Testing multiple range limit hypotheses in an endemic salamander (Ambystoma barbouri)

Background/Question/Methods

At the very core of ecological and evolutionary biology is understanding the distribution and abundance of the planet’s biodiversity.  Factors causing limits to species’ geographic ranges are thus a central question in biology, especially because the factors that limit the extent of many species’ ranges are more complex than simple geographic barriers. The streamside salamander (Ambystoma barbouri) has a restricted, circular distribution in the Eastern United States with no apparent barriers. We explored three major range limit hypotheses using three core-to-edge transects of Ambystoma barbouri populations genotyped at 12 microsatellite loci. We first determined the appropriate center of the species range by using an ecological niche model. Next we investigated the hypothesis that landscape features restrict ranges by identifying non-permissible habitat at the range edges using tools in the field of landscape genetics. Third, we looked for evidence for the abundant-center hypothesis by measuring genetic diversity, effective population sizes, and genetic differentiation along the transects. Finally, we tested the hypothesis that gene flow to the edge prevents local adaptation by identifying asymmetric migration using Bayesian assignment tests and coalescent approaches. 

Results/Conclusions

Predictions made by all three hypotheses were supported in this system and the center of the distribution was identified as the core based on habitat suitability. There was an increased proportion of resistant habitat at the edges of the core, although it differed based on transect. Furthermore there were trends of decreased genetic diversity, effective populations sizes, and genetic connectivity moving from core to edge in two out of the three transects. Finally we found support for asymmetric migration along all three transects. These results show that there are interactions between major range limit hypotheses and that different processes may explain range limits at different edges. We hope to soon assess the relative contribution of each hypothesis to explaining the range limit by utilizing genomic data.  Understanding multiple factors that restrict range expansion is critically needed for guiding management. With a better understanding of the mechanisms limiting species ranges, we can predict which species are most at risk of extinction, select locations for protected areas, and assess potential biodiversity impacts of various disturbances.