West Mojave Plan FEIR/S - Desert Managers Group

“higher density” and “lower density” tortoise areas is an important one relative to the plan’seffectiveness of minimizing and mitigating take.3.3.2.3 Tortoise Life HistoryThe following life history information is taken from U.S. Fish and Wildlife Service(2002c). The desert tortoise is a large, herbivorous reptile found in portions of the California,Arizona, Nevada, and Utah deserts. It also occurs in Sonora and Sinaloa, Mexico. In California,the desert tortoise occurs primarily within the creosote, shadscale, and Joshua tree series ofMojave Desert scrub, and the lower Colorado River Valley subdivision of Sonoran desert scrub.Optimal habitat has been characterized as creosote bush scrub in which precipitation ranges from2 to 8 inches, diversity of perennial plants is relatively high, and production of ephemerals ishigh (Luckenbach 1982, Turner and Brown 1982, Schamberger and Turner 1986). Soils must befriable enough for digging of burrows, but firm enough so that burrows do not collapse. InCalifornia, desert tortoises are typically associated with gravelly flats or sandy soils with someclay, but are occasionally found in windblown sand or in rocky terrain (Luckenbach 1982).Desert tortoises occur in the California desert from below sea level to an elevation of 7,300 feet,but the most favorable habitat occurs at elevations of approximately 1,000 to 3,000 feet(Luckenbach 1982, Schamberger and Turner 1986).Chambers Group (1994) has reported that, in the Alvord Slope area, tortoises were mostcommon on low to moderate slopes of 0 to 10%. They were most abundant on valley floors,bajadas, and lower portions of hills. Preferred substrates included sand, gravel, and desertpavement in plains, washes, fans, and hills.Adult desert tortoises are most active in California during the spring and early summerwhen annual plants are most common although juvenile tortoises have been observed outsideburrows throughout the year, including December through January when adults are generally in astate of hibernation (Dave Morafka, pers. comm.). Additional adult activity occurs duringwarmer fall months and occasionally after summer rainstorms. Adult desert tortoises spend mostof the remainder of the year in burrows, escaping the extreme conditions of the desert. Furtherinformation on the range, biology, and ecology of the desert tortoise can be found in Burge(1978), Burge and Bradley (1976), Hovik and Hardenbrook (1989), Luckenbach (1982),Weinstein et al. (1987), and USFWS (1994b).Tortoise activity is heavily influenced by the amount and timing of rainfall. Annualplants, which make up most of the tortoise’s diet in the western Mojave Desert, vary dependingon the timing of winter precipitation and the ensuing temperatures. Annual forbs, which arerelatively more nutritionally balanced for tortoises, generally emerge following early winter rainswith relatively warmer temperatures preceding and during the spring growing season. If winterrains do not come until late January or February, and temperatures are relatively cooler, nativeand non-native annual grasses will often emerge instead of native forbs. Such forage,particularly non-native grasses, offers little nutritional quality to tortoises.Chapter 3 3-73