Habitat and Distribution
Amargosa vole critical habitat (49 FR 45160) encompasses an area of 4,520 acres in southeastern Inyo County, California: T2ON R7E Sec.4, 5, N ½ and SE ¼ Sec.9, NW ¼ Sec.10, SW ¼ SW ¼ Sec.15, E ½ Sec.16,NW ¼ Sec.22; T21N R7E S ½ Sec.28, S ½ and NW ¼ Sec.29, Sec.32, 33. Within these areas, the major constituent elements that are known to require special management considerations or protection are marsh vegetation (primarily bulrushes of the genus Scirpus), springs, and some open water along the Amargosa River, which provide escape cover and an adequate food supply.
Critical habitat consists of all extant vole populations and significant areas of potential habitat from just north of Tecopa Hot Springs to the northern Amargosa Canyon, just south of Tecopa. The type locality (where the type specimen was found), near Shoshone, is not within critical habitat. No information exists suggesting that critical habitat boundaries should be revised.
The Amargosa vole has been found in isolated wetland habitats where bulrush is a dominant perennial overstory species. These form discontinuous narrow bands along the Amargosa River, broken by more “characteristic” desert vegetation dominated by creosote bush (Larrea tridentata), burrobush (Ambrosia dumosa) and desert holly (Atriplex hymenelytra). Perennial tributary spring sources interspersed along this section of the Amargosa River additionally create mesic habitat “islands” of cattails and bulrush, ranging in size from less than 1 to over 5 acres. Gould and Bleich (1977) located five individual voles in five separate areas where bulrush densities ranged from “moderate” to “high.” Four of the five sites were on slopes of less than 20 percent. The remaining site was on level ground. Bleich (1979b) subsequently captured 14 voles at a single site within “moderate” density bulrush habitat on level ground. Virtually all known trapping sites (six of seven) were closely associated with standing perennial surface water. No estimates of plant species composition were provided in these studies. Rado and Rowlands (1984) later described the vegetational composition at two sites where the Amargosa vole was captured. Information obtained using 8 the releve method (Mueller-Dombois and Ellenberg 1974) included estimates of vegetational cover by canopy height. A successful vole-trapping site in Amargosa Canyon was dominated by an overstory of bulrush, arrow weed (Pluchea sericea), seep-weed (Suaeda torreyanna), quailbush (Atriplex lentWormis), and southern reed (Phragmites australis). Understory vegetation included yerba mansa (Anemopsis calWornica) and saltgrass. Saltcedar (Tamarix ramosissima) was introduced at some point in the past and has recently become extremely dense in areas considered suitable vole habitat in northern Amargosa Canyon. Constituent vegetation at another vole site approximately 3 miles to the north in the Tecopa Lake Basin consisted of a less diverse plant assemblage dominated by an overstory of bulrush and an understory of yerba mansa, saltgrass, and reeds (Juncus sp.).
The current distribution of the Amargosa vole extends discontinuously from a tributary spring site located in the SW ¼ NE ¼ of Sec.33, T21N R7E; south to the SW ¼ SW ¼ of Sec.15, T20N R7E, San Bernardino Baseline Meridian. The distribution of this rodent appears to coincide principally with isolated bulrush-cattail “pockets” that are not subjected to regular inundation during heavy summer thunderstorms. Although the precise area of wetland habitats in the Tecopa Lake Basin and Amargosa Canyon has not been determined, such areas do not exceed 500 acres each. Amargosa voles have recently been live-trapped in wetland habitats from the following localities: T21N R7E, Sec.33; T2ON R7E Sec.4, 5, 9, 15, San Bernardino Baseline Meridian (Bleich 1979b, Rado and Rowlands 1984, Murphy and Freas 1989). Wetland habitats above 1,370 feet (410 meters) elevation (upland) are not susceptible to inundation by seasonal flooding; habitats below 1,370 feet (410 meters) elevation (lowland) are vulnerable to flooding.
Occupied habitat patches above 1,370 feet (410 meters) elevation include the marsh created by the developed warm spring east of Tecopa Hot Springs Road immediately north of the community of Tecopa Hot Springs on land administered by the Bureau ofLand Management (BLM). These occupied patches also include the narrow tule marsh running through the Tecopa Hot Springs County Park west of Tecopa Hot Springs road, and the tule marsh on both sides of Tecopa Hot springs Road east of the railroad grading and immediately north of the community of Tecopa on lands administered by BLM. The property owned by The Nature Conservancy is expected to provide additional secure habitat for voles (Murphy and Freas 1989). These habitat patches will be the focus of initial recovery efforts to achieve the interim recovery objective.
The Amargosa River drainage, extending downstream for approximately 5 linear miles (8 kilometers) from the southernmost documented Amargosa vole locality, has been inventoried unsuccessfully for the species (Rado and Rowlands 1984, Rado 1985). A narrow but almost continuous band dominated by cattails and bulrush existed at the time of these surveys. Prior investigators hypothesized that these habitats are intermittently submerged by flash flood waters through the Amargosa Canyon. Thus, although the habitat is seemingly favorable, the flooding creates an unstable situation that may limit vole dispersal and colonization (Rado and Rowlands 1984, Murphy and Freas 1989).