Reintroduction Ecology of the Endangered Riparian Brush Rabbit (Sylvilagus bachmani riparius)

The riparian brush rabbit (Sylvilagus bachmani riparius ) is an endangered species found in dense, brushy habitat in the California's Central Valley. Between July 2002 and July 2005, we reintroduced 325 captive-bred individuals to unoccupied habitat within their historic range using a soft-release strategy and monitored them with radiotelemetry. Longer time in soft-release pens resulted in increased monthly survival. Rabbits were most susceptible to post-release mortality during the first 4 weeks following reintroduction and both body mass and length of time in the soft-release enclosure influenced this relationship. When we controlled for release mortality, subsequent monthly survival probabilities were most strongly influenced by release year and by a catastrophic flooding event. Male home range sizes tended to be larger than those of females, whereas both sexes had similar core area sizes. Home range and core area size estimates were larger in the breeding season than during the non-breeding season, and home range estimates decreased in size over the duration of the study. Following a wildfire in July 2004, analysis of home range data for 11 males and 11 females present in or adjacent to burn areas revealed no differences between 90-day pre- and post-fire periods, or between the sexes. Of 1,143 possible dyads, 534 (47%) exhibited a mean of 86% overlap in 95% fixed-kernel home ranges. Overlap did not differ by dyad type, but was greater in non-breeding than breeding seasons. Rabbits exhibited scale-independent habitat selection, disproportionately favoring thick understory cover such as sandbar willow mixed with dense shrubs. Viability analysis suggested that the risk of quasi-extinction was high (>98%) for scenarios that did not incorporate either continued supplementation or a combination of supplementation and alternate release strategies. Reintroduction programs should employ an adaptive management approach with ongoing monitoring of target animals and concurrent analysis to allow managers to adjust methods as conditions dictate. Efforts to augment existing escape cover and flood refugia should be continued at strategic locations throughout the Refuge. Finally, integral components of habitat and potential limiting factors must be identified to comprehensively address their influence on demographics and the cause of brush rabbit decline.