Sensitive, Threatened, and Endangered Mammals of Riparian and other Wetland Communities in California^[1]

Daniel F. Williams and Kerry S. Kilburn^[2]

Abstract.—Studies of the distribution, habitat requirements, and population status of species and subspecies of mammals in California were conducted in order to identify taxa threatened with extinction. Investigations were limited to taxa without current state or federal Rare, Threatened, or Endangered status. Twenty-one species and subspecies of mammals confined to or dependent upon riparian and other wetland communities were identifed as being especially vulnerable to loss of habitat and facing potential threats of extinction. These taxa are grouped into four categories depending upon the apparent nature and proximity of the threats to their populations. Destruction of riparian and other wetland communities is the principal factor jeopardizing all 21 taxa. Preservation of these and other members of riparian and other wetland communities can probably be accomplished most efficiently by an integrated approach that focuses on preserving biotic communities rather than single species. Herein we outline the elements of such a plan.

Introduction

Of the 502 recent native species and subspecies of land mammals in California (Hall 1981), approximately 25% (133 taxa) are limited to or largely dependent upon riparian and other wetland communities. No other general type of mammalian habitat in California approaches riparian and other wetland communities in importance to mammals, and none has been so diminished in extent and degraded in quality (Warner 1979). As a result, populations of mammalian species dependent upon freshwater and tidal riparian wetland communities have declined markedly in size in nearly every region within California. Whitetailed deer (Odocoileus virginianaochroura and O . v . couesi ) have become extinct in California in this century (Williams in press). Populations of one tideland species, salt marsh harvest mice (Reithrodontomysraviventris raviventris and R . r . halicoetes ) and one freshwater wetland species, the Amargosa vole (Microtuscalifornicusscirpensis ), are listed as Endangered (California Department of Fish and Game 1980); several other riparian and wetland species are seriously jeopardized by destruction and degradation of their habitats.

Concern over the rapid loss of biotic communities within California and the resulting threats to wildlife prompted the Nongame Wildlife Investigations Unit of the California Department of Fish and Game (DFG) to commission a study of potentially threatened populations of mammals within the state. Williams initiated that study in 1979 and filed the final report to DFG in September 1981 (Williams in press). The investigation was limited to native species of land mammals without state or federal Rare, Threatened, or Endangered status. Populations of 52 species and subspecies were identified as being potentially threatened with extinction; 21 of these are limited to or principally dependent upon riparian and wetland communities.

This report summarizes the investigations into the current population status of those mammalian species confined to or dependent upon riparian and other wetland communities in California.

Methods

Taxa believed to be extinct and species concurrently being investigated by the DFG were excluded from the investigation. To be included on the final list of concern, the entire California population of a taxon had to be potentially

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] Daniel F. Williams is Professor of Zoology and Kerry S. Kilburn is a student in the Department of Biological Science; both are at California State College, Stanislaus, Turlock.

jeopardized with extinction, given continuation of current trends in diminishment and degradation of habitat, harvest or persecution, or other factors threatening populations. Thus, locally depleted populations of more wide-ranging taxa were excluded, with a single exception: an isolated population of the Sierra Nevada mountain beaver (Aplodontiarufa californica ), living in tufa deposits along Lee Vining Creek near where it historically entered Mono Lake (Steele in press), was included because of its unique habitat.

Initially, a working list of 86 candidate species and subspecies was assembled. Information on life history, habitat requirements, past and present distributions, systematic status, probable population status, and the nature of potential threats was gathered for each candidate, from the literature and other sources (see below). Letters and questionnaires requesting information on potentially threatened mammals were sent to all members of the American Society of Mammalogists residing in California, and to other, selected persons in state and federal agencies, universities, and natural history museums. A number of persons were contacted by telephone for inquiries about selected taxa and developments affecting wildlife habitats in certain areas of the state. Williams visited and obtained distribution data from 17 natural history museums which collectively contain the great majority of mammal specimens from California. Distribution data on some taxa were also obtained from 18 other mammal research collections (see Williams in press).

Most areas in the state where loss of habitat was thought to pose a threat to one or more candidate species were visited. Some preliminary field work to determine presence and abundance of candidate species was conducted, but time and funding did not permit detailed or extensive field work. Rather, the objective of the investigations was to develop a list of species, in priority categories, which would be used to determine the disbursement of limited funds for detailed investigations in the field and for other administrative decisions.

Common and scientific names used in this report are from Williams (1979). Vernacular names for subspecies are included because subspecies can be accorded state and federal Rare, Threatened, or Endangered status. These names are from Grinnell (1933), or those coined by the authors of more recently described species and subspecies, or, when no name was available, by Williams (in press). See Williams (ibid .) for detailed remarks on the taxonomy used here.

