Body size distributions of anurans are explained by diversification rates and the environment

Aim:
Body size frequency distributions are often skewed to the right, with a greater frequency of small‐sized species. Right skewness can appear when speciation is biased towards small species and extinction towards large ones. In contrast, limits imposed by environmental constraints will select taxa to co‐occur in assemblages and can modify size distributions to the left or to the right. We analysed whether the shape of size distributions of anurans is related to diversification rates and how the environment might also be creating the observed patterns.
Location:
The Western Hemisphere.
Time period:
Current.
Major taxa studied:
Anurans.
Methods:
We computed diversification rates from a dated phylogeny and obtained data for body size, spatial distribution and the climate (annual temperature, potential evapotranspiration and normalized difference vegetation index) inhabited by 2,598 anuran species to analyse the relationship between diversification rates and body size. We then used an assemblage‐based approach to test whether the climate is acting together with species evolutionary history to generate skewed distributions of body sizes at local sites.
Results:
Skewness was positively correlated with size‐biased speciation and extinction, whereas it was negatively correlated with net diversification. After the inclusion of environmental predictors, the explanatory power of our models increased significantly. When ranking the predictors, evolutionary rates remained first, followed by a secondary role of environmental variables.
Main conclusions

Despite the high frequency of right‐skewed body size distributions, anuran assemblages of the New World showed a variation of skewness that can be explained by faster net diversification (speciation minus extinction) at modal sizes. We conclude that size‐biased evolutionary rates and the environment both have a role in defining the proportion of small and large sizes at local scales.