Brush rabbit Sylvilagus backmanii cinerascens Class: Mammalia Order: Lagomorpha Family: Leporidae Dudek and Associates Species Accounts Status: Federal: None State: None Data Characterization The MSHCP data base includes a total of 166 records for the brush rabbit. Of the 166 records, 151 (91%) are precision 1 (i.e., an "x" and "y" coordinate that allows for a relatively precise location), nine (5%) are precision 2 (one "x" or "y" coordinate or equivalent that allows a reasonably precise location) and the remaining six (4%) are precision codes 3 and 4 that do not allow for a precise location of the record. Most of the records are from the last 20 years; 114 (69%) since 1990 and 47 (28%) from the 1980s. The primary concern about the data is the accuracy of field identification between the brush rabbit and the co-occurring desert cottontail (Sylvilagus audubonii), which occurs in more xeric conditions, is somewhat larger (0.6-1.2 kg versus 0.6-0.8 kg), lighter in color and has longer ears (76-102 mm in length versus 51-66 mm). The desert cottontail is more common in arid valleys, but there may be a tendency to identify the two species in the field by habitat association, attributing brush rabbits to areas with heavier brush. Habitat and Habitat Associations Brush rabbits inhabit dense, brushy cover, most commonly in chaparral vegetation (Chapman 1974). They also occur in early successional stages of oak and conifer habitats (Zeiner et al. 1990). Brush rabbits do not dig their own dens, but use the burrows of other species, brush piles, or "forms." In the San Francisco Bay area, Connell (1954) found that brush rabbits concentrate their activities at the edge of brush and exhibit much less use of grass areas. Use of interior brush also was used irregularly and Connell suggests that the brush-herb ecotone is better habitat than continuous chaparral. Chapman (1971) also found that brush rabbits at a study site near Corvallis, Oregon rarely left brushy cover. Brush may be used more in the drier seasons while grasses are used in the wetter seasons in relation to growth of annual herbaceous vegetation. Use of habitat also probably is related to the breeding season. Within the MSHCP planning area, 70 (42%) of the occurrence are in chaparral (chaparral, red shank chaparral), 23 (14%) in coastal sage scrub (Diegan coastal sage scrub, Riversidean sage scrub), 27 (16%) in non-native grassland (possible confusion with the desert cottontail?), six (15%) in oak woodland (coast live oak woodland and Engelmann oak woodland), one (<1%) in southern cottonwood-willow riparian, one (<1%) in freshwater marsh, two (1%) in yellow pine forest, 17 (10%) in agriculture (dairy & livestock, grove/orchards, field croplands), and 19 (11%) in urban/residential/exotic landscapes. Biogeography The brush rabbit, S. bachmani, is a Pacific coastal species that occurs west of the Cascades and Sierra Nevadas from southern Oregon to Baja California, Mexico. It is generally absent from the dry Central Valley, except for a small population of S. b. riparius known only from the west side of the San Joaquin River in Stanislaus County. Marginal records for the subspecies S. b. cinerascens include San Emigdio Canyon; Reche Canyon; Dos Palmas Springs; Santa Rosa Mountains; and Baja California (Hall 1981). They occur from sea level to at least 2,070 meters (Chapman 1974). Range The brush rabbit occurs in appropriate habitat throughout western Riverside County. Populations appear to be centered around Sage, the Santa Rosa Plateau, and the foothills of the San Jacinto Mountains. Additional localities include Domenigoni Valley, Harford Springs area, Vail Lake area, Potrero Valley, Lake Mathews, Lakeview Mountains, the Badlands, Reche Canyon, Banning/Beaumont, Calimesa, and Garner Valley. Key Populations in Planning Area Throughout the planning area in appropriate habitat. Biology Genetics: The brush rabbit has a diploid chromosome number of 48 (Chapman 1974). No other relevant genetic studies were found. Diet: Brush rabbits are herbivorous and graze on a wide variety of grasses and annual forbs, including clovers (Trifolium spp.), foxtail barley (Hordeum murinum), bromes (Bromus spp.), wild oats (Avena spp.), and thistles (Sonchus asper, Circium lanceolatum) (Chapman 1974; Zeiner et al. 1990). They prefer the newly grown tips of plants (Chapman 1974). Brush rabbits also feed on species such as creeping eragrostis (Eragrostis hypnoides), spikerush (Eleocharis palustris), wild rose (Rosa californica), Mexican tea (Chenopodium ambrosoides), marsh baccharis (Baccharis douglasii), rush (Juncus spp.), the roots of poison hemlock (Conium maculatum), and the stems and leaves of California blackberry (Rubus ursinus) (Chapman 1974). Their diet and foraging behavior vary in relation to season; they forage on annual, herbaceous vegetation during the wetter season and perennial brush species during the drier season. Daily Activity: Brush rabbits mostly are crepuscular, with activity greatest around dusk and dawn. They are less active at night and occasionally active during the day (Zeiner et al. 1990). They often stay just inside brushy cover and then venture into open grassy areas to