The relationship between inter- and intraspecific variation in critical thermal limits and the distribution of ant biodiversity, now and (maybe) in the future

Background/Question/Methods

Understanding the factors that limit the distribution of species and patterns of biodiversity is at the core of ecological and biogeographical research. The complex relationship between environmental conditions and biogeographical distributions often relies on large-scale climatic information and macroecological data from museum records or field guides. Few studies, however, incorporate actual physiological mechanisms and measurements of the studied organisms to understand patterns of diversity and predict their distribution in a changing world. In this study, we examined trait-based physiological responses to varying thermal regimes to better predict the effects of a changing climate on ant distributions and community structure.  Through controlled laboratory experiments coupled with field-collected data on the distributions of ants, we investigated critical thermal minima and maxima of ant species along both elevational and latitudinal gradients. Ants were collected at 30 sites in Great Smoky Mountains National Park (400m-1800m) as well as 25 sites from Florida to Maine, spanning approximately 15 degrees latitude.     

Results/Conclusions

Among-species variation in critical thermal limits was correlated with both the elevational and latitudinal ranges of ant species. Interestingly, we documented considerable intraspecific variation in thermal tolerance in several ant species, including the most common forest ant species in eastern deciduous forests (Aphaenogaster rudis). Generally speaking, populations from high elevations exhibit more variation in thermal tolerance and lower critical thermal minima than populations from low elevations.  Additionally, critical thermal maximum is invariant among populations, but critical thermal minimum declines significantly with elevation and perhaps latitude. Taken together, these results suggest that macroecological or niche models that ignore intraspecific variation in key physiological or functional traits do so at their peril.  But, it might be possible to disentangle the underlying factors that constrain the distributions of ant species, which may be critical in predicting the response of biodiversity to ongoing climatic changes.