The intuitive and oft-cited center-periphery hypothesis (CPH) predicts that genetic diversity and demographic rates will decline when observing populations from the core to the edge of a species distribution. Although the idea of geographically marginal populations being less fit than core populations is deeply entrenched and often assumed to be true in the minds of many ecologists, a thorough analysis has been lacking to actually test the major tenets of CPH. Pironon et al. (2017) sought to remedy this by reviewing 200+ empirical studies on CPH. Remarkably, Pironon et al. found only limited support for CPH. Approximately half of the reviewed studies demonstrated a decrease in genetic variation in core vs. edge populations. Even more interesting, only 20% - 30% of studies found populations to exhibit decreasing demographic rates from core vs. edge populations. Therefore, a majority of the reviewed cases did not support the two primary postulates of CPH. The key takeaway from this review is that geographic marginality is often not the same as ecological marginality. A refinement of CPH is therefore in order. Pironon et al. suggest the integration of ecological and geographic factors as a way forward beyond the traditional CPH. For example, a species distribution during glaciation will likely have a different distribution of geographic and ecological cores and peripheries compared to its post-glacial distributions. Therefore, a population that was previously a geographic and ecological core may now be geographically marginal, but still an ecological core. The interactions between historical and present local ecological factors may provide a better explanation for the variation observed in population genetics and demographic rates as opposed to one factor or the other. The findings of Pironon et al. (2017) underline the necessity of thinking at smaller ecological scales and larger temporal scales if there is to be a more complete understanding of the mechanisms which lead to the emergence of geographic species distributions.
Citation:
Pironon S., Papuga G., Villellas J., Angert A.L., Garcia M.B., and Thompson J.D. (2017). Geographic variation in genetic and demographic performance: new insights from an old biogeographical paradigm. Biological Reviews. 92: 1877 - 1909.