Smaller effective population size near habitat edge in a specialist herbivore

Background/Question/Methods

Habitat fragmentation is a major threat to species’ survival across terrestrial, aquatic, and marine habitats. Species’ responses to fragmentation may be greatly influenced by their responses to habitat edges, the boundary between two habitat types or landscape features.  While studies have focused on the effects of habitat patch area and connectivity, the effect of habitat edges on a population’s genetic variation has not been well studied.  The maintenance of genetic variation is critical to population persistence and the survival of threatened or endangered species because it reduces the likelihood of inbreeding depression and enables adaptation to changing environments through selection on pre-existing genotypes.  A population’s ability to maintain genetic variation is often assessed via estimates of effective population size (N_e), which are based on genetic drift in a Wright-Fisher model. Habitat edges may alter genetic variation and N_e through numerous processes, such as genetic drift, selection, or gene flow, and these effects may be particularly pronounced in specialist species. Tumidagena minuta is a specialist herbivore that lives isolated within patches of the marsh hay, Spartina patens.  In 2011, T. minuta were collected from both interior and edge locations within S. patens patches in a mid-Atlantic salt marsh.  We are genotyping individuals using nine microsatellite markers and then estimating N_efor each location via linkage disequilibrium. 

Results/Conclusions

Within two patches, the interior samples produced N_e estimates of 895.0 and infinite, and the edge samples produced estimates of 108.6 and 351.8, respectively.  This reduction in N_e near the habitat edge indicates that genetic drift may act more strongly in edge habitats relative to interior habitat.  For populations experiencing a decline due to habitat fragmentation and increased edge to interior habitat ratios, this may lead to a further reduction in genetic variation as alleles are lost through drift.  This suggests that edges may be an important consideration when assessing the long-term impact of habitat fragmentation on populations' persistence and designing effective management strategies.