feed (Connell 1954; Chapman 1974). They spend a considerable time sunning, usually following rain or fog (Chapman 1974). Connell (1954) found that males made their greatest daily moves February to March, while females' longest moves were December to February. Average daily moves were 81±15 feet by males and 61±10 feet by females. Survival: Annual survival rates in brush rabbits are relatively low, but appear to be age-related. Based on live-trapping, Connell (1954) estimated an annual survival rate of males and females combined of only 15% over 12 months. He concluded that mortality or emmigration (these two causes of disappearance could not be separated in the field study) of young-of-the-year accounted much for much of the "mortality" in the population, with males disappearing at a higher rate in the first six months after first capture (86%) compared to females (67%). In contrast, only two of seven (29%) of adults trapped in April and May were not retrapped in September. The longest-lived brush rabbits in the wild in the Connell study were more than two years old, and individuals may survive in captivity for more than three years. Brush rabbits are preyed on by a variety of species, including bobcat (Felis rufus), coyote (Canis latrans), gray fox (Urocyon cinereoargenteus), long-tailed weasel (Mustela frenata), raptors, and some snakes such as rattlesnakes (Crotalus spp.), and gopher snake (Pituophis melanoleucus). They avoid predation by remaining close to brush and brambles and remain motionless for a period of time when in the open (Chapman 1974). Ectoparasites also may contribute to mortality. For example, Connell (1954) found bot fly larvae (Cuterebra sp.) on a female that appeared lethargic when carrying the larvae and he reported from another study by Mossman the death of a male that had much neck bleeding where two bot fly larvae were lodged. Socio-Spatial Behavior: A trapping study of brush rabbits 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 (Connell 1954). Based on the maximum diameter of movement, Connell estimated circular average home ranges of 0.95 acre for males and 0.34 acre for females. In the Oregon study by Chapman (1971), brush rabbits had relatively small home ranges. Circular average home ranges based on home range diameters were as follows: 0.27 acre for adult males; 0.41 acre for juvenile males; 0.21 acre for adult females; and 0.17 acre for juvenile females. The size and shape of home ranges of brush rabbits in Oregon reflected the size and shape of bramble clumps. The minimum size of permanently occupied clumps was about 460 sq meters (0.1 acre) (Chapman 1974). Smaller clumps may be occupied, but only if in proximity to larger clumps. Although the home range estimates above are based on circular ranges calculated from range diameters, 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. These runways may be used by other small mammals such as voles (Microtus spp.) and western harvest mouse (Reithrodontomys megalotis). Intraspecific socio-spatial behavior appears to be variable and may reflect local resource conditions. Chapman (1974), for example, cites a study by Zoloth (1969) of brush rabbit behavior on Año Nuevo Island. Several rabbits were observed to feed in the same area simultaneously, but maintained inter-individual distances of one to 24 feet before aggressive chases occurred. On the other hand, Connell (1954) reported that females tended to not overlap while males showed relatively extensive overlap. Connell characterized females as "semiterritorial" but did not observe whether they defended territories. It is not known whether aggregations of brush rabbits serve some kind of social function (e.g., mutual predator detection or social communication) or whether they simply occur as a result of patchy resource distribution. Connell (1954) estimated population sizes of 0.9 to 2.3 rabbits per acre, but population densities can be expected to vary widely depending on local resource conditions, habitat patchiness, natural fluctuations in population cycles, etc. Reproduction: The peak breeding season for brush rabbits in California is from December to May and possibly into June (Chapman 1974). However, based on an analysis of reproductive organs from rabbits taken in San Francisco Bay area, Mossman (1955) determined that males are capable of breeding from possibly October through July and most likely from November through June. He concluded that males probably are not fertile from July through October, the driest part of the year. Females exhibit pregnancy from the first week of December until about the end of June (Mossman 1955). In Oregon, the breeding season is the same length, but runs from about February to August (Chapman and Harman 1972). Gestation is 27±3 days. Three and possibly four litters of 2-5 offspring are produced annually (Chapman and Harman 1972; Mossman 1955) and an annual production of 15.2 young per female was calculated by Chapman and Harman (1972). Chapman and Harman conclude that brush rabbits are not as fecund as some other cottontail species that may produce up to 35 offspring per year. Litter sizes are smaller in Oregon than California and there appears