Mammal Species of the World
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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).
Sylvilagus bachmani ranges from the Columbia River in Oregon southward to the tip of Baja, California and from east of the Cascade and Sierra Nevada ranges and desert areas west throughout much of California (Wilson & Ruff 1999).
Biogeographic Regions: nearctic (Native )
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: (20,000-2,500,000 square km (about 8000-1,000,000 square miles)) Western North America, from western Oregon south to tip of Baja California, Mexico; west to the Sierra-Cascade axis. Sea level to 2070 m (Chapman 1974).
Brush rabbits are a small to medium sized cottontail. The pelage is evenly dark, consisting of steel gray, black, and orange. The ears are fairly small with a slight point. The tail is not prominent, on the top it is the same dark brown and white underneath.
Sylvilagus bachmani ranges in length from 11 inches to 14 1/2 inches. Females are generally a little bigger than males. The dental formula is 2/1, 3/2, 3/3, totalling 28. (Wilson & Ruff 1999, Verts & Carraway 1998).
Range mass: 0.48 to 0.92 kg.
Average mass: 0.71 kg.
Other Physical Features: endothermic ; bilateral symmetry
Length: 38 cm
Weight: 965 grams
Size in North America
Average: 336 mm
Range: 303-369 mm
Range: 511-917 g
Habitat and Ecology
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).
As their name implies, brush rabbits are primarily found in areas with dense brushy cover. Brush rabbits rarely leave the brush for extended periods of time.
Terrestrial Biomes: chaparral ; forest
Comments: Dense scrub and brushy edges of habitats, chaparral, and cactus. Also brushy areas on sand dunes and in bramble thickets. Usually near dense vegetative cover. Seldom uses burrows. May use forms connected by runways. Female constructs nest for young; nest measures about 75 by 150 mm, lined with fur and dried grass.
Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Brush rabbits are herbivores whose diet varies with the season. Grasses make up a large portion of their diet. However, brush rabbits feed on other species of plants, including leaves, forbs and scrubs such as wild rose and blackberries. Whenever available, green clover is preferred (Chapman 1974, Wilson & Ruff 1999).
Primary Diet: herbivore (Folivore )
Comments: In the spring, summer, and fall feeds primarily on grasses and other herbaceous vegetation. In the winter feeds on woody vegetation in addition to whatever herbaceous vegetation is available.
Predators include bobcats, coyotes, minks, weasels, skunks, and great horned owls.
Life History and Behavior
Perception Channels: tactile ; chemical
Comments: Primarily crepuscular. Most active from sunset to 0200 hours and from about 0600 to 1030 (Chapman 1974).
Lifespan, longevity, and ageing
Brush rabbits usually have three litters a year; although four litters are possible. Brush rabbits are not as fecund as other members of the genus. Breeding season for brush rabbits begins in December and lasts until May or June for rabbits in California. Brush rabbits in Oregon breed from February to August. Many breed again soon after giving birth.
The gestation period is about 27 days, with litters sizes of usually 2 to 4 young. The young are born altricial and stay in a lined and covered nest in the ground for about 14 days, opening their eyes on approximately the 10th day. The mother has 4 pairs of mammae and comes to feed her young at night. Maturity is reached at about 4 or 5 months after birth. Young brush rabbits are able to breed the following breeding season. (Chapman 1974, Wilson & Ruff 1999, Verts & Carraway 1998).
Range number of offspring: 2 to 6.
Average number of offspring: 3.5.
Average gestation period: 27 days.
Range weaning age: 14 to 21 days.
Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual
Average birth mass: 28.17 g.
Average gestation period: 27 days.
Average number of offspring: 3.6.
Average age at sexual or reproductive maturity (female)
Sex: female: 154 days.
Parental Investment: altricial
Breeding season lasts approximately 7 months (from December-May or June in California; February-August in Oregon (Chapman 1974). May produce 3 or 4 litters of 1-6 (usually 3) young/litter. Gestation lasts about 27 days.
Molecular Biology and Genetics
Statistics of barcoding coverage: Sylvilagus bachmani
Public Records: 0
Specimens with Barcodes: 5
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
- 1996Lower Risk/least concern (LR/lc)
One subspecies of riparian brush rabbits, S. bachmani riparius, are considered endangered throughout their range under the U.S. Endangered Species Act. This subspecies occurs in the San Joaquin Valley of California and is also considered endangered by the state of California. Population declines are due to many factors, such as loss of natural habitat, wildfire, and disease.
US Federal List: endangered
CITES: no special status
IUCN Red List of Threatened Species: least concern
National NatureServe Conservation Status
Rounded National Status Rank: N5 - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
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).
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).
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).
Relevance to Humans and Ecosystems
Brush rabbits are sometimes considered a pest because they can cause damage to crops and decorative vegetation. Brush rabbits have also been blamed for depredations on the seedlings planted for forest regeneration, however this is questionable (Verts & Carraway 1998).
Like many other rabbits, brush rabbits are hunted for sport and food. Some may be captured and raised for pets.
