Common-garden experiment reveals outbreeding depression and region-of-origin effects on reproductive success in a frequently translocated tortoise

Abstract

Human-mediated animal movement can expose wildlife populations to novel environments. Phenotypic plasticity can buffer against the challenges presented by novel environments, while adaptation to local ecosystems may limit resilience in novel ecosystems. Outbreeding depression during the mixing of disparate gene pools can also reduce reproductive success after long-distance movement. Here, we use a 'common-garden' population of gopher tortoises (Gopherus polyphemus), translocated from numerous sites across the state of Florida, USA, to a mitigation site in the north-west (panhandle) region to assess whether geographic origin, outbreeding effects, and behavioral plasticity influence reproductive success in this threatened keystone species. We found that females from north-east Florida produced clutches with lower hatching success than females from other regions. We detected regional differentiation in nest site selection behavior in the common environment of the translocation site, though these differences did not mediate the regional effect on hatching success. We also found evidence for outbreeding depression: hatching success declined with increasing parental geographic and genetic distances, dropping from 93% to 67% across the range of observed parental genetic distances. Together, these results suggest that newly admixed populations may suffer reproductive costs due to historical population differentiation, and that undetected outbreeding depression could significantly hamper conservation efforts for this species and others undergoing a variety of human-mediated movements. Animals are frequently translocated to mitigate habitat loss. Studying a translocated gopher tortoise population with individuals of diverse geographic origin, we found evidence of outbreeding depression: parents originating from more geographically separated sites produced eggs less likely to hatch. We also observed that nest site selection partially depended on female region of origin when reproducing at a common site, suggesting regional differentiation and imperfect behavioral plasticity in response to environmental change. These results suggest that outbreeding depression may be more common than previously thought, and that subtle geographic differentiation may present challenges for large-scale, long-distance translocation as a mitigation strategy.image