West Mojave Plan FEIR/S - Desert Managers Group

Boarman (pers. comm., Nov 2002) has suggested that the lower rainfall levelsexperienced in the second half of the 20 th Century may have reduced productivity, therebyreducing the capacity of the land to sustain as many tortoises are previously. Oftedahl (pers.comm., Nov 2002) has suggested that long-term cattle grazing may have depleted the naturalseed bank of plants with a high potassium excretion potential (see discussion above) and that itmay be impossible to regain that seed bank, even if grazing is discontinued.Nor does the Recovery Plan indicate how or when the baseline population numbersshould be established. In fact, that baseline is currently being established through line distancesampling surveys, which were initiated in the western Mojave Desert in 2001. It may take up tofive years to determine a statistically valid baseline population. If so, the baseline wouldrepresent a snap shot of tortoise densities in the year 2005.3.3.2.5 Threats to Tortoises: Mortality FactorsAvailable literature presents many threats that are known or suspected to affect tortoisesand their habitats. Dr. William Boarman (2002) identified 22 impacts that may affect tortoisesthroughout the listed population: agriculture, collecting, construction, disease, drought, energyand mineral development, fire, garbage and litter, handling and manipulation, invasive weeds,landfills, livestock grazing, military operations, noise, non off-highway vehicle recreation, offhighwayvehicles, predation, roads and highways, urbanization and development, utilitycorridors, vandalism, and wild horses and burros. Dr. Boarman’s analysis is included in itsentirety as Appendix J.Dr. Boarman’s discussion of threats is general and is not restricted to physical impactsand miscellaneous threats that are known to occur in the West Mojave planning area. Thefollowing discussion focuses on threats present within the planning area. It addresses (1) directand indirect anthropogenic (i.e. human-caused) mortality factors, (2) natural mortality factors,and (3) carcass observations and die-offs suggested by recent data. The relationship between offhighway vehicles and tortoises, an issue that has received a high level of public interest, isaddressed separately in Section 3.3.2.6 (below).A detailed analysis of carcass observations is presented in Appendix L. The reader isencouraged to review that analysis as an adjunct to summary carcass observations presented inthe following sections.3.3.2.5.1 Direct and Indirect Anthropogenic Mortality FactorsThere are both direct and indirect anthropogenic mortality factors (see Boarman 2002 fordiscussion). Direct mortality factors have immediate results (incidental mortality duringconstruction, removal of animals from the desert), whereas indirect mortality factors occur overtime, and are not always easily associated with the direct mortality factors from which they arise.Direct Anthropogenic Mortality Factors: These include blading a pipeline right-ofway,tract home development, and similar land disturbances where native vegetation is removedand tortoises residing in the area are either crushed or forced to move into adjacent areas ofChapter 3 3-95