Positive Impacts: food ; body parts are source of valuable material
|This article needs additional citations for verification. (January 2014)|
The brush rabbit (Sylvilagus bachmani), or western brush rabbit, is a species of cottontail rabbit found in western coastal regions of North America, from the Columbia River in Oregon to the southern tip of the Baja California peninsula. Its range extends as far east as the eastern sides of the Sierra Nevada and Cascade mountain ranges.
The brush rabbit inhabits dense, brushy cover, most commonly in chaparral vegetation. It also occurs in oak and conifer habitats and it will live in brush or grassland, and form networks of runways through the vegetation. The brush rabbit does not dig its own burrow or den, but uses the burrow of other species, brush piles, or forms. In the San Francisco Bay Area, it was found that the brush rabbit concentrates its activities at the edge of brush and exhibits much less use of grassy areas. It uses the interior brush of the wilderness and it was also found that this may be a better environment for it than the chaparral one. Studies done on the brush rabbit in Oregon also showed that it rarely left the brushy areas it inhabits. Brush may be used more in the drier seasons while grasses are used in the wetter seasons in relation to growth of annual vegetation. Use of habitat also probably is related to the breeding season.
The brush rabbit is smaller than many of the other cottontails, and unlike most of them, the underside of its tail is grey rather than white (which may be why its common name does not include the word "cottontail"). The upperside of the brush rabbit's fur varies from light brown to gray in color, while the underside is usually always white. Adult rabbits measure anywhere from 10–14 inches long and rarely weigh over two pounds.
Large numbers of geographically defined subspecies have been proposed, including in Oregon, ubericolor; in California, cinerascens, mariposae, riparius, tehamae and trowbridgii; and in Baja California, cerrosensis, exiguus, howelli, peninsularis and rosaphagus. Subspecies bachmani, macrorhinus and virgulti are less geographically restricted. Sylvilagus bachmani riparius, the riparian brush rabbit, is highly endangered; formerly numerous along the San Joaquin River and Stanislaus River, it is now reduced to a population of a few hundred in the Caswell Memorial State Park. Of the various proposed subspecies, only the following are currently recognized; the others are synonyms: S. b. ubericolor, S. b. cinerascens, S. b. bachmani, S. b. exiguus, S. b. howelli, S. b. cerrosensis.
It has been noted that numbers of the eastern cottontail were brought west to reproduce and provide a food source for the settlers. The interbreeding of the two species has occurred where the Brush Rabbit has in parts of Oregon developed the white cottontail although retaining its smaller size.
Brush rabbit mating, as with other rabbits, may occur year-round but peak breeding seasons are between February and August. The gestation period of the brush rabbit female is about 22 days. A female brush rabbit can have as many as five litters per year but two to three is more common. One to seven young are born per litter and they are altricial. The average number born per litter is three.
A trapping study[which?] of the brush rabbit in the Berkeley Hills in northern California indicated that males had larger home ranges than females at all times of the year, and especially in May when females were moving the least. It is estimated the home ranges of the Brush Rabbit average just under 1-acre (4,000 m2) for males and just under 0.5 acres (2,000 m2) for females. The shape of these home ranges are usually circular but depending on the vegetation can be different in size and shape. Range use probably is not circular in shape or uniform, but rather consists of a series of runways that directly connect high use areas within brush habitat. Intraspecific socio-spatial behavior appears to be variable and may reflect local resource conditions. Several rabbits have been observed to feed in the same area simultaneously, but maintained inter-individual distances of one to 24 feet (7.3 m) before aggressive chases occurred. It has been shown that females tended to not overlap while males showed relatively extensive overlapping and this may indicate that females are territorial. Groups of brush rabbits may serve social purposes, such as predator detection, but this has not been proven.
Predators and survival techniques
Its predators include the cougar, the coyote, foxes, the bobcat, weasels, and various raptors and snakes. Its survival strategies include remaining immobile, when in brushy areas, and zig-zag running when found and/or in open spaces.
Human interaction and impact
The brush rabbit is not hunted as are many other cottontail species, probably because of its small size. It is not a major cause of damage to crops or other human developments in its habitat. Certain subspecies of the brush rabbit are considered endangered and are protected by state and federal laws.
|Wikimedia Commons has media related to Sylvilagus bachmani.|
- Hoffman, R. S.; Smith, A. T. (2005). "Order Lagomorpha". In Wilson, D. E.; Reeder, D. M. Mammal Species of the World (3rd ed.). Johns Hopkins University Press. p. 208. ISBN 978-0-8018-8221-0. OCLC 62265494.
- Mexican Association for Conservation and Study of Lagomorphs (AMCELA), Romero Malpica, F.J., Rangel Cordero, H. & Williams, D.F. (2008). "Sylvilagus bachmani". IUCN Red List of Threatened Species. Version 2009.2. International Union for Conservation of Nature. Retrieved 1 February 2010.
Names and Taxonomy
Comments: This is the only species of SYLVILAGUS known to have retained the putative ancestral karyotype (2n=48) shared by all known LEPUS and by ROMEROLAGUS (see Hoffmann, in Wilson and Reeder 1993).