to be a negative correlation between litter size and latitude and a positive correlation between breeding and the rainy season (Chapman and Harman 1972). In Oregon brush rabbits, it was determined that 16.3% of ova failed to implant and 15.5% of embryos were resorbed (Chapman and Harman 1972). Based on intervals between litters, females exhibit a postpartum estrus and can become pregnant shortly after giving birth (Chapman and Harman 1972). Brush rabbits prepare a nest cavity approximately 75 by 150 mm lined with fur and small amounts of grass (Chapman 1974). The young are only fed at night and spend about two weeks in the nest. They mature in about four to five months, but females at least probably do not breed in their natal season (Chapman and Harman 1972; Mossman 1955). Dispersal: Brush rabbits appear to be sedentary, but very little specific dispersal data were found for this species. Based on radiotelemetry data for brush rabbits near Corvallis, Oregon, Chapman (1971) concluded that dispersal movements were relatively small. Young brush rabbits were repeatedly trapped at the same trap sites and radio data indicated that they never left the bramble clump in which they were first trapped. Radio-tracking of adult rabbits also indicated that they rarely left the clumps in which they were trapped. In addition, the distances over which brush rabbits were able to successfully home were shorter than other species of Sylvilagus and many other mammals. Threats Disease: Known ecto- and endoparasites of brush rabbits are Hoplopsyllus powersii and Hoplopsyllus minutus, tapeworms (Moscouyia pectinata-americana, Taenia pisiformis), and pinworms (Nematoda: Passalurus ambiguous) (Chapman 1974). Connell (1954) also reports brush rabbits carrying bot fly larvae, which appear to affect their health and in at least one case may have directly caused the death of a male rabbit. Habitat Loss and Fragmentation: Little data are available on population status and trends of the brush rabbit in western Riverside County, but it can be assumed that the rabbit is vulnerable to urbanization, hunting, and loss of large, contiguous habitat patches. Special Biological Considerations Little is known of the effects of habitat loss and fragmentation on the viability of brush rabbit populations. A computer simulation study of the New England cottontail (Sylvilagus transitionalis) metapopulations in response to habitat loss and environmental correlations (based on increased vulnerability to predation) showed a rapid decline or extinction of populations (Litvaitus and Villafuerte 1996). This study demonstrated the importance of a habitat management program that maintains habitat connections and habitat suitability (e.g., maintaining early successional habitat). Such factors should be considered in the designation and management of habitat for the brush rabbit in western Riverside County. However, more information on minimum habitat requirements, annual survival, population growth, and dispersal patterns in this species is crucial for examining these effects. A radiotelemetry study of orientation and homing by brush rabbits in Oregon by Chapman (1971) provides some useful behavioral information that should be considered for design and evaluation of habitat linkages. Chapman observed that rabbits largely restricted their homing routes to brushy cover regardless of the direction or distance. When crossing between clumps, rabbits invariably chose the shortest distance between clumps. Rabbits' homing movements were impeded by human activity and vehicles and they were reluctant to cross roads. Chapman also demonstrated that brush rabbits show little natural dispersal and tend to stay in the clumps in which they were first trapped regardless of age. These findings have important implications for reserve design. Brush rabbits probably will require continuous suitable habitat because they appear unlikely to move long distances through unsuitable habitat. Small, isolated patches of habitat probably are unlikely to support viable populations of brush rabbits. Literature Cited Chapman, J.A. 1971. Orientation and homing of the brush rabbit (Sylvilagus bachmani). Journal of Mammalogy 52:686-699. Chapman, J.A. and A.L. Harman. 1972. The breeding biology of a brush rabbit population. Journal of Wildlife Management 36:816-823. Chapman, J.A. 1974. Sylvilagus bachmani. In: Mammalian Species No. 34:1-4. Published by the American Society of Mammalogists. Connell, J.H. 1954. Home range and mobility of brush rabbits in California chaparral. Journal of Mammalogy 35:392-405. Hall, E.R. 1981. The Mammals of North America. John Wiley and Sons, New York. 2 Vol. 1181 pp. Litvaitis, J.A. and R. Villafuerte. 1996. Factors affecting the persistence of New England cottontail metapopulations: the role of habitat management. Wildlife Society Bulletin 24:686-693. Mossman, A.S. 1955. Reproduction of the brush rabbit in California. Journal of Wildlife Management 19:177-184. Zeiner, D.C., W.F. Laudenslayer, Jr., K.E. Mayer, and M. White. 1990. California Wildlife, Volume III, Mammals. California Statewide Wildlife Habitat Relationships System. Department of Fish and Game, Sacramento, California.