West Mojave Plan FEIR/S - Desert Managers Group

concluded, “Conflicting evidence and incomplete data sets make analysis of raven-tortoiseinteractions a difficult task. We cannot determine the effect of ravens on tortoise populationsthroughout the Mojave Desert. However, it appears that ravens may decrease juvenile tortoisenumbers in localized areas.” BLM (1990) concluded, “At this point the contribution of avianpredation to tortoise population mortality remains unknown… Whether they [common ravens]can significantly depress a tortoise population is open to question.”Efficacy of Raven Management Previously Applied: BLM implemented the first focusedraven reduction program in 1989, using firearms and poison to eradicate ravens at two sites inthe West Mojave (Rado 1990). In 1989, between 106 and 120 ravens were poisoning withStarlicide-treated baits at the 29 Palms Marine Corps Base landfill (Rado 1990). Radoconcluded that the BLM’s raven control program had substantially reduced the number of ravensat both the base landfill and at the DTNA. However, there have been no follow-up studies to seewhat current raven populations are in these two places, so long-term effects of the reduction areunknown.Boarman (1992) has reported that covering refuse with at least 6 inches of soil, thereplacement of open landfills with enclosed transfer stations, and other measures would result inless available food, and may lead to reduced raven numbers. He felt that the most effectivecontrol for long-term raven management must address anthropogenic food sources and requirelow maintenance (Boarman 1992).Drought: Boarman (2002) concluded that drought might cause episodic tortoisemortality that is punctuated by periods of low mortality during years with more abundantrainfall). He speculated that drought-induced stress in concert with other threats (e.g., disease,predation) might have resulted in significant mortality (Peterson 1994a).One obvious effect of drought is the lack of available water for tortoises and other desertadaptedwildlife. Boarman (2002, pers. comm. from Dr. Kenneth Nagy) reported that tortoisescan probably survive one to two years without drinking water but will start dying of dehydrationafter that. In very dry years, there is no production of annual plant species, which provide asubstantial portion of a tortoise’s annual intake of water. In somewhat wetter years, annualproduction may be restricted to a few species, or there may be only a few individual plants of awide array of species that germinate. The nutritional quality of these species becomes relativelymore important when they are in limited supply and comprise a tortoise’s entire dietary intakefor a given season. Boarman (2002) reports that Turner et al. (1984) and Avery (1998) foundthat tortoises might survive drought periods by eating less nutritious cacti and shrubs.Desert Washes and Drought: The 261 tortoises observed during sign count surveyswere observed in only six different plant communites. Tortoise occurrence within each of the sixcommunities and the percent of the planning area occupied by each community (WMP 1996vegetation map) are given in Table 3-20 (listed in descending order of tortoise occurrence):Chapter 3 3-108