Is Phylogeographic Congruence Predicted by Historical Habitat Stability, or Ecological Co-associations?

Abstract

Comparative phylogeographic studies can distinguish between idiosyncratic and community-wide responses to past environmental change. However, to date, the impacts of species interactions have been largely overlooked. Here we used non-genetic data to characterize two competing scenarios about expected levels of congruence among five deadwood-associated (saproxylic) invertebrate species (i.e., a wood-feeding cockroach, termite, and beetle; a predatory centipede, and a detritivorous millipede) from the southern Appalachian Mountains - a globally recognized center of endemism. Under one scenario, abiotic factors primarily drove species' responses, with predicted congruence based on the spatial overlap of climatically stable habitat areas estimated for each species via ecological niche modeling. The second scenario considered biotic factors to be most influential, with proxies for species interactions used to predict congruence. Analyses of mitochondrial and nuclear DNA sequences focused on four axes of comparison: the number and geographic distribution of distinct spatial-genetic clusters, phylogeographic structure, changes in effective population size, and historical gene flow dynamics. Overall, we found stronger support for the ecological co-associations scenario, suggesting an important influence of biotic factors in constraining or facilitating species' responses to Pleistocene climatic cycles. However, there was an imperfect fit between predictions and outcomes of genetic data analyses. Thus, while thought-provoking, conclusions remain tentative until additional data on species interactions becomes available. Ultimately, the approaches presented here advance comparative phylogeography by expanding the scope of inferences beyond solely considering abiotic drivers, which we believe is too simplistic. This work also provides conservation-relevant insights into the evolutionary history of a functionally important ecological community.