Endangered Species Recovery Program

riparian brush rabbit
Sylvilagus bachmani riparius

Status

U.S.A. and California Endangered

Life History

The riparian brush rabbit is a small cottontail that is secretive by nature. Historically, riparian brush rabbits inhabited dense, brushy areas of Valley riparian forests, marked by extensive thickets of wild rose (Rosa spp.), blackberries (Rubus spp.), and willows (Salix spp.). Thriving mats of low-growing vines and shrubs serve as ideal living sites where they build tunnels under and through the vegetation. Suitable existing habitat for riparian brush rabbits is characterized by an abundance of woody ground litter and fewer willows, signifying areas of higher ground not subject to regular or heavy flooding. Willows are mostly found where flooding occurs regularly. Because there is no connecting habitat above flood level in the sites with potential habitat outside of the one known population at Caswell Memorial State Park (MSP), rabbits cannot successfully occupy their preferred habitat.

Riparian brush rabbits feed at the edges of shrub cover rather than in large openings. Their diet consists of herbaceous vegetation, such as grasses, sedges, clover, forbs, and buds, bark, and leaves of woody plants. One preferred plant is green clover (Trifolium wormskiolodii). They consume herbaceous plants found along trails, fire breaks, or at the edge of brushy areas, and they eat the leaves, bark, and buds of many types of woody shrubs and vines within and at the edges of thickets.

The approximate breeding season of riparian brush rabbits occurs from January to May. Although males are capable of breeding all year long, females are only receptive during this period. After 27 to 30 days, the female gives birth to a litter of 3 or 4. In favorable years, females may produce 3 or 4 litters, resulting in 9 to 16 young. The young are born in a shallow burrow or cavity lined with grasses and fur and covered by a plug of dried vegetation. With seminaked bodies and eyes closed, the young remain in the nest for about 2 weeks, after which their eyes open. It is not until 4 or 5 months after birth that they are mature. Although these rabbits have a high reproductive rate five out of six rabbits typically do not survive to the next breeding season.

For the most part, riparian brush rabbits remain hidden under protective shrub cover. They seldom venture more than 1 meter (3.3 ft) from cover and refrain from too much movement. A typical response if danger presents itself is to retreat back into the cover rather than to be pursued in open areas. They have a limited ability to climb trees and bushes, which proves helpful during floods. They are active throughout the year, and especially during twilight hours near dawn and dusk. Depending on the season, they are active from two to four hours. The least active hours are between 1030 and 1600 hours; however, they occasionally are active during this time, on overcast or cloudy days. Under certain conditions, basking in the sun takes place in the early morning and afternoon, either on a log or dry form (resting place for rabbits). Ideal basking sites are typically no more than a few inches from cover and less than about 46 cm (18.1 in) above ground, with a partial, low overstory of small trees or vines for protection from aerial predators.

Distribution

Historically, riparian brush rabbit are known to have occurred in riparian forests along the San Joaquin River and Stanislaus rivers in Stanislaus and San Joaquin counties. They probably also occupied streamside communities along the other tributaries of the San Joaquin River on the Valley floor. One population estimate was about 110,000 individuals residing in this historic range.

Today, the largest remaining fragment of habitat and only extant population are found along the Stanislaus River in Caswell Memorial State Park, San Joaquin County, California. No other sightings of riparian brush rabbits outside the Park have been reported in over 40 years. The last population estimate was 213 to 312 individuals at Caswell MSP in January 1993. Anecdotal information suggests that the current (January 1997) population size is much lower because more than 80% of the Park has been flooded since about January 1, and recent sign of rabbits on the few narrow spots above water was extremely sparse.

The dramatic decline of the riparian brush rabbit began in the 1940s with the building of dams, constructed for irrigation and flood control, on the major rivers of the Central Valley. Protection from flooding resulted in conversion of floodplains to croplands and the consequent reduction and fragmentation of remaining riparian communities. The most serious problem, however has been the lack of suitable habitat above the level of regular floods where the animals could find food and cover for protection from weather and predators.

Aside from the periodic threats from flooding, wildfire poses a major threat due to long-term fire suppression in the Park and the consequent increase in fuel from dead leaves, woody debris, and decadent, flammable shrubs. Other factors that could affect this population are diseases common to rabbits in California, such as tularemia, plague, myxomatosis, silverwater, encephalitis, listeriosis, Q-fever, and brucellosis. Competition with the more fecund and vagile desert cottontail potentially is another threat.

Classification

Order LAGOMORPHA, Family LEPORIDAE, Genus Sylvilagus, Species bachmani.

Subspecies

The riparian brush rabbit, Sylvilagus bachmani riparius, is 1 of 13 subspecies of the brush rabbit, 8 of which are found in California.

Recent Synonyms

None

Other Common Names

None

References

California Dept. of Fish and Game. 1990. Brush Rabbit, Sylvilagus bachmani. Pp. 84-85, in California's Wildlife, Vol. III, Mammals (D.C. Zeiner, W.F. Laudenslayer, Jr., K.E. Mayer, and M. White, eds.). The Resource Agency, Sacramento, CA, 407 pp.; Larsen, C.J. 1993. Report to the Fish and Game Commission: status review of the riparian brush rabbit (Sylvilagus bachmani riparius) in California. California Dept. Fish and Game, Sacramento, Nongame Bird and Mammal Sec. Rep. 93-12:1-39; Williams, D.F., and K.S. Kilburn. 1992. The conservation status of the endemic mammals of the San Joaquin faunal region, California. Pp. 329-345, in Endangered and sensitive species of the San Joaquin Valley, California (D.F. Williams, S. Byrne, and T.A. Rado, eds.). California Energy Commission, Sacramento, CA, 388 pp.

Size

Total length
300-375 mm (10.58-13.23 inches)
Weight
500-800 g (1.1-1.8 lbs)

Identification

Features that distinguish the riparian brush rabbit from the desert cottontail (S. audubonii) include size and coloration. The riparian brush rabbit is smaller and darker grayish-brown, though populations of desert cottontails living along the Valley rivers are about the same color as the riparian brush rabbit (which is lighter colored than many of the other subspecies). The tail of the brush rabbit is small and inconspicuous compared to the desert cottontail, and its ears are uniformly colored. The tail of the desert cottontail shows much white viewed from behind, and the inner (medial) tips of the ears are black. When looked at from above, the cheeks of the brush rabbit protrude whereas those of the desert cottontail are slightly concave.

Authors of Profile

T.M. Sandoval, D.F. Williams, and G.W. Colliver