Collaborative Research: Hidden Dimensions of Diversity in Woodland Salamanders: Investigating Ecophysiological Evolution in a Classic Non-Adaptive Radiation

One of the most persistent patterns in biology is that species richness is unequally distributed across the tree of life: whereas some lineages are exceptionally prolific in generating new species, others are species-poor, even over long evolutionary timespans. It is generally thought that, in order to be species-rich, a lineage must also be ecologically diverse. Yet, this is not always the case. This proposal focuses on woodland salamanders (genus Plethodon), a highly diverse amphibian lineage in the Appalachian region of eastern North America. This lineage is renowned for high species richness despite little ecological diversity. Nonetheless, the story of the Plethodon radiation has largely been studied from a single niche axis – that of structural habitat use and corresponding morphology. Just as important, but largely ignored, is thermal and hydric habitat use, and how interactions with climatic factors may generate biodiversity. As lungless salamanders, these organisms perform a delicate dance with their thermal and hydric environments to ensure cutaneous respiration. This award centers around discovering how physiological diversity is structured across the lineage, and how physiological evolution contributes to the high species diversity of Plethodon. This award is important because it will advance our understanding of how biodiversity originates. Specifically, the researchers will unpack how diversification (or lack thereof) along several ecological axes sculpts broad-scale patterns of species richness. The award is also important because it will provide detailed and updated information about salamander vulnerability to ongoing environmental change. This project will provide training opportunities for undergraduates and high school students. Theory suggests that disparity in biodiversity reflects ecological differences among lineages. At one extreme, adaptive radiation is characterized by rapid and prolific diversification into numerous ecological niches and, at the other end, lineages with low ecological diversity also tend to have few species. So-called ‘non-adaptive’ radiations occupy an enigmatic middle ground characterized by high species richness despite low ecological diversity. Perhaps most famous among these radiations are the plethodontid (lungless) salamanders. This lineage is renowned for long-term morphological stasis, suggesting that diversification has been driven by neutral processes. Using a series of phylogenetic causal models, the researchers will explore evolutionary patterns of physiological diversity, and determine their causal mechanism(s). Then, using evolutionary rate matrices, the researchers will compare rates and patterns of morphological and physiological evolution. Then, the researchers will explore the physiological correlates of species’ distributions to explore why the Appalachian region is so species rich in lungless amphibians. Lastly, using a series of new mechanistic models, the researchers will explore how salamander distributions will be altered over the next century. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.