Diefenbach, D.R., L.A. Hansen, R.J. Warren, M.J. Conroy, and M.G. Nelms. 2009. Restoration of bobcats to Cumberland Island, Georgia, USA: Lessons learned and evidence for the role of bobcats as keystone predators. Pages 423-435 in A. Vargas, C. Breitenmoser and U. Breitenmoser (editors). Iberian Lynx Ex situ Conservation: An Interdisciplinary Approach. Fundación Biodiversidad, Madrid, Spain.
We translocated 32 bobcats (Lynx rufus; 3.1 bobcats/10 km2) to a coastal barrier island, Cumberland Island, Georgia, USA, during 1988-1989 to restore a native predator. In this paper, we summarize our methods of capturing and handling bobcats and results of post-release monitoring of movements, survival, and reproduction. We use this as an opportunity to (1) identify key lessons we learned that could be useful for future felid reintroductions, (2) demonstrate the importance of post-reintroduction monitoring to learn more about the role of predators in ecosystem functioning, and (3) summarize insights we have gained about bobcat prey selection and social organization in a solitary felid. Also, we present previously unpublished data that provide evidence for bobcats initiating a top-down trophic cascade on the Cumberland Island forested ecosystem. We captured bobcats from the coastal plain of Georgia in the hope that these bobcats would have gene complexes adapted to the environment on Cumberland Island. Captured bobcats were temporarily held in a facility on the mainland so that groups of 4-6 individuals could be translocated and released on the island in fall 1988 and 1989. All reintroduced bobcats were vaccinated for three common feline diseases and fitted with radio-collars. Post-release monitoring included radio-telemetry to monitor movements, spatial organization, and survival, re-capture to assess body condition and replace radio-collars, prey surveys to assess food abundance, and scat collection to assess prey use. We analyzed white-tailed deer (Odocoileus virginianus) harvest data from 1980-1997 to assess changes in the deer population (body weights, age-sex structure, and abundance) because deer were a dominant prey species during our post-release monitoring of 1988-1991. Also, because browsing by deer suppressed oak (Quercus spp.) regeneration prior to the bobcat reintroduction, we investigated whether declines in deer abundance resulted in increased numbers and size of seedlings and root sprouts of these tree species. We reintroduced three males and 11 females in fall 1988 and 12 males and six females in fall 1989. One female released in 1988 returned to the mainland, another died from physical injuries, and one male released in 1989 swam into the Atlantic Ocean and presumably drowned. However, annual survival of adults was 93% (SE = 2.6%) for the first three years and recaptured bobcats exhibited an average weight gain of 0.8 kg (12% increase). We documented reproduction of 10 kittens in 1989 (4 litters), one litter of two kittens in 1990, and one litter of two kittens in 1991. Bobcat prey abundance varied spatially and seasonally, but we detected no effect of season or year on diet composition during 1989-90. Marsh rabbits (Sylvilagus palustris), white-tailed deer, and hispid cotton rats (Sigmodon hispidus) were the only prey species identified in bobcat scats in all seasons and we considered these species principal prey. Only the presence of marsh rabbits and cotton rats in bobcat diets were correlated with abundance, which suggested bobcats exhibited a functional response to these prey species. Diet species diversity and richness were negatively correlated with marsh rabbit abundance, which agrees with predictions of a diet optimization model in which alternate prey species (raccoon [Procyon lotor], feral hogs [Sus scrofa], and cotton deermice [Peromyscus gossypinus]) increased with a decrease in abundance of a preferred prey species. By 1997-1998, prey use changed, in which white-tailed deer and marsh rabbits occurred less frequently in scats and all other species occurred more frequently. Female bobcats reintroduced during the first year of the reintroduction exhibited little change in the location or size of their home range (95% fixed kernel utilization distribution), but failed to exclude newcomers reintroduced in 1989 from either their home range or core areas (50% fixed kernel utilization distribution). No bobcats retained areas of exclusive use from conspecifics of the same sex. We observed increasing intrasexual overlap among females during 1989-91, such that each home range overlap was equivalent to each female sharing her home range with >2 other females. Overlap of core areas was equivalent to each female sharing her core area with nearly one other female. Mean eviscerated body weights of harvested white-tailed deer increased 