West Mojave Plan FEIR/S - Desert Managers Group

Tortoise Distribution within the Revised Range: The TCS distribution map (Map 3-11on attached CD Rom) was generated exclusively based on 1998 to 2002 sign count surveys.Map 3-11 uses TCS categories that have been traditionally used to estimate tortoise densities,which are useful to determine relative tortoise abundance and distribution. These categoriesinclude: 0, 1 to 3, 4 to 8, 9 to 16, 17 to 28, and 29 to 50 TCS/transect (i.e., which is mostlyreported as “TCS/mi 2 ). For convenience, the six categories are occasionally segregated into twocategories of relatively low sign counts (0 to 8) or relatively high sign counts (9 to 50). Theaverage number of TCS in DWMAs was found to be 5 mi 2 , which was used as the demarcationto differentiate above- and below-average density areas.This map reveals that there are few regions within DWMAs where tortoises arecompletely extirpated, or for other reasons, do not occur. It identifies areas where high signcounts were found on 261 transects, or conversely, no sign was found (transects along the SierraNevada, west of the aqueduct). In many cases, low-density areas may be adjacent to orsurrounded by relatively higher density areas.Natural Absences of Tortoises: Several safe assumptions can be made about tortoisedistribution in the western Mojave Desert since the 1970s:• Tortoises are mostly absent from dry lakebeds.• Tortoises are absent from areas above 5,000 feet elevation, and nearly so at 4,500 feet. In1999, only 5 of 609 (0.8%) transects with tortoise sign occurred above 4,000 feet;similarly, in 2001, only 12 of 991 (1.2%) transects with tortoise sign occurred above4,000 feet. In 1998, all 875 transects were located below 4,500 feet.• Tortoises may be naturally sparse in the northern portion of the range, from the AvawatzMountains, through China Lake Naval Air Weapons Station, up to Rose Valley alongHighway 395. Much of this area is protected on military installations or otherwiseinaccessible to most casual desert visitors, yet no surveys since the 1970's have foundsignificant areas of above-average tortoise sign. Weinstein (1989) found that latitude wasa contributing factor to tortoise occurrence, and that in general densities decreased withincreasing northern latitudes.There are also places where local geological and hydrological factors may be responsiblefor relatively low tortoise numbers. The lavic flows associated with Black Mountain, north ofHarper Lake, may be sufficiently unsuitable that tortoises are naturally uncommon, althoughthere is an apparent abundance of tortoises at Pisgah Crater, a similar formation. During 1994,on the south-central and southwestern portions of Edwards Air Force Base (between SouthRogers Dry Lake and Rosamond Dry Lake, including Buckhorn Dry Lake), the only tortoisesfound were restricted to a small hill that rose above the surrounding saltbush scrub, which wasvegetated by the only creosote bush scrub observed in the region (LaRue, pers. obs.).The more difficult question is, what is the evidence that tortoises occupied all areaswhere they are now presumed extirpated (missing)? Tortoises are extirpated from large portionsChapter 3 3-93