Habitat fragmentation has been widely recognized as a fundamental conservation issue. Fragmentation affects the quantity and quality of the habitat, and the movements of individuals within and between habitat patches. Florida Scrub-Jay (Aphelocoma cœrulescens, FSJ) dispersal is especially likely to be strongly affected by fragmentation: FSJs are strong habitat-specialists exhibiting short dispersal distances. Here, we used a panel of 13 study sites fragmented to different degrees to explore how FSJ dispersal changes as fragmentation increases. To do so we used an indirect genetic approach: using 20-locus microsatellite data, we compared FSJ gene flow patterns within these 13 contrasted landscapes, and inferred whether dispersal is influenced by differences in landscape structure. Within each site, we characterized the isolation-by-distance (IBD) patterns by estimating the slope of the regression of inter-individual genetic distances over geographic distances; these slopes can be considered as estimates of gene flow levels. We tested the hypothesis that decreases in habitat proportion and increases in the sizes of the gaps separating habitat patches both increase FSJ dispersal.
The significant relationship between IBD slopes and landscape indices showed that FSJ gene flow, and hence effective dispersal, decrease as habitat amount decreases and habitat gap size increases. At first sight, our results contradict those of an earlier mark-recapture study which found that FSJ dispersal rate was similar in a fragmented and continuous study sites, but that dispersal distance was higher in the fragmented site. But considered in concert, the combination of these direct and indirect estimates of FSJ dispersal strongly suggests that FSJs disperse on average longer distances as fragmentation increases, but cannot perform rare long-distance dispersal movements as they do in continuous landscapes. This study highlights the utility of combining direct and indirect studies of dispersal when exploring patterns of effective dispersal across a complex landscape.