Sylvilagus bachmani 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Mammalia Lagomorpha Leporidae

Scientific Name: Sylvilagus bachmani (Waterhouse, 1839)
Common Name(s):
English Brush Rabbit, Riparian Brush Rabbit
Spanish Conejo Matorralero
Taxonomic Notes: There are 13 recognized subspecies: Sylvilagus bachmani bachmani, S. b. cinerascens, S. b. peninsularis, S. b. cerrosensis, S. b. ubericolor, S. b. exiguus, S. b. mariposae, S. b. virgulti, S. b. howelli, S. b. macrorhinus, S. b. riparius, S. b. tehamae, and S. b. rosaphagus (Hall 1981).

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2008
Date Assessed: 2008-06-30
Assessor(s): Mexican Association for Conservation and Study of Lagomorphs (AMCELA), Romero Malpica, F.J., Rangel Cordero, H. & Williams, D.F.
Reviewer(s): Smith, A.T. & Boyer, A.F. (Lagomorph Red List Authority)
Sylvilagus bachmani is abundant throughout most of its range, and most populations do not appear to be experiencing significant decline (Chapman and Ceballos 1990).The subspecies S. b. riparius has been listed as an endangered species by the state of California, USA (State of California 1994) and the U.S. Fish and Wildlife Service (Williams et al. 2004) due to decline primarily caused by habitat degradation, wildfire, and flood.
Previously published Red List assessments:

Geographic Range [top]

Range Description:Sylvilagus bachmani is distributed along the Pacific coast of North America. It reaches the Columbia River in the north (Oregon-Washington border) and in the south it extends to the southern tip of Baja California. It extends east from the Pacific Ocean to the western Sierra Nevada mountain range (Chapman 1974).

The subspecies S. b. riparius occurs only in Caswell Memorial State Park (MSP) on the Stanislaus River, and the South Delta area of the San Joaquin River, including Paradise Cut and Tom Paine Slough. The park size is 253 acres, and population on the South Delta occurs on privately owned land (Williams and Hamilton 2002). Even though there is other ideal habitat for S. b. riparius in MSP they are unable to reach it because there is no connecting habitat above flood level in MSP (Sandoval, Williams, and Colliver 2006). It occupies an elevational range of 0-2,070 m (Chapman 1974).

In Caswell Memorial State Park, Stanislaus River, San Joaquin County, California, USA, S. b. riparius occurs in about 90% of the park's 102 ha when populations are high, but about 20-40% of the Park at other times (Williams 1988, Williams 1993).

The South Delta population of S. b. riparius, in the vicinity of Mossdale and Lathrop, San Joaquin County, CA, USA, exists on about 122 ha of private land within an area of about 2,927 ha. Populations are found along Paradise Cut, Tom Paine Slough, Grantline Canal, and the San Joaquin River. Rabbits also are found along the narrow right-of-ways of two railroads running through the area. Habitat for S. b. riparius is distributed in discontinuous, narrow strands of riparian vegetation along streams, sloughs, and railroad beds adjacent to intensely cultivated fields. Most land in this area is planned for urban and industrial development within the next 1-10 years. Existing habitat is periodically cut or burned for weed and flood control (Williams and Hamilton 2002).

The re-introduced population of S. b. riparius is located in the San Joaquin River National Wildlife Refuge in Stanislaus County, California. This refuge has about 285 ha of habitat for brush rabbits. An additional few hundred hectares are actively being restored from cropland to natural, riparian vegetation. Refugia from flooding are also being established on the refuge for rabbits and other terrestrial wildlife (Hansen 2006).
Countries occurrence:
Mexico (Baja California, Baja California Sur); United States (California, Oregon)
Additional data:
Upper elevation limit (metres):2070
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Except for the Riparian brush rabbit (subspecies riparius) which is state and federally listed as endangered (Williams et al. 2004), Sylvilagus bachmani is considered common where it is known to exist, throughout Oregon, California, and Baja California (Chapman and Ceballos 1990).

