Variation in thermal physiology can drive the temperature-dependence of microbial community richness

Predicting how species diversity changes along environmental gradients is an enduring problem in ecology. Current theories cannot explain the observation that microbial taxonomic richness can show positive, unimodal, as well as negative diversity-temperature gradients. Here we derive a general empirically-grounded theory that can explain this phenomenon by linking microbial species richness in local communities to variation in their temperature-driven competitive interaction and growth rates. It predicts that richness depends on variation in shape of the thermal performance curves of these metabolic traits across species in the community. Specifically, the shape of the microbial community temperature-richness relationship depends on how the strength of competition across the community and the degree of variation in growth rates changes across temperature. These in turn can be predicted from the variation in thermal performance across the community. We show that empirical variation in the thermal performance curves of metabolic traits across extant bacterial taxa is indeed sufficient to generate the variety of community-level temperature-richness responses observed in the real world. Our results provide a new mechanism that can help explain temperature-diversity gradients in microbial communities, and provide a quantitative framework for interlinking variation in the thermal physiology of microbial species to their community-level diversity.