Endangered Species Recovery Program
Taxonomy.-- The brush rabbit was described as Lepus bachmani by Waterhouse in 1838 and renamed L. trowbridgii by Baird in 1855, and redescribed with the currently accepted specific name of Sylvilagus bachmani by Lyon in 1904 (Larsen 1993). The species is found west of the Cascade-Sierra crest from the Columbia River to the tip of Baja California (Williams and Basey 1986). Thirteen subspecies of brush rabbit are recognized. The riparian brush rabbit, S. b. riparius, is one of eight subspecies found in California. It was described by Orr (1935) based on a specimen from the west side of the San Joaquin River about 3 kilometers (2 miles) northeast of Vernalis in Stanislaus County, California.
Description.-- Brush rabbits are small, brownish rabbits that can be distinguished from their relative, the desert cottontail, by a smaller, inconspicuous tail and uniformly colored ears (no black tip) (Figure 66). The adult riparian brush rabbit is about 300 to 375 millimeters (10.58 to 13.23 inches) long, and can be distinguished from other subspecies by its relatively pale color, gray sides, darker back, and the fact that, viewed from above, its cheeks protrude outward rather than being straight or concave (Orr 1940).
Figure 66. Illustration of a riparian brush rabbit. Drawing by Wendy Stevens (© CSU Stanislaus Foundation).
Historical Distribution.-- Historically, the riparian brush rabbit is believed, based on the presence of suitable habitat, to have been found associated with riparian forests along portions of the San Joaquin River and its tributaries on the Valley floor, from at least Stanislaus County to the Delta (Orr 1935).
Current Distribution.-- By the mid-1980s, the riparian forest within the former range of the riparian brush rabbit had been reduced to a few small and widely scattered fragments, totaling about 2,100 hectares (5,189 acres). At 104.5 hectares (258.2 acres), Caswell Memorial State Park, on the Stanislaus River in southern San Joaquin County, is the largest remaining fragment of suitable riparian forest (Warner 1984) and home to the only extant population of riparian brush rabbit (Figure 67) (Williams and Basey 1986).
Food and Foraging.-- Avoiding large openings in shrub cover, riparian brush rabbits frequent small clearings where they feed on a variety of herbaceous vegetation, including grasses, sedges, clover, forbs, shoots, and leaves. Grasses and other herbs are the most important food for brush rabbits, but shrubs such as California wild rose (Rosa californica), marsh baccharis (Baccharis douglasii), and California blackberry (Rubus ursinus) also are eaten. When available, green clover (Trifolium wormskioldii) is preferred over all other foods (Orr 1940).
Reproduction and Demography.-- Breeding of riparian brush rabbits is restricted to approximately January to May, putting this species at a competiive disadvantage to the desert cottontails outside the park that breed all year. Gestation is about 27 days, the usual litter size is three or four, and females produce three to four litters during the season. On average, a female may produce 9 to 16 young each year. Although this is a relatively high reproductive rate, it is lower than many other cottontail species, and five out of six rabbits do not survive to the next breeding season (Mossman 1955, Chapman and Harman 1972).
The population at Caswell Memorial State Park may have reached its lowest numbers after a flood in 1976, when survivors were removed to dry land from trees and shrubs by Park personnel in boats. After flooding in 1986, the population was estimated at between 10 and 20 individuals (Williams 1988). In 1993 the population was estimated by Williams (1993) at 213 to 312 individuals, and considered to be at carrying capacity under prevailing environmental conditions. Surveys were conducted in May 1997 after extensive winter flooding at Caswell State Park. Although one riparian brush rabbit was sighted, none were live-trapped. However, in the fall 1997/spring 1998 trapping session, one riparian brush rabbit male was live-trapped.
Behavior and Species Interactions.-- Brush rabbits are closely tied to cover, and usually remain for several seconds to minutes just inside dense, brushy cover before venturing into the open. They seldom move more than a meter from cover, then remain motionless, watching for signs of danger. When pursued, they leap back into the cover of shrubs instead of heading into open ground (Chapman 1974). They will not cross large, open areas, and hence are unable to disperse beyond the dense brush of the riparian forest at Caswell Memorial State Park (Williams 1988).
The riparian brush rabbit can climb into bushes and trees, though its climbing is awkward and its abilities limited. This trait probably has significant survival value, given that the riparian forests that are its preferred habitat are subject to inundation by periodic flooding (Chapman 1974, Williams 1988).
Individuals are intolerant of each other when they come too close, but there is no well defined territoriality. Young are more tolerant of approach by another rabbit than are adults (Chapman 1974).
When weather conditions are appropriate, individuals spend considerable time in the early mornings and afternoons on a log or a dry form (a resting place for a rabbit) basking in the sun. Favored basking sites are a few inches from cover no more than about 46 centimeters (18 inches) above ground, and protected by a partial, low-stratum canopy (Williams 1988, D.F. Williams unpubl. observ.).
