Is phylogeographic congruence predicted by historical habitat stability, or ecological co-associations?

Comparative phylogeographic studies can uniquely distinguish idiosyncratic versus community-wide responses to past environmental change. However, to date, 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 saproxylic invertebrate species (i.e., a wood-feeding cockroach, termite and beetle; a predatory centipede, and a detritivorous millipede) from the southern Appalachians mountains—a topographically complex unglaciated landscape. Under one scenario, abiotic factors primarily drove species' responses, with predicted congruence based on spatial overlap of climatically stable habitat areas estimated for each species via ecological niche modeling. The other scenario considered biotic factors to be most influential, with proxies for actual or potential direct interactions used to predict congruence. Analyses of mitochondrial and nuclear DNA sequence datasets for each species focused on four axes of comparison: the number and distribution of spatial-genetic clusters, phylogeographic structure, changes in long-term effective population size, and historical gene flow dynamics. Overall, we found stronger support for 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 this scenario's predictions and outcomes of empirical data analyses. Thus, our conclusions are compelling, but tentative. This work advances comparative phylogeography by expanding the scope of inferences beyond abiotic drivers, and provides insights into the evolutionary history of a functionally important ecological community, within a globally recognized center of endemism.