Endangered Species Recovery Program

6. Buena Vista Lake Shrew (Sorex ornatus relictus)

• Taxonomy
• Description
• Identification
• Historical Distribution
• Current Distribution
• Conservation Efforts
• Food and Foraging
• Reproduction
• Demography
• Behavior and Species Interactions
• Activity Cycle
• Habitat and Community Associations
• Reasons for Decline
• Threats to Survival
• Conservation Efforts
• Conservation Strategy

Taxonomy.-- The Buena Vista Lake shrew (Sorex ornatus relictus) was described by Grinnell (1932b) from the type specimen collected near Buena Vista Lake, Kern County, California. This shrew is one of nine subspecies of the ornate shrew (Sorex ornatus) (Merriam 1895, Hall 1981, Junge and Hoffmann 1981).

The systematic status of the Buena Vista Lake shrew is uncertain because only a few specimens have been available for comparison and a review of the systematics of the species has not been completed (Maldonado 1992). An evaluation of the systematics of the group, using DNA analysis, is currently underway. Preliminary results indicate that the Buena Vista Lake shrew is a distinct evolutionary unit of ornate shrew (J. Malonado pers. comm.).

Description.-- Ranges of external measurements from the type specimen and two additional specimens are: total length, 98 to 105 millimeters (3.86 to 4.13 inches); tail length, 35 to 39 millimeters (1.38 to 1.54 inches); hind foot length, 11.5 to 13 millimeters (0.45 to 0.51 inch); and ear length from the notch, 6.5 to 8.5 millimeters (0.26 to 0.33 inch). Weights ranged from 4.1 to 7.6 grams (0.14 to 0.27 ounce). The upper surface of the Buena Vista Lake shrew is blackish-brown, with a pepper-and-salt pattern of buffy brown and black, the black predominating. The sides are more buffy brown than the upper surface. The lower surface is smoke gray. The tail is not noticeably bicolored and darkens towards the end, both above and below (Grinnell 1932b).

Identification.-- The Buena Vista Lake shrew (Figure 64) differs externally from S. ornatus ornatus, whose range surrounds that of S. o. relictus. The coloration of the Buena Vista Lake shrew is distinctly darker, grayish-black, rather than brown. The body size is slightly larger, but the tail is shorter. The teeth are essentially the same, but the third and fifth unicuspids (teeth behind the incisors that have a single main cusp) are even smaller relative to the other teeth (Grinnell 1932b).

Figure 64. Illustration of a Buena Vista Lake shrew. Souce: Daniel F. Williams.

Historical Distribution.-- The Buena Vista Lake shrew formerly occurred in wetlands around Buena Vista Lake, and presumably throughout the Tulare Basin (Grinnell 1932b, 1933a; Williams and Kilburn 1984, Williams 1986). As early as 1933, Grinnell (1933a) found the distribution of this species to be much restricted due to the disappearance of lakes and sloughs. Since Grinnells (1932b) report, Buena Vista Lake and the surrounding lakes and Valley Freshwater Marshes have been drained and cultivated. Further, canals in the area are steep-sided and kept free of vegetation (Williams and Kilburn 1992).

Current Distribution.-- Little is known about the current distribution of the Buena Vista Lake shrew. It was rediscovered in 1986 by Robert Hansen during excavations on the Kern Lake Preserve (Figure 65) (D.F. Williams unpubl. observ.). The status of this population was assessed in the early 1990s (Center for Conservation Biology 1990, Maldonado 1992) and most recently in 1995 (Maldonado 1998). Two shrews were also collected in 1992 and one in 1994 at the Kern National Wildlife Refuge (J. Allen pers. comm.). Water management practices at the Refuge have focused on waterfowl, and riparian habitat has not received adequate water over the years to maintain riparian diversity (Engler in litt. 1994). Any other extant populations found within the Tulare Basin may or may not be representative of the Buena Vista Lake shrew. No other recent records of this shrew are known, though only a few biological surveys have included attempts to capture shrews (Clark et al. 1982, Germano in litt. 1992, T. Kato pers. comm., S. Tabor pers. comm.).

Figure 65. Recent distributional records for the Buena Vista Lake shrew (Sorex ornatus relictus)

Conservation Efforts.-- The Buena Vista Lake shrew is a Federal candidate for listing as threatened or endangered (USFWS 1996), and is a California State Mammalian Species of Special Concern (Williams 1986).

