Climate-Driven Reshuffling of Butterfly Communities: Body Size Declines and Community Homogenization in a Rapidly Warming Switzerland

Climate change has been reshaping natural communities by driving species to change their ranges in response to warming temperatures. In Switzerland, the rate of warming since 1950 has approached nearly double the global average, making these impacts important for community composition and ecosystem function. Using the Community Weighted Length Index (CWLI) as the body size estimator, previously extracted by museum specimens, we studied two decades of butterfly community data that have been inventoried by the Biodiversity Monitoring Switzerland program. We checked for deviations from the expectation of random trends in the CWLI using null models. We modeled the long-term patterns at each site and the overall long-term patterns through linear regression and mixed-effects models alongside the model of the relationship between wing length, elevation, and abundance. CWLI of Swiss butterfly assemblages dropped by 5.2% over two decades, suggesting that warmer temperatures are differentially affecting large-bodied species. While CWLI trends were significantly declining at most sites (73%), a subset of sites (27%) showed trends of increasing CWLI, sometimes associated with cooler microclimates either in forests or at higher elevation. Our results further indicate a progressive homogenization of butterfly communities, marked by smoother CWLI slopes and reduced CWLI range over time. These results underline complex relations of climate-driven range shifts with body-size dynamics and community restructuring, emphasizing the importance of understanding the processes to mitigate biodiversity loss in thermally sensitive ecosystems.