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Stanislaus River Report
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Stanislaus River Basin and Calaveras River Water Use Program|
Threatened and Endangered Species Report - March 1995
Bay Delta and Special Water Projects Division, CA Dept of Fish and Game
Riparian Brush RabbitSylvilagus bachmani riparius
Federal Category 1 candidate for listing by the U.S. Fish and Wildlife Service as Threatened or Endangered, and listed as Endangered by the State of California
Life HistoryThis subspecies of the brush rabbit is a small brownish cottontail-like rabbit with a white belly, relatively short ears, and a small inconspicuous tail. The hind legs are short and hind feet are slender and not covered with long or dense hair. The white belly and ventral tail hairs are gray near the skin and the ears lack dark tips (Orr 1940, Ingles 1965, Chapman 1974). Adult riparian brush rabbits are about 13 inches long and can be distinguished from other subspecies by their relatively pale color, gray sides, darker back (Orr 1935), restricted range and habitat requirements, and skull characteristics. When looking down at the head from above, the riparian brush rabbit cheeks protrude outward rather than being straight or curving inward as in other subspecies (Orr 1935, 1940).
Adult riparian brush rabbits forage on herbaceous vegetation in general, including grasses, sedges, clover, forbs, shoots, and leaves. Vegetation is eaten in available areas within or very close to brushy cover, along trails, and fire breaks. They seldom venture more than several yards from brushy cover, and do not forage in large open areas (Larson 1993). Foraging activity occurs during the early morning and early evening hours.
Breeding occurs from December to April and young riparian brush rabbits are born between January and May. The gestation period for brush rabbits is 27 days and the females may produce several litters during one season. Females average nine to 16 offspring per year which remain in the nest for about 24 days. Their eyes open at ten days but they remain in the nest for another two weeks. The nest is a shallow burrow lined with grasses and fur and covered by a plug of residual vegetation (Larson 1993).
Brushy clumps smaller than 400 square yards are rarely occupied. Because they inhabit such dense riparian habitat, they construct a maze of tunnels and runways through the vegetation. Visual stimuli is thought to be used for orientation, therefore any animals displaced more than 350 yards from their home range may never return to their original location (Basey 1990).
Factors contributing to the mortality of the riparian brush rabbit are predators such as owls, hawks, foxes, snakes, and even feral dogs and cats. They are also susceptible to diseases and parasites that typically affect all rabbit species, many of which are contagious and fatal. Because the population is so restricted in terms of geography and size, the genetic viability is thought to be very low. Chance environmental events resulting in flooding or wildfire may also cause direct mortality. Five out of six rabbits of the genus Sylvilagus are not expected to survive from one year to the next which makes it especially hard for such a small population like the riparian brush rabbit to recover.
Findings and ConclusionsThe vast habitat destruction, fragmentation, and degradation of the San Joaquin Valley native riparian forest habitat within their historic range has been the major cause of decline for the riparian brush rabbit (Williams and Basey 1986, Basey 1990). The continued growth of the valley which once contained about 91,426 acres has been reduced to approximately six percent of historic levels. The historic range of the riparian brush rabbit encompassed 89,665 acres. Caswell Memorial State Park is the largest remaining area of suitable habitat (261 acres) within the historic range and houses the only known remaining population of riparian brush rabbits.
Because this species inhabits such dense riparian habitat, any disturbance has an adverse effect on the population. Along the Stanislaus River at Caswell Memorial State Park are adjacent areas of suitable habitat for the riparian brush rabbit but, because of human impacts such as thinning and clearing for recreation use, the brush rabbits do not inhabit these areas.
Any project involving the Stanislaus River needs to consider potential negative impacts to the riparian forest occurring at Caswell Memorial State Park and consequently to the riparian brush rabbit.