Common mammalian associates of riparian brush rabbits are riparian woodrats, roof rats, western gray squirrels (Sciurus griseus), American opossums (Didelphis virginiana), striped skunks, feral cats (Felis sylvestris), gray foxes, coyotes, and feral dogs (Basey 1990, Williams 1988). Predators of riparian brush rabbits include red-tailed (Buteo jamaicensis), Swainsons (B. swainsoni), and red-shouldered hawks (B. lineatus), owls, feral cats, gray foxes, coyotes, and dogs.
Activity Cycles.-- Riparian brush rabbits are most active during the twilight hours around dawn and dusk. Depending on season, the main activity periods last 2 to 4 hours. The least activity is from about 10:30 a.m. to 4:00 p.m. (Chapman 1974).
Habitat and Community Associations.-- Riparian brush rabbits live in the brushy understory of Valley riparian forests. Forest with a closed canopy, however, generally lacks sufficient understory of shrubs for their needs. Where mats of low growing wild roses, wild grape (Vitis californica), and blackberries are found in savanna-like settings, brush rabbits live in tunnels through the vines and shrubs.
Sites inhabited by riparian brush rabbits usually have a mix of roses, blackberries, marsh baccharis, and grape vines, with high volumes of roses and coyote bushes (Baccharis sp.) in comparison to uninhabited sites. There are significantly moe ground litter and surface area of roses and significantly fewer willows in the canopy and understory (none) at sites inhabited by riparian brush rabbits than sites occupied by desert cottontails. Presence of more surface litter and lack of willows in the understory signify areas of higher ground that are not flooded regularly or heavily (Williams and Basey 1986).
Reasons for Decline.-- Two phenomena jointly have been the primary cause of the decline of the riparian brush rabbit. Both had their origin in the completion, beginning in the 1940s, of large dams for irrigation and flood control on the major rivers of the Central Valley. The first was the destruction and fragmentation of the San Joaquin Valley riparian forest by conversion to various urban and agricultural uses, and its degradation through a variety of other human activities. By the mid-1980s, this community had been reduced to only about 5.8 percent of its original extent. There probably is less today (Larsen 1993).
The second, more specific phenomenon was the conversion of land within the floodplains from shrub-dotted pasture land to vineyards, orchards, and row crops, with attendant land clearing and leveling, and the building and maintenance of levees. The land along rivers no longer exhibits the small patches of shrub-covered upland that once provided rabbits refuge from flooding and predation (Williams and Basey 1986, Williams 1988).
Threats to Survival.-- The primary threat to the survival of the riparian brush rabbit is the limited extent of its existing habitat and the fact that there is only one extant population. Periodic flooding still occurs along all major rivers in the Valley (Kindel 1984). The increased predation to which these animals are exposed while taking refuge on cleared levees (Nolan 1984) or in exposed bushes or trees contributes directly to population decline and an elevated risk of extinction. With behavioral restrictions on its freedom of movement (low mobility) and the dearth of habitat suitably protected from frequent floods down-stream of Caswell Memorial State Park, there is little chance that individuals that escape drowning or predation will meet mates or reproduce.
The long-term suppression of fire in Caswell Memorial State Park, combined with prolonged drought, has caused the buildup of high fuel loads. The dense, brushy habitat to which the rabbits are restricted is thus highly susceptible to catastrophic wildfire that would cause both high mortality and severe destruction of habitat. Recovery of the riparian brush rabbit population from such a devastating event would be improbable.
Like most rabbits, the riparian brush rabbit is subject to a variety of common diseases, including tularemia, plague, myxomatosis, silverwater, encephalitis, listeriosis, Q-fever, and brucellosis. These contagious, and generally fatal, diseases could be transmitted easily to riparian brush rabbits from neighboring populations of desert cottontails (Williams 1988). In a widespread, genetically heterogeneous population, such an outbreak would be of minimal concern. However, in this small, remnant brush rabbit population, this kind of epidemic could quickly destroy the entire population.
Dependence on nearly continuous shrub cover, low mobility, and competition with the more fecund and mobile desert cottontail (Ingles 1941, Chapman 1971, Chapman and Wilner 1978) are significant threats to the riparian brush rabbit in the ecotone communities between the riparian shrublands and the open, dry plant communities of the San Joaquin Valley (Williams 1986).
Given the biology and behavior of riparian brush rabbits and the smallness and highly fragmented distribution of the remnant of their habitat, natural dispersal cannot be expected. Thomas (1990) suggested that, to assure the medium- to long-term persistence of birds or mammals, the geometric mean of population size should be about 1,000 for species with normally varying numbers and about 10,000 for species exhibiting a high variability in population size. With its maximum population size limited by the size of its habitat well below either of these suggested minimms, the riparian brush rabbit population is at a high risk of imminent extinction from several consequent threats related to population genetics and dynamics and environmental variability.