The S. b. riparius population at Caswell Memorial State Park in San Joaquin County, California, USA, was statistically estimated in 1993, when there were 241 rabbits (approximate 95% confidence interval = 170-608) (Williams 1993). Similar censuses were conducted in 1997-2004, resulting in capture of 0 (1997) to 16 (2002) rabbits. Capture rates in the best year (2002) were only 27% of that of 1993, and captures and recaptures were too few in any one year since 1993 to use capture-recapture population models to meaningfully estimate population size (Williams et al. 2004).

Population censuses for the S. b. riparius South Delta metapopulation have not been conducted because of restrictions on activities on private land. However, capture rates for short-term assessments of presence and to capture breeders for a controlled propagation program suggest that this metapopulation has remained at fairly high densities for the past 7 years (when it was discovered). Capture rates for the South Delta metapopulation varied from 127% to 626% of the capture rate of the Caswell MSP population in 1993. The South Delta population probably consists of about 300-600 rabbits at peaks in population cycles (Lloyd et al. 2004).
Current Population Trend:Stable
Additional data:
Population severely fragmented:No

Habitat and Ecology [top]

Habitat and Ecology:Sylvilagus bachmani requires dense brush cover, occupying disjunct bramble clumps, with the size of the clumps affecting home range size and suitability for occupation (Chapman 1974). Home ranges are small (less than 2,000 sq. meters) and are dependent upon habitat uniformity (Chapman and Ceballos 1990). The bramble habitat contains runways and occasionally burrows formed by other species (Chapman and Ceballos 1990). Their diet consists predominantly of grasses, but it will consume alternative vegetation when seasonally available (Chapman 1974).

The breeding season for S. bachmani varies from north to south, but appears to be uniform in length (Chapman 1974). In California, S. bachmani breeds from December to May or June, and in Oregon between February and August (Chapman and Ceballos 1990). Litter size varies regionally, with an annual mean of 2.87 in Oregon, 3.50 in north and central California, and 4.00 in west-central California (Chapman 1974). Fecundity of S. bachmani is lower than other Sylvilagus species, producing about 15 young annually (Chapman 1974). Gestation time is approximately 27 days (Cervantes et al. 2005). Total length is 30.0-37.5 cm (Cervantes et al. 2005).

The subspecies S. b. riparius occupies both old-growth riparian forest, dominated by Valley Oak (Quercus lobata) and riparian communities dominated by thickets of willows (Salix spp.), wild roses (Rosa spp.), blackberries (Rubus spp.), and other successional trees and shrubs and when available dense tall stands of herbaceous plants adjacent to patches of riparian shrubs in the northern San Joaquin Valley, California. S. b. riparius stays close to dense stands of vegetation into which it retreats for escape, resting and nesting. They do not normally burrow or use burrows (Williams and Basey 1986, Williams 1988, Williams and Hamilton 2002). S. b. riparius eats a great variety of woody and herbaceous plants, preferring green foliage of shrubs, grasses and forbs. Much of its foraging is concentrated at the edge between thickets of woody plants and more open ground with herbaceous plants. A favored habitat is willow thickets along stream banks (Orr 1940, Chapman 1974).

Use and Trade [top]

Use and Trade: This species is hunted for food and sport. <2-5% of the total population is utilized.

Threats [top]

Major Threat(s): Hunting for food and the establishment of human settlements pose a threat to Sylvilagus bachmani, but the extent and severity are not quantified.

Decline in numbers and threats of extinction of subspecies S. b. riparius stem principally from actions involved in developments of irrigated agriculture in the San Joaquin Valley of California. Foremost of these developments were stream impoundment, channelization of streams on the valley floor and the San Joaquin River delta, and clearing and cultivation of natural communities. Riparian communities in the San Joaquin Valley have been reduced to about 1% of their historical extent, are found almost entirely within the levees of channelized streams. Most existing patches have been extensively degraded by wood cutting, livestock, invasion of exotic species, greater levels of flooding within levees, and lack of natural riverine processes (U.S. Fish and Wildlife Service 1998; U.S. Fish and Wildlife Service 2000).

