Time and environment explain the current richness distribution of non-marine turtles worldwide

Abstract

Ecological, historical, and evolutionary hypotheses are important to explain geographical diversity gradients in many clades, but few studies have combined them into a single analysis allowing a comparison of their relative importance. This study aimed to evaluate the relative importance of ecological, historical, and evolutionary hypotheses in explaining the current global distribution of non-marine turtles, a group whose distribution patterns are still poorly explored. We used data from distribution range maps of 336 species of non-marine turtles, environmental layers, and phylogeny to obtain richness estimates of these animals in 2° × 2° cells and predictors related to ecological, evolutionary and historical hypotheses driving richness patterns. Then we used a path analysis to evaluate direct and indirect effects of the predictors on turtle richness. Ancestral area reconstruction was also performed in order to evaluate the influence of time-for-speciation in the current diversity of the group. We found that environmental variables had the highest direct effects on non-marine turtle richness, whereas diversification rates and area available in the last 55 million yr minimally influenced turtle distributions. We found evidence for the time-for-speciation effect, since regions colonized early were generally richer than recently colonized regions. In addition, regions with a high number of colonization events had a higher number of turtle species. Our results suggested that ecological processes may influence non-marine turtle richness independent of diversification rates, but they are probably related to dispersal abilities. However, colonization time was also an important component that must be taken into account. Finally, our study provided additional support for the importance of ecological (climate and productivity) and historical (time-for-speciation and dispersal) processes in shaping current biodiversity patterns.