5.0–7.6 kg for males and 2.0–4.9 kg for females between the pre-reintroduction years (1984-1989) and post-reintroduction years (1990-1997). Eviscerated body weights were 11.0 kg greater in 1997 compared to 1989 (average difference of means by age-sex class). Estimates and indices of deer abundance indicated that following reintroduction of bobcats the population declined and remained low. Population estimates indicated hunter success rates declined following the bobcat reintroduction, even though the number of hunters prior to the reintroduction ( = 270 hunters/year) differed little from after the reintroduction of bobcats ( = 278 hunters/year). On nine plots containing 87 oak trees, where oak regeneration at each tree was measured in 1990, the number of trees with seedlings or root sprouts increased from 52 to 86 and the average number of seedlings per plot increased by 153.5. On plots that contained seedlings and sprouts in both 1990 and 1997, average height increased 4.6 cm (95% CI = 4.0–5.2). We were able to successfully capture and translocate approximately 15 bobcats per year, although we were not able to capture enough bobcats to control the sex ratio of the population of reintroduced bobcats; at the conclusion of the translocations in 1989 the reintroduced population consisted of 14 males and 15 females, although it was female-biased (11females, 3 males) after the translocations in 1988. This introduced potential confounding effects when interpreting results of our post-release monitoring of food habits and spatial organization. We strongly recommend that future reintroduction projects establish a means of conducting ‘soft’ releases of animals whereby animals are held in captivity at the release site and allowed to leave captivity following a waiting period. Even holding bobcats overnight might have prevented the disorientation of the released bobcat that swam into the Atlantic Ocean and presumably drowned. A dramatic population decline in marsh rabbits, caused by above-normal rainfall from a hurricane, allowed us to detect changes in bobcat diets and identify a functional response to prey abundance and evidence for diet optimization. The frequency of occurrence of deer in bobcat diets year-round (23–47%) was greater than reported for other studies in the southeastern United States (0–8%). Although we did not have sufficient data to identify the shape of these functional relationships, bobcat diets in 1997-1998 had lower occurrence of marsh rabbits and deer and a more even distribution of occurrence of all prey species in their diet. Also, there was no evidence the frequent occurrence of deer in the diet was because of a lack of food availability because bobcat survival was high, recaptured bobcats exhibited weight gains, and bobcats maintained normal home range sizes. Results of the analysis of spatial organization of bobcats were consistent with the hypothesis that bobcats maintain home ranges via a system of land tenure established by prior rights. However, we observed significant intrasexual overlap of both home ranges and core areas. Furthermore, we observed declining reproduction with an increase in home range overlap. We believe that successful reproduction in bobcats may be related to access by females to exclusive use areas even under conditions of adequate or good food availability. Under the conditions of this study (moderate bobcat density, adequate food availability, and limited dispersal) bobcats exhibited no evidence of an ability to exclude other adult individuals from their home ranges or core areas. Our observations of bobcat use of deer as a primary prey species following their reintroduction, a decline in deer abundance, and an increase in oak regeneration indicated that bobcats caused a trophic cascade effect on the island. We did not expect to observe such strong trophic level changes on the island ecosystem because deer generally are not considered primary prey for bobcats. However research prior to the restoration of bobcats indicated deer were abundant and deer browsing suppressed tree regeneration, and apparently deer were suitable prey for bobcats because of their abundance and small size. We believe the results of this study provide strong justification for post-release monitoring of a reintroduced species to be able to understand why a reintroduction project succeeds or fails. Moreover, the role of predators in ecosystems in poorly understood, especially vertebrate predators, and restoration projects of predator populations should consider monitoring trophic level characteristics of ecosystems. If such a monitoring program were developed to test theories of community population dynamics, there would be much potential to better understand food webs of terrestrial ecosystems and trophic level inter-relationships.