Refer also to Williams (ibid .) for a detailed account of methods, remarks on systematic status, recommendations for management actions, and documentation of distribution records.

Results

Twenty-one species and subspecies of mammals of riparian and other wetland communities were found to face potential threats of extinction. The major factor jeopardizing each of these populations is loss and degradation of habitat. Each taxon is assigned to one of four categories according to the apparent proximity of the threats to remaining populations (table 1). The categories are described below.

Category l.—Species are considered to be potentially endangered as defined by the federal Endangered Species Act of 1973.^[3] Immediate action to stop loss and degradation of habitat for these species is needed. Field investigations to establish status and baseline population data should be carried out as rapidly as possible.

Category 2.—Species may be threatened or endangered as defined in the federal Endangered Species Act, but the threats of extinction seem less imminent than for species in category 1. Priority in management actions should be given to halting loss and degradation of habitat and establishing baseline data on populations.

Category 3.—Species probably do not warrant Endangered status now and appear not to be under proximate threats of extinction. If current trends in loss and degradation of habitat continue, however, they could quickly become endangered. These species may merit Rare (state) or Threatened (federal) status under current regulations. The principal administrative actions required are to initiate field investigations into population status and to consider the habitat needs of these species in land development and resource management plans.

Category 4.—Species are considered to be sensitive or vulnerable to disturbances, including loss and degradation of habitat, overharvesting, and other factors. Principal administrative actions needed include special considerations for these species in land development and resource management decisions, and protection from overharvest.

Distribution

Table 1 also briefly lists the distribution of each taxon. Note that five species found principally or wholly along the Colorado River in California are considered to be jeopardized. Of the five, only the Yuma mountain lion (Felisconcolor browni ) probably ranges far beyond the immediate vicinity of the river valley (ibid .), although it appears to be dependent upon the riparian community. Two of the species are restricted to the tidal marshes in the coastal region of the Los Angeles Basin, and two others are confined to the salt marsh communities in the San Francisco Bay area. Four of the poten-

[3] P.L. 93–205.

tially jeopardized species are found in the wetland and riparian communities of the San Joaquin Valley. Of these four, only the golden beaver (Castorcanadensissubauratus ) ranges beyond the San Joaquin Valley. The golden beaver is the only species listed here which may also face a serious threat from overharvesting.

These four areas, the Colorado River Valley, the San Joaquin Valley, and the tidal marshes of the Los Angeles Basin and San Francisco Bay, are viewed as special problem areas in terms of loss and degradation of riparian and wetland communities. Degradation of riparian and other wetland communities has, however, diminished mammalian habitats throughout all areas of California (table 1).

Jeopardized Species

The habitats of the 21 taxa listed in table 1 are briefly outlined below. In many cases, little or no data were available for the taxa of concern.

Salt Marsh Wandering Shrew

The salt marsh wandering shrew (Sorexvagrans halicoetes ) occupies the medium-high marsh about 1.8–2.4 m. (6–8 ft.) above sea level and lower marsh areas not regularly inundated, characterized by abundant driftwood and other debris scattered among Salicornia . It requires dense cover, abundant food (invertebrates), suitable nesting sites, and fairly continuous ground moisture (Johnston and Rudd 1957).

San Bernardino Dusky Shrew

The San Bernardino dusky shrew (Sorexmonticolusparvidens ) is probably similar in its habitat association to the populations in the Sierra Nevada; no information on its specific requirements, however, is available. In the Sierra Nevada, dusky shrews are associated with riparian and wetland communities from the upper mixed conifer zone to the timberline (Williams in press).

Buena Vista Lake Shrew

The Buena Vista Lake shrew (Sorexornatus relictus ) occupied marshes on the perimeter of the historic Lake Buena Vista (Grinnell 1933). It may occupy dense vegetation along streams and sloughs and around the perimeter of tule marshes in the Tulare Basin, although nothing has been recorded about its habitat (Williams in press).

Southern California Salt Marsh Shrew

The Southern California salt marsh shrew (Sorex ornatussalicornicus ) occurs in coastal marshes and probably requires fairly dense groundcover, nesting sites above mean high tide and free from inundation, and fairly moist surroundings. Nothing has been recorded about its habitat requirements (ibid .).

