Résumé
Range shifts of many species are now documented as a response to global warming. But whether these observed changes are occurring fast enough remains uncertain and hardly quantifiable. Here, we developed a simple framework to measure change in community composition in response to climate warming. This framework is based on a community temperature index (CTI) that directly reflects, for a given species assemblage, the balance between low- and high-temperature dwelling species. Using data from the French breeding bird survey, we first found a strong increase in CTI over the last two decades revealing that birds are rapidly tracking climate warming. This increase corresponds to a 91 km northward shift in bird community composition, which is much higher than previous estimates based on changes in species range edges. During the same period, temperature increase corresponds to a 273 km northward shift in temperature. Change in community composition was thus insufficient to keep up with temperature increase: birds are lagging approximately 182 km behind climate warming. Our method is applicable to any taxa with large-scale survey data, using either abundance or occurrence data. This approach can be further used to test whether different delays are found across groups or in different land-use contexts.