Principal existing threats to S. b. riparius can be summarized in order of importance as: 1) stochastic environmental processes, wildfire and flood; 2) additional habitat loss and degradation due to urbanization and conversion to agriculture; 3) increased predation from domestic and feral cats and dogs due to urban development adjacent to existing inhabited sites; and 4) genetic and demographic stochasticity in small populations. While all of these are proximate threats, wildfire and flood are probably the most severe threats to the population at Caswell MSP. A less imminent threat is natural successional processes that are reducing habitat quality. Because the park has multiuse objectives, and because up-stream impoundments have greatly modified the natural flooding regime, secondary successional patches, especially those caused by scouring floods, are disappearing from the Park. The changes are noticeable and may be associated with the decline in S. b. riparius within the park (Williams and Hamilton 2002).

For the South Delta metapopulation of S. b. riparius, wildfire is a serious threat to each subpopulation, but would unlikely affect all subpopulations simultaneously. Flooding poses a more immediate metapopulation threat, because the entire area is approximately at sea level or below, rivers are channelized, and there are few or no refugia above flood level. Over the past several decades there have been numerous breaks in levees and widespread flooding. The most recent was in 1997 when only the top of the railroad beds were not under water. Typically, the only areas not flooded are built-up areas for railroads, levee tops, multilane highways and other developed sites. Likely these areas will be inundated at some time in the future when flooding is greater than it has been in recent decades. Urbanization also is viewed as an imminent threat because most of the known populations are adjacent to areas undergoing development planning, with construction slated to begin within 1-10 years. However, this threat may be mitigated by appropriate design and management of conservation lands set aside for brush rabbits and other organisms (Williams and Hamilton 2002).

Conservation Actions [top]

Conservation Actions: There are no known conservation measures in place for the species as a whole.

The San Joaquin Valley population of subspecies Sylvilagus bachmani riparius has been listed by the state of California as an endangered species (U. S. Fish and Wildlife Service 2000), and has been the subject of a recovery plan involving habitat management and captive breeding (U.S. Fish and Wildlife Service 1998, Hansen 2006).

The following timeline includes recent conservation measures implemented for S. b. riparius:
1) Distribution and status review in 1986 (Williams and Basey 1986).
2) Listing as a California Species of Special Concern (Williams and Basey 1986).
3) Ecology and habitat management plan for Caswell Memorial State Park developed (Williams 1988).
4) Populations estimate made for Caswell MSP in 1993, the only known population at the time (Williams 1993).
5) Listed as an endangered species by the State of California in 1994 (State of California 1994).
6) Featured species in the multi-species Recovery Plan for Upland Species of the San Joaquin Valley, California (U.S. Fish and Wildlife Service 1998).
7) Habitat management plan focused on wildfire and flooding developed (Close and Williams 1998).
8) Listed as a USA endangered species by the U.S. Fish and Wildlife Service in 2002 (Williams et al. 2004).
9) Controlled Propagation and Reintroduction Plan for Riparian Brush Rabbits developed for the U.S. Fish and Wildlife Service (Williams and Hamilton 2002).
10) Habitat acquisition and restoration begun by multi-agency CalFed Program with U.S. Fish and Wildlife Service as the land owner and manager, starting in 2000 and continuing to date (Williams et al. 2004).
11) Controlled propagation facility constructed beginning in 2001 and completed in 2002 by U.S. Bureau of Reclamation (Williams et al. 2004).
12) Controlled propagation of riparian brush rabbits initiated in December 2001 with 3 males and 3 females (Williams et al. 2004).
13) Forty-nine young rabbits from controlled propagation facility released in the wild in unoccupied historical habitat between July and October 2002 (Williams et al. 2004).
14) In December 2002, 18 adult rabbits introduced into 3 controlled propagation pens (3 of each sex, each pen), producing 284 young, of which 214 had been released in the wild as of March 2004 (Williams et al. 2004).

Citation: Mexican Association for Conservation and Study of Lagomorphs (AMCELA), Romero Malpica, F.J., Rangel Cordero, H. & Williams, D.F. 2008. Sylvilagus bachmani. The IUCN Red List of Threatened Species 2008: e.T41302A10435277. . Downloaded on 17 October 2017.
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