Suisun Shrew

The Suisun shrew (Sorexornatussinuosus ) appears to require dense, low-lying cover where invertebrates are abundant. It typically inhabits tidal marshes characterized, in order of decreasing tolerance to inundation, by Spartina foliosa , Salicorniaambigua , and Grindeliacuneifolia , and brackish marshes dominated by Scirpuscalifornicus and Typha latifolia . Suitability of habitat for shrews is determined by growth forms of the plant community, rather than species composition (Rudd 1955).

Santa Catalina Shrew

The Santa Catalina shrew (Sorexwillettti ) is probably found at least in the larger, stream-bearing canyons of Santa Catalina Island (von Bloeker 1932) and is possibly widely distributed, at least seasonally. Nothing is recorded about its habitat requirements (Williams in press).

Arizona Myotis (Bat)

The Arizona myotis (Myotisoccultus ) is most commonly associated with pine forests at 1,800–2,700 m. (6000–9000 ft.) outside California (Barbour and Davis 1969). It is known in California only from the low desert along the Colorado River (Williams in press). In most area, its roosts have been found beneath bridges and in attics of buildings (Barbour and Davis 1969), and it probably also roosts in hollows in trees and protected crevices in rocks (Williams in press).

Arizona Cave Myotis (Bat)

The Arizona cave myotis (Myotisvelifer velifer ) inhabits arid zones in the southwestern United States (Barbour and Davis 1969). Optimal foraging habitat seems to be the dense, linear stands of mesquite, tamarisk, and catclaw acacia bordering the still water and oxbow ponds along the floodplain of the Colorado River (Vaughan 1959). Preferred roost sites in California appear to be mine tunnels and caves (Stager 1939; Vaughan 1959).

Riparian Brush Rabbit

The riparian brush rabbit (Sylvilagusbachmani riparius ) is confined to dense thickets of brush such as wild rose (Rosa sp.), willows (Salix sp.), and blackberries which occur close to the San Joaquin River (Orr 1940).

Oregon Snowshoe Hare

The Oregon snowshoe hare (Lepusamericanus klamathensis ) is found primarily in riparian areas with thickets of deciduous trees such as willows and alders and in dense thickets of young conifers, particularly young firs (Williams in press).

Sierra Nevada Snowshoe Hare

The Sierra Nevada snowshoe hare (Lepusamericanustahoensis ) lives only in boreal zones, typically inhabiting riparian communities with thickets of deciduous trees and shrubs such as willows and alders (Orr 1940).

Sierra Nevada Mountain Beaver

The Sierra Nevada mountain beaver (Aplodontiarufacalifornica ) was recently discovered living along a freshwater seep near where Lee Vining Creek historically entered Mono Lake. Vegetation supported by the seep was characteristic of the herbaceous plants and woody shrubs of the riparian zone of the Great Basin sagebrushsteppe province. The area surrounding the seep predominately supported big sagebrush (Artemisia tridentata ) and rabbit bush (Chrysothamnus viscidiflorus ) (Steele in press).

Point Arena Mountain Beaver

The Point Arena mountain beaver (Aplodontiarufanigra ) primarily occupies thickets of thimbleberries on north-facing slopes (Camp 1918).

Point Reyes Mountain Beaver

The Point Reyes mountain beaver (Aplodontiarufaphaea ) is found in hillsides below 300-m. (1,000-ft.) elevation, in seepage areas overgrown with sword ferns and thimbleberries (Grinnell 1933).

Sonora Beaver

The Sonora beaver (Castorcanadensis repentinus ) inhabits slow- to moderate-flowing waters of the main channels of the Colorado River and the sloughs, canals, and oxbow lakes along the river and in the Imperial Valley (Williams in press).

Golden Beaver

The golden beaver (Castorcanadensis subauratus ) inhabits slow- to moderate-flowing streams, ponds, and lakes. Its principal requirement seems to be sufficient food, consisting of roots, bulbs, grasses, cattails, and other herbaceous plants, and bark and twigs of willows, cottonwoods, alders, and other woody plants (Grinnell etal . 1937).

Southern Marsh Harvest Mouse

The southern marsh harvest mouse (Reithrodontomys megalotislimicola ) is strictly confined to marshy areas, generally coastal salt marshes dominated by Salicornia . Adjacent weedy areas and marshes in brackish sites may also be inhabited (von Bloeker 1932).

Colorado River Cotton Rat

The Colorado River rat (Sigmodonarizonae plenus ) appears to be restricted to "isolated sections of alluvium bottom along the Colorado River" (Goldman 1928). Within this zone, it inhabits areas supporting sedges, rushes, cane, and other grasslike plants (Williams in press).

