate in the interim,leaving no indication of tortoise populations even 20 years before the first study plots weresurveyed.10 For example, tortoise sign was found on only 1.5 mi 2 in a 20 mi 2 area surveyed in this region in 2002.Chapter 3 3-94