Food and Foraging.-- The specific feeding and foraging habits of the Buena Vista Lake shrew are unknown. In general, shrews primarily feed on insects and other animals, mostly invertebrates (Harris 1990, Williams 1991, Maldonado 1992). Food probably is not cached and stored, so the shrew must forage periodically day and night to maintain its high metabolic rate.

Reproduction.-- Nothing is known specifically about the reproduction and mating system of the Buena Vista Lake shrew. In general, the reproductive period of the ornate shrew extends from late February through September and early October (Rudd 1955, Brown 1974, Rust 1978). The breeding season of the Buena Vista Lake shrew may begin in autumn and end with the onset of the dry season in May or June. In high-quality habitat in permanent wetlands, the breeding season may be extended (Center for Conservation Biology 1990, Williams in litt. 1989). Up to two litters are produced per yar containing four to six young (Owens and Hoffman 1983).

Demography.-- Little is known about population numbers, home range, or territoriality of the Buena Vista Lake shrew or ornate shrews in general. Twenty-five Buena Vista Lake shrews were captured during four trapping sessions from December 1988 through May 1989. Only one animal was recaptured (Freas 1990). In captivity, ornate shrews defend nest sites (Newman 1976). Population densities of the taxonomically related species, S. vagrans vagrans, in western Washington, varied from about 25.8 per hectare (10.12 per acre) in fall and winter to 50.2 per hectare (20.32 per acre) at the high point in summer (Newman 1976). Though no values are available for S. ornatus, trapping results suggest that S. o. relictus exists at much lower densities, probably no more than 10 to 15 per hectare (4 to 6 per acre) at the high point. Assuming a density of 13 per hectare (5.3 per acre), and a desired population size of no less than 5,000 individuals, approximately 400 hectares (1,000 acres) of occupied habitat would be required for long-term conservation.

Behavior and Species Interactions.-- Pairs of ornate shrews lived together in captivity without antagonism if adequate food and water were provided (Owen and Hoffmann 1983). Although shrews were not observed burrowing in leaf litter on cage floors, they are thought to burrow in natural settings (Rudd 1953). During hot weather in dry habitats, the ornate shrew may restrict its daytime activity to burrows of other animals (Pearson 1959).

Activity Cycle.-- Ornate shrews are active day and night (Pearson 1959, Newman and Rudd 1978, Rust 1978). Nocturnal activity predominates, especially during the breeding season, in the Suisun shrew (S.o. sinuosus; Rust 1978). The intensity and distribution of activity within a 24-hour period varies with sexual maturity (Rust 1978).

Habitat and Community Associations.-- Ornate shrews in general tend to be associated with the structure of the vegetation rather than with species composition of the community (Owen and Hoffmann 1983). Buena Vista Lake shrews occupied Valley Freshwater Marshes on the perimeter of Buena Vista Lake and probably occurred throughout the Tulare Basin (Williams 1986), though most of the marshlands were drained or dried up prior to the discovery of the shrew in 1932 (Grinnell 1932b). Recent captures on the Kern Lake Preserve occurred in areas with a dense wetland vegetative cover and an abundant layer of detritus (decomposed vegetation) (Center for Conservation Biology 1990, Maldonado 1992). Plant species associated within these areas include Fremont cottonwood (Populus fremontii), willows (Salix spp.), glasswort, alkali heath, wild-rye grass Elymus sp.), and Baltic rush (Juncus balticus). Animal species captured on the Preserve, but only in the xerophytic community, were deer mice and California pocket mice (Center for Conservation Biology 1990).

Reasons for Decline.-- Loss and fragmentation of habitat are the major causes for decline and threat to the Buena Vista Lake shrews survival (Williams and Kilburn 1984, 1992). The conversion of natural lands to agriculture and diversion of fresh water supplies have eliminated most of the riparian habitat that once supported the shrew, reducing the subspecies to what may be a single remaining population. By the early 1930s, the former Tulare, Buena Vista, Goose, and Kern lakes were virtually dry and open for cultivation (Griggs 1992). Historical Buena Vista Lake now is cultivated, and Kern Lake has been reduced to 13.4 hectares (33 acres) with a small pond and artificially-maintained wetland, and a more xerophytic community of annual and perennial saltbushes, saltgrass, and annual grasses and forbs (Center for Conservation Biology 1990, Williams and Kilburn 1992).

