Ecophysiological analysis of species variation in geographic range size

Background/Question/Methods

Understanding the factors that limit species’ geographic distributions is fundamental in the fields of ecology and evolution. It is also central to predicting whether species ranges will shift, contract, or remain stable in response to rapid climate change. Yet, despite far-reaching implications, it is poorly understood why some species have very restricted distributions, while others are more widespread. In this study, we use eastern North America’s species-rich plethodontid salamander fauna to dissect the causes of one the most widespread and poorly understood patterns in biogeography—the tendency of species’ latitudinal extents to increase from the equator to the poles. This pattern may be caused by greater seasonal variation towards the poles driving the evolution of broader climatic tolerances (climate variability hypothesis). To test the role of seasonality in shaping distributions, we determined critical thermal maximum (CTMax) and critical thermal minimum (CTMin) for 19 species of plethodontid salamander acclimated in the laboratory.

Results/Conclusions

Significant differences were found between species for both CTMax and CTMin. In addition, thermal-tolerance breadth (measured by CTMax – CTMin) was found to be positively related to latitudinal extent, a result which supports key predictions of the climate variability hypothesis. Further, CTMin was found to be negatively correlated with latitude, whereas CTMax had no correlation. This indicates that tolerance to cold temperature is likely an important driver of northern range expansion. This study is one of only a few that sets out to compare physiological traits and latitudinal extent in a group of ecologically similar, closely related species, and offers valuable insight into the factors most critical in shaping species’ geographic ranges.