Gaining knowledge of how ecosystems provide essential services to humans is of primary importance, especially with the current threat of climate change. Yet, little is known about how increased temperature will impact the biodiversity-ecosystem functioning (BEF) relationship. We tackled this subject theoretically and experimentally. We developed a BEF theory based on mechanistic population dynamic models, which allows the inclusion of the effect of temperature. Using experimentally established relationships between attack rate and temperature, the model predicts that temperature increase will intensify competition and consequently the BEF relationship will flatten or even become negative. We conducted a laboratory experiment with natural microbial microcosms and the results were in agreement with the model predictions. The experimental results also revealed that an increase of both temperature average and variation had a more intense effect than an increase of temperature average alone. Our results indicate that, under climate change, high diversity may not guarantee high ecosystem functioning.