Demographic expansion and panmixia in a St. Martin endemic, Anolis pogus, coincides with the decline of a competitor.

Understanding patterns of differentiation at microgeographic scales can enhance our understanding of evolutionary dynamics and lead to the development of effective conservation strategies. In particular, high levels of landscape heterogeneity can strongly influence species abundances, genetic structure, and demographic trends. The bearded anole, Anolis pogus, is endemic to the topographically complex island of St. Martin and of conservation concern. Here, we examined genetic diversity and inbreeding, assessed which features of the landscape influence population abundances, tested for population genetic structure across St. Martin, and inferred historical demographic trends. We found minimal inbreeding or low genetic diversity in A. pogus. We found that suitable habitat occurs broadly across the island and that population abundances were largely predicted by canopy cover. However, there was no signature of population genetic structure across the distribution, in contrast to the co-distributed anole species (Anolis gingivinus). Historical demographic trends in A. pogus were in sharp contrast to A. gingivinus, with effective population sizes of A. pogus increasing in the recent past, whereas A. gingivinus population sizes have declined. We posit that declines in a competitor species allowed for population size expansion in A. pogus. Overall, these analyses suggest that A. pogus is unlikely to be of immediate conservation concern. Further, we highlight the role of demographic history and ecological interactions in shaping population structure.