ECR Feature: Evan Whiting on species richness in snakes and lizards

Evan Whiting is a PhD candidate at the University of Minnesota. He is a herpetologist interested in understanding the diversification and biogeography of reptiles. Evan shares his recent work on latitudinal and environmental clines (or lack thereof) across and within different snake and lizard clades in continental North America.

Evan Whiting enjoying a hike in Florida, looking for lizards and other wildlife.

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Institute. University of Minnesota

Academic life stage. PhD candidate

Major research themes. Palaeontology, Herpetology, Ecomorphology, Biogeography, Systematics

Current study system. I’m currently studying squamate reptiles (lizards and snakes), one of the most diverse groups of modern vertebrate animals (>11,000 recognized species). I’ve always been fascinated by squamates and other reptiles, which have an incredible evolutionary history and wide range of different ecologies. Squamates are ectothermic (‘cold-blooded’), so their body temperature and metabolism depend on external conditions in their habitat, like heat from sunlight and shade from trees. Thus, squamates are useful barometers for the climate and environment, so understanding their past and present distributions can tell us a lot about their ecosystems, as well as how they may respond to climate change.

Green Anole (Anolis carolinensis), one of the most widespread species of anoles in continental North America.

Recent paper in JBI. Whiting, E.T. and D.L. Fox. 2020. Latitudinal and environmental patterns of species richness in lizards and snakes across continental North America. Journal of Biogeography.

Motivation behind this paper. Most modern terrestrial vertebrate animal groups, including squamates, follow a latitudinal diversity gradient, with a greater number of species at lower latitudes. Studies addressing these gradients have typically focused on explaining broader patterns and larger groups, rather than breaking them down into their constituent clades, which might not all follow the same biogeographic and environmental patterns. We wanted to compare the latitudinal gradients of different types of North American lizards and snakes on a clade-by-clade basis. We wanted to determine if they are all driven by the same environmental factors (temperature, moisture/precipitation, topography), and assess how the geographic distributions of different squamate clades relate to the major North American biomes.

Prairie Rattlesnake (Crotalus viridis), a viperid snake widely distributed throughout the Great Plains region of North America.

Key methodologies. To study the latitudinal diversity gradients of lizards and snakes, we first used GIS software to build an equal-area grid system (100 × 100 km cells) spanning all of continental North America, from Canada to Panama. Our grid includes data for 10 climatic and topographic variables, as well as species richness data compiled from the geographic ranges of over 1,000 modern squamate species. Using the data from our new grid, we then constructed latitudinal diversity gradients and multiple linear regression models for each lizard and snake clade in our study to test for potential relationships between species richness and different climatic and topographic variables. We also documented squamate diversity across all major North American biomes and investigated whether species richness is strongly associated with biome area or biome variety, which we used as a proxy for habitat heterogeneity.

Major results. We found that lizards and snakes both exhibited strong latitudinal diversity gradients overall, although the snake gradient was substantially stronger than the lizard gradient. When we broke these larger gradients down, we found that many of the individual clades in our study did not follow strong latitudinal diversity gradients. Additionally, our multiple linear regression models demonstrated that not all the individual lizard and snake clades followed the same environmental patterns as squamates overall, although species richness was strongly positively correlated with temperature variables for most clades. We also found that lizards were generally more strongly associated with habitat heterogeneity than snakes, with a few exceptions. Our statistical results and grid-based species richness maps show that different squamate clades follow different latitudinal and environmental patterns, thereby demonstrating the importance of deconstructing latitudinal diversity gradients to evaluate their underlying structures and environmental drivers.

Southern Watersnake (Nerodia fasciata), a natricine colubrid snake native to the southeastern United States.

Challenges and unexpected outcomes. This was a completely new type of research project for me, so learning how to assemble the GIS data and code/conduct all of my analyses in R was admittedly quite challenging and outside of my comfort zone (i.e., bones and fossils). I certainly learned a lot though, particularly during the very helpful and constructive revision process. One of the most unexpected outcomes we found was that our regression models were not heavily influenced by underlying spatial structure in the species richness data, indicating that climate and topography are very strong predictors of squamate species richness. Another somewhat surprising outcome we found was that most of the individual lizard and snake clades in our study did not exhibit particularly strong latitudinal gradients, despite squamates overall displaying a very strong latitudinal diversity gradient.

Next steps? Our research complements and opens the doors for additional questions, such as how quickly different squamate clades respond to environmental perturbations and the geologic antiquity of modern biogeographic patterns. We know from previous studies that many of the modern latitudinal diversity gradients we observe for groups like mammals did not exist in the deep past, but we do not know if this was also the case for ancient lizards and snakes. This necessitates further systematic work on fossils from different regions and time intervals in order to improve our understanding of squamate evolution and distributions, which translates to lots of exciting future research opportunities and discoveries!

If you could study any organism on Earth, what would it be? I would like to continue studying reptiles, as they are my favorite animals and there is still so much that we do not know about them. I’m especially excited about studying fossil reptiles, which offer a unique opportunity to travel back in time and address questions about evolution, ecology, and biogeography in the deep past.

Green Iguana (Iguana iguana), a common iguanian lizard native to the Neotropics.

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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