Species thermal tolerances and macro-scale geographic distributions

Background/Question/Methods

Many ecological hypotheses aiming to explain macro-ecological patterns such as richness–climate relationships or variations in species’ range sizes rely on the assumption that individual species’ geographic ranges are strongly limited by physiological tolerances. In particular, temperature is generally thought to strongly limit species’ geographic ranges, leading to assemblage-level patterns such as richness–temperature relationships (e.g. through tropical niche conservatism). Here, I test the assumption that species’ thermal tolerances can predict their geographic ranges using published data on birds, mammals, reptiles, amphibians, insects and arachnids. Then, I test to what extent species’ thermal tolerances can predict macro-scale richness–temperature relationships and inter-specific variations in range size.

Results/Conclusions

Lower and upper physiological thermal limits are very poor predictors of the minimum and maximum temperatures occupied within a species’ range. Similarly, inter-specific variations in the breadth of thermal tolerances poorly predict variations in range size. And, although species’ thermal tolerances do predict a positive richness–temperature relationship, they cannot accurately predict empirical richness–temperature relationships. In fact, the relationship predicted by species’ thermal tolerances is generally much steeper than what we observe empirically. Thus, the proportion of the species’ pool (defined by climatic tolerances) that occurs in a fixed-size area is generally large in cold areas, but is increasingly small in warmer areas.