West Mojave Plan FEIR/S - Desert Managers Group

Both agriculture and mechanized military maneuvers have resulted in a gradual trendfrom coarser to finer substrates (Krzysik 1994; LaRue and Boarman, in prep.). When the naturalvegetative cover is removed for agricultural (and other) purposes, the soil is far more susceptibleto wind erosion than if left undisturbed. The potential problem with converting relatively coarsesubstrates to finer materials is that the MGS tends to prefer the coarser materials, compared tothe round-tailed ground squirrel, which prefers sandier substrates (Hafner and Yates 1983).Gustafson (1993) reports that the contact zone between the two species is approximately240 km (149 miles) between northern Fort Irwin and Victorville, with the Mojave River inbetween. MGS-round-tailed hybrids have been observed in the Helendale area (Wessman 1977;LaRue, pers. obs. 1997), adjacent to the Mojave River, and much of the river basin has beenconverted to agricultural fields (WMP data). Although agriculture is not likely to significantlyexpand outside the Mojave River, the Fort Irwin expansion would extend west into the knownrange of the MGS. If similar impacts are observed in the expansion area as has been documentedon the existing installation (i.e., conversion of gravelly soils to sandier soils; Krzysik 1994), thereis the potential for expanding preferred round-tailed ground squirrel habitat and reducing MGShabitat. If so, the potential for hybridization between the two species will be facilitated where itpresently may not occur.The only occurrences of hybrid (Wessman 1977) and suspected hybrid (Krzysik 1994;LaRue, 1997 pers. obs.) ground squirrels have been in the areas of Fort Irwin and Helendale.Gustafson (1993) reported that hybridization likely occurred in these areas due to ecological andbehavioral changes in one or the other species that resulted from agricultural disturbances in theHelendale area and military maneuvers at Fort Irwin. Dr. Recht (2001 pers. comm.) has recentlytrapped the round-tailed ground squirrel in the Superior Valley, 10 or more miles inside theknown range of the MGS. This suggests that there is potential for hybridization to occur wellinto the known range, and not just along the edges.Gustafson (1993), citing Hafner (1992) discussed the low vagility (the potential for ananimal to disperse) of the MGS, which results, in part, from the species being active for onlythree to four months of a given year. Hafner (1992) concluded that low dispersal potential, onthe local population level, may be on average about 5 m per year, and that this low vagilitywould preclude the MGS from rapidly expanding into suitable habitats where it was previouslyextirpated. Since that time, the Leitners have found that juvenile MGS may disperse up to fourmiles from their natal burrows in a given year. So, although dispersal abilities may be morepronounced than previously thought, the quality (including sandiness) of habitat may still limitrecolonization potential.Military Maneuvers: Military maneuvers may affect the MGS through direct mortality,crushing burrows (and animals within them), or decreasing shrub cover (Laabs 1998). Krzysikand Woodman (1991) reported that coarse-grain soils were pulverized by mechanized equipmentat Fort Irwin. Army maneuvers in the main corridors caused a reduction in the particle size,which led to dust storms and reduced human visibility to within several meters. China LakeNAWS reported that opening the Mojave B Range to Army maneuvers would create so muchdust (which they compared to that blowing south out of the Owens Valley) that the Navy’smission would have been compromised by this use (Range Systems Engineering Office andChapter 3 3-160