ECR Feature: Tom Radomski on the Rapoport Effect in North American salamanders

Tom Radomski is a PhD candidate at the University of Minnesota. He is a biogeographer with an interest in the range size and limits of salamanders. Tom shares his recent work on the “Rapoport Effect” in North American salamanders.

Tom Radomski

Personal links. Twitter.

Institute. University of Minnesota.

Academic life stage. PhD candidate.

Major research themes. Geographic Range Size, Geographic Range Limits, Salamanders.

Current study system. I work on salamanders in eastern North America. I don’t have a deep reason for why – as Marge Simpson once said, “I just think they’re neat.” That said, two things that make many salamanders interesting for biogeography is that they have weird ecophysiological properties and they are very dispersal limited. For the former, they exclusively breathe through their skin while simultaneously maintaining water balance through the skin, which creates a complicated problem with interesting solutions. For the latter, dispersal-limitation leads to interesting patterns where suitable habitat is inaccessible, even when it’s relatively close in geographic space.

Recent paper in JBI. Radomski, T., Kuchta, S. R., & Kozak, K. H. (2022). Post‐Pleistocene dispersal explains the Rapoport effect in North American salamanders. Journal of Biogeography49(6), 1048-1060.

A baby South Mountains Gray-cheeked Salamander (Plethodon meridianus).

Motivation behind this work. During my master’s work with Shawn Kuchta at Ohio University, we found that two northern mitochondrial clades within the Eastern Red-backed Salamander (Plethodon cinereus) had huge geographic distributions, while further south, mitochondrial clades within the same species had much smaller distributions. We interpreted this as recent post-glacial range expansion in the two northern clades. When I looked up other published phylogeographic studies on salamanders of North America, the same thing appeared. This was reminiscent of a pattern called a ‘Rapoport effect’, in which species further from the equator tend to have larger geographic distributions. It seemed intuitive that if post-glacial range expansion could result in intraspecific mitochondrial clades with large geographic distributions, then maybe it could explain the same pattern across entire species. I wanted to evaluate this expectation by examining the ranges of 169 temperate North American salamander species.

Key methodologies. Our test of the hypothesis that post-glacial range expansion contributes to variation in species’ range size was done in two parts.

First, we used range maps to demonstrate that there is a Rapoport effect in North American salamanders. In other words, we asked if species’ range sizes get bigger the further North they are. Then we did a few unusual things. We removed parts of species’ ranges that are in post-glacial areas and found that this weakened the Rapoport effect (i.e., weaker correlation between range size and latitude). This supported our hypothesis that post-glacial colonization was important for forming a Rapoport effect in North American salamanders.

Second, we built ecological niche models for species that live far south of glacial areas and projected the models to northern post-glacial areas. Ecological niche models take data about the locations in which a species occurs and the environment at those locations, and then produces an estimate of where suitable habitat for the species is located. Basically, we asked if species with small distributions could be distributed more broadly than we observe in nature. This fit our hypothesis that species’ range sizes are constrained by the ability of species to access suitable habitat, which in our case was the availability of post-glacial areas.

Photos from the field. (Left) “While checking temperature sensors in South Mountains State Park, North Carolina, some low clouds rolled into forest”. (Right) “While I was getting data from a temperature sensor, I found a Dusky salamander taking refuge. I like to think it was my field assistant that day.” – Tom Radomski.

Major results. There are many studies on Rapoport effects that (1) test for the correlation of range size and latitude or (2) try to understand range size as a function of climate. However, our angle was different in that we focused on postglacial range expansion. A few previous researchers have found evidence for post-glacial range expansion being an important contributor to Rapoport effects. However, that body of research was smaller than I had expected. Our work therefore highlighted the potential importance of post-glacial range expansion and available habitat in determining current range sizes.

Unexpected outcomes. We found that post-glacial colonization was a much bigger factor in shaping range size for salamanders in eastern North America than in western North America. We think this has to do with the more complex topography of western North America and the steep aridity gradients as one goes further inland.

Next steps? There is a great review in which the authors claimed that research on Rapoport effects was over (Gaston et al. 1998). Despite that review, work on Rapoport effects has increased tremendously over time. I’d like to synthesize some ideas about Rapoport effects – particularly what they do and do not mean. Rapoport effects are typically ascribed to be the result of climatic tolerance (i.e., poleward species are climatic generalists so they should have large ranges). But there are a lot of other reasons they could appear. I’ve argued post-glacial range dispersal, but also the relative amount of different habitat types is probably important, as well as legacies of speciation and extinction.

If you could study any organism on Earth, what would it be? That’s an easy one: salamanders.

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