Threats to Survival.-- The Buena Vista Lake shrew is a limited local endemic subspecies (Williams and Kilburn 1992), has never been found to be locally abundant, and lives in very restricted areas of marshy wetland habitat (Bradford 1992). Because the sole population is small (only 1 individuals as of 1995) and occurs in a single small location (30 acres at the former Kern Lake Preserve), the Buena Vista Lake shrew is extremely vulnerable to natural or human-made environmental impacts. Kern Lake Preserve is privately owned by the J.G. Boswell Company, and was privately managed by The Nature Conservancy until recently. The partnership between The Nature Conservancy and J.G. Boswell Company was terminated in early 1995, and efforts by USFWS to negotiate a Conservation Agreement with J.G. Boswell Company have failed (Reed Tollefsun pers. comm., K. Freas pers. comm.). Thus, the shrews only known habitat is without protection, and there is a possibility that the water supply that maintains the pond and wetland plant community will be diverted elsewhere for irrigated agriculture. Elevated concentrations of selenium also represent a serious human-made environmental threat to the Buena Vista Lake shrew. Ornate shrews captured at Kesterson National Wildlife Refuge showed selenium concentrations three to twenty-five times greater than those found for any other small mammal at the same site (Clark 1987). High selenium levels have been measured in evaporation ponds within the agricultural lands immediately surrounding the former Kern Lake Preserve (California Department of Water Resources in litt. 1997). Potential dietary selenium concentrations, from sampled aquatic insects, are within ranges toxic to small mammals (Olson 1986, Skorupa et al 1996), and could potentially adversely affect the shrew. Such effects could include, but may not be limited to, reduced reproductive output or premature death (Eisler 1985, Skorupa et al 1996). The Buena Vista Lake shrew also faces high risks of extinction from random catastrophic events (e.g. floods, drought and inbreeding). There are no known viable populations of Buena Vista Lake shrews outside the former Kern Lake Preserve for recolonization if a catastrophic event were to occur at this site. While the species still occurs within its limited range, it is not known whether or not the population is declining, how habitat conditions may be affecting the population, nor how small population size may be affecting genetic and behavioral stability.

Conservation Efforts.-- Establishment of the Kern Lake Preserve, through an agreement between the owner, J.G. Boswell Company, and The Nature Conservancy provided protection of habitat for the Buena Vista Lake shrew and several candidate plant species from 1985 to 1995. The Nature Conservancy sponsored a population census for the species on the Preserve in 1988-1989 (Center for Conservation Biology 1990). More recently, USFWS sponsored a study to determine current status of the shrew at the Preserve and to try to locate other populations (J. Maldonado pers. comm.).

In 1994 and 1995, USFWS worked with the J.G. Boswell Company and The Nature Conservancy in an attempt to reverse The Nature Conservancys decision to no longer manage the Preserve. USFWS has been working to develop a prelisting conservation agreement. Currently, there is an impasse: there is no conservation agreement for the property and no active management of habitat for the species that live there (J.A. Medlin in litt. 1995b).

Conservation Strategy.-- The Kern Lake site should be preserved in perpetuity for the Buena Vista Lake shrew. In addition, greater efforts to locate and protect other extant populations of Buena Vista Lake shrews within the Tulare Basin are needed. Remnant patches of suitable habitat that might support the Buena Vista Lake shrew include areas within the Buena Vista Lake Aquatic Recreation Area, the Buena Vista Golf Course, and along the Buena Vista Slough, Goose Lake Slough and the Kern River west of Bakersfield (J. Maldonado pers. comm., Williams in litt. 1994). Additional areas of suitable moist locations that might provide remnant shrew habitat occur within the Pixley National Wildlife Refuge west of the former Tulare Lake bed, as well as around the former Goose Lake bed (Harris 1990). Areas south along Jerry Slough east of Buttonwillow Ridge may provide remnant shrew habitat as well (P. Collins pers. comm.).

Critical to conservation is the establishment of habitat hat can support expansion and introduction efforts. Areas appropriate for habitat establishment include wetland areas within the Kern Water Bank Habitat Conservation Plan. Wetland creation and water conveyance facilities such as canals and ditches will provide habitat for this species, although it is unlikely that this habitat would become occupied in any other way than by deliberate introduction. Introductions would be under cooperative agreement with the resource agencies and Kern Water Bank Authority, or by other means (USFWS in litt. 1997b). Two other areas are the State Tule Elk Reserve near Tupman, another area where negotiations are underway to secure a permanent water supply (J. Single pers. comm.), and the Kern National Wildlife Refuge. Expansion of habitat, introduction efforts, and the protection of the Buena Vista Lake shrew should be an objective of any future National Wildlife Refuge and Ecological Reserve development and management plans.

The status of the Buena Vista Lake shrew should be reevaluated within 3 years of recovery plan approval.