Conservation Efforts.-- In 1986, after surveys along rivers within its historical range indicated that there was only a single, small extant population in Caswell Memorial State Park (Williams and Basey 1986), the riparian brush rabbit was designated as a "Mammalian Species of Special Concern" by the CDFGs Wildlife Management Division. It was given Federal category-1 candidate status by USFWS in 1985 (USFWS 1985d) and remained a candidate for listing in USFWSs most recent Notice of Review (USFWS 1996). The riparian brush rabbit was proposed for listing by the USFWS on November 21, 1997 (USFWS 1997). The subspecies was listed as endangered by the State of California in May 1994 (Title 14, Division 1, California Administrative Code, Section 670.5, Animals of California declared to be endangered or threatened).
Besides the passive protection afforded to the species by the status of Caswell as a State Park, the California Department of Parks and Recreation funded a study of ecology and habitat management of riparian brush rabbits (Basey 1990, Williams 1988) and a small mammal inventory (Cook 1992). California Department of Parks and Recreation, Bureau of Reclamation, and USFWS, through the Endangered Species Recovery Program, funded a population assessment in the winter of 1993 and 1996-1997 (Williams 1993). The California Department of Parks and Recreation has expanded fire trails in Caswell Memorial State Park, which provides additional edge habitat for rabbits and better access to fight fires. The agency also has an on-going control program for feral animals, has curtailed ground-squirrel control (brush rabbits will eat treated bait meant for ground squirrels), and is involved in ongoing planning for habitat protection for wildlife in the park.
The only other management activity focused on the riparian brush rabbit at this time is a project to establish an experimental population on the Kings River in Fresno County, outside of the historical range of the subspecies. This effort was initiated when the Endangered Species Recovery Program suggested to the Bureau of Reclamation that establishing a population of riparian brush rabbits on public property along the Kings River could be one option for partially meeting their mitigation responsibilities under the Friant Biological Opinion. Besides Bureau of Reclamation, potential participants in this cooperative project include Caltrans, Endangered Species Recovery Program, Fresno County, COE and CDFG.
Conservation Strategy.-- For optimal survival of riparian brush rabbits at Caswell Memorial State Park, expansion of the existing park and management of riparian brush rabbit habitat is necessary. Habitat management includes revitalizing decadent shrubs, reducing fire hazards, and providing refuges and reducing predation during periodic flooding. Park expansion, however, would require willingness from adjacent landowners to sell or dedicate the property for expansion of the riparian community, which has not been the case in the past, and may not be a practical option. Yet, even should this be achieved, expansion and enhancement of habitat of the park will not be sufficient to secure the survival of the species.
Important to conservation of the riparian brush rabbit is the establishment of other viable populations within its historical range. For successful establishment, studies on appropriate management, habitat restoration techniques, and reintroduction or introduction methods are important. Reintroduction methods may include researching genetic diversity among remaining individuals and implementing a captive breeding program. Potential translocation sites exist on State and Federal lands, and lands covered by Federal wildlife habitat easements along or adjacent to several stretches of the Stanislaus and San Joaquin Rivers. Until new populations are established, there must be close and constant vigilance to detect any immediate threat from fire, flooding, or disease and to allow emergency acton to prevent extinction of the species.
The major problems with existing potential habitat outside Caswell Memorial State Park, including that with wildlife habitat easements and part of the National Wildlife Refuge system, are frequent flooding and lack of sufficient connected habitat (Williams and Basey 1986). A substantial amount of this property could become useable habitat for brush rabbits by providing protection from flooding. Dikes or raised areas with cover to shelter from high water, cessation of wood cutting, and stopping the removal of logs and limbs, and curtailment of livestock grazing are needed along several stretches of the Stanislaus River downstream from Caswell Memorial State Park.
An element in the conservation strategy is restoration of riparian habitat on Bureau of Reclamation property along the Kings River in Fresno County. This area is outside the historical geographic range of the riparian brush rabbit. Its importance is paramount, however, because there is not another site in public ownership that offers the potential for quickly restoring sufficient habitat to support a population. Establishment of a second population is important to prevent a single flood, wildfire, or other disaster from causing extinction of the rabbit.
Conservation Actions.-- Because of the small size of remaining blocks of potential habitat, and the severely limited dispersal capability of the riparian brush rabbit, it is likely to require continuing special protection of its habitat and population. Realization of this limitation should remove barriers to the rapid establishment of as many populations in remnant habitat as possible, and sustaining those populations by reintroduction should any one become extinct. In furtherance of these objectives, the needed actions are: