Abstract
Global warming and the loss of biodiversity through human activities (e.g., land-use change, pollution, invasive species) are two of the most profound threats to the functional integrity of the Earth's ecosystems. These factors are, however, most frequently investigated separately, ignoring the potential for synergistic effects of biodiversity loss and environmental warming on ecosystem functioning. Here we use high-throughput experiments with microbial communities to investigate how changes in temperature affect the relationship between biodiversity and ecosystem functioning. We found that changes in temperature systematically altered the relationship between biodiversity and ecosystem functioning. As temperatures departed from ambient conditions the exponent of the diversity-functioning relationship increased, meaning that more species were required to maintain ecosystem functioning under thermal stress. This key result was driven by two processes linked to variability in the thermal tolerance curves of taxa. First, more diverse communities had a greater chance of including species with thermal traits that enabled them to maintain productivity as temperatures shifted from ambient conditions. Second, we found a pronounced increase in the contribution of complementarity to the net biodiversity effect at high and low temperatures, indicating that changes in species interactions played a critical role in mediating the impacts of temperature change on the relationship between biodiversity and ecosystem functioning. Our results highlight that if biodiversity loss occurs independently of species' thermal tolerance traits, then the additional impacts of environmental warming will result in sharp declines in ecosystem function.