San Joaquin Valley Wood Rat

The San Joaquin Valley wood rat (Neotomafuscipesriparia ) is strictly confined to riparian communities. Nothing specific has been recorded about the habitat of this subspecies, but dusky-footed wood rats generally occur in areas supporting mixtures of trees and brush (ibid .).

White-footed Vole

The white-footed vole (Arborimusalbipes ) seems generally to be associated with small streams in forested areas and very small clearings, created by fallen timber and supporting herbaceous growth (Maser and Johnson 1967). Thickets of alder may be essential habitat for this species (Williams in press).

Yuma Mountain Lion

The Yuma mountain lion (Felisconcolor browni ) primarily inhabits the dense vegetation of the bottomland along the Colorado River; it has also been found in adjacent, rocky uplands (ibid .). Aside from adequate numbers of deer for food, the habitat requirements for this species are essentially unknown.

Discussion and Conclusions

Destruction of riparian and other wetland communities is pandemic in California. This loss and degradation is expected to increase with the increasing human population, unless measures are quickly adopted to protect remaining communities. This will be difficult to accomplish, considering the excessive human competition for limited amounts of water and the many conflicting demands placed upon riparian ecosystems. Loss of riparian and other wetland communities along the Colorado River, the low-elevation segments of the Sacramento and San Joaquin rivers, and the tidal marshes of San Francisco Bay and the Los Angeles Basin poses threats to a number of unique taxa. Biotic communities in these areas should receive priority attention in land development and resource management decisions.

Devising procedures to ensure preservation of biological diversity while permitting needed development of other natural resources is the problem before us. The present approaches are neither efficient nor cost-effective, and efforts are generally fragmented among a plethora of administrative units, resulting in duplication of efforts, gaps in coverage, competition for money and influence, and conflict.

A number of the 21 taxa treated here should be given protection as federally listed Threatened and Endangered species; this would provide for preservation of essential habitat and initiate actions directed at securing and increasing jeopardized populations. Their preservation could be accomplished most efficiently, however, by concentrating conservation efforts on their biotic communities rather than emphasizing singlespecies management. This would also provide more security to essential members of biotic communities not normally accorded protected status (e.g., lower plants, most invertebrates).

An integrated development/conservation approach which focuses upon preserving representative segments of each unique community while other resource- and land-use goals are being developed is needed. This approach would lessen the need for official listing of most of these species and save much of the money and duplication of effort now expended on management of

endangered species on a one-by-one basis. It would also ensure that resource and land development objectives are compatible with national and state goals, including the maintenance of the health and well-being of this and future generations of people.

The state should be divided into management areas based upon known and projected capabilities for resources, rate and degree of projected development, and political and administrative boundaries. We believe that resource management areas based upon major watersheds would be the most efficient approach. A task force consisting of representatives of political and resource management agencies within the management area, USDI Fish and Wildlife Service biologists, conservationists, and development interests should be established to oversee preparation of an integrated conservation/development plan for each management area. The widest possible input from the public should be sought during the formative stage of the resource area management plans.

Guidelines and procedures for implementing conservation/development plans and for amending plans in response to changing resource needs and land conditions should be established. Coordination of activities and development of goals should be accomplished at the state level by a group with a similar composition to that described for the management areas. Federal resource management agencies should set national and regional land- and resource-use goals in cooperation with the state.

Goals for use of land and resources in each management area should be defined. Unique biota and sensitive species should be identifed; resource development goals should include habitat needs of vegetation and wildlife. Harvest and use patterns of renewable resources (timber, grazing, land, wildlife, water, etc.) should be designed to optimize productivity while maintaining community diversity—this would require definition of long-term objectives for land and resource use.

Planning and implementation of long-term management of natural resources, including vegetation and wildlife, could be greatly streamlined and economized by merging land and resource management functions now fragmented among several federal agencies: Department of Agriculture (e.g., forests, soil conservation, environmental quality); Department of Commerce (e.g., marine fisheries, ocean resources, coastal management); Department of Defense (military bases and reservations); Energy Department; Department of the Interior (e.g., rangeland, minerals and mining, national parks, water, fish and wildlife). Sufficient legislation and precedents already exist, however, to implement interagency, multigovernmental programs, such as those proposed without reorganization or new legislation.

Unless actions similar to those proposed here are undertaken immediately, we are confident that populations of all 21 taxa treated here will diminish, perhaps to the point of extinction for many. These losses alone would be substantial. More alarming, however, is that these species are members of biotic communities which are rapidly diminishing. Riparian and other wetland communities are vital to most wildlife species; their degradation and loss will represent a catastrophic loss of biological diversity.

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