Factors associated with survival of reintroduced riparian brush rabbits in California

Abstract

The riparian brush rabbit (Sylvilagus bachmani riparius) is an endangered species found in dense, brushy habitat in the California's Central Valley. We implemented a reintroduction program to bolster populations at a Federal Wildlife Refuge and to assess factors influencing mortality and subsequent survival of released individuals. 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 their subsequent survival 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 during this acclimation period, subsequent monthly survival probabilities were most strongly influenced by release year (year 1 vs. years 2 and 3) and by a catastrophic flooding event; length of time in the soft-release enclosure remained an important variable in longer-term survival. Cause of mortality was unknown for the majority of deaths (61.9%), but predation (including presumptive predation) was the greatest known cause of death in translocated rabbits (26.4%). 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.

Short-Term Success in the Reintroduction of the Red-Humped Agouti Dasyprocta Leporina, an Important Seed Disperser, in a Brazilian Atlantic Forest Reserve

“…[44,45]). The weight at release is thus an important predictor of post-release survival [14,46]. We therefore do not recommend shortening the acclimatization period.…”

Factors Influencing Mortality Associated with the Translocation of Northern River Otters (Lontra canadensis) in Utah

“…Other species, for example, have also shown a relationship between body mass/age class at release and survival (e.g., Menu et al 2001, Hamilton et al 2010, Bodinof et al 2012, Buuveibaatar et al 2013. The combination of multiple stressors can compound the risk of mortality.…”

Incorporating Allee effects into reintroduction strategies

“…The other difference between the two strategies is that we can control the numbers released whereas attempts to reduce post-release mortality and dispersal are often ineffective. For example, while and Deredec and Courchamp (2007) suggest that Allee effects can be reduced by penning animals at release sites to prevent dispersal, this has been ineffective or detrimental in many cases (e.g., Short et al 1992;Castro et al 1994;Clarke et al 2002;Hardman and Moro 2006) although there are examples of penning reducing both post-release dispersal (e.g., Bright and Morris 1994;Tuberville et al 2005) and mortality (e.g., Bright and Morris 1994;King and Gurnell 2005;Hamilton et al 2010). Other methods for reducing post-release mortality include both predator training (Biggins et al 1999) and anti-predator training (van Heezik et al 1999;Shier and Owings 2006).…”

“…Management to reduce post-release mortality and dispersal An alternative to increasing the size or density of the release group is to undertake management designed to reduce post-release dispersal and mortality, which can be defined as the dispersal and mortality occurring during the acclimation period immediately after release (Hamilton et al 2010). Organisms are subject to stresses associated with the translocation process and adjustment to the novel environment at this stage, so it is common to have significantly elevated mortality (e.g., Tavecchia et al 2009;Hamilton et al 2010) and/or dispersal (e.g., Hardman and Moro 2006;Le Gouar et al 2008;van Heezik et al 2009) during the acclimation period.…”

“…Organisms are subject to stresses associated with the translocation process and adjustment to the novel environment at this stage, so it is common to have significantly elevated mortality (e.g., Tavecchia et al 2009;Hamilton et al 2010) and/or dispersal (e.g., Hardman and Moro 2006;Le Gouar et al 2008;van Heezik et al 2009) during the acclimation period. The initial population (N 0 ), which is often defined as the number of individuals surviving to the first breeding season, is therefore often much lower than the number of individuals released (N r ).…”