Thermophilization and reshuffling of montane leaf beetle communities over a two-decade period

Given the sensitivity of mountain biodiversity to human pressure, it is essential to quantify changes in montane biological communities and contrast them with expectations based on potential drivers of change. This need is particularly pressing for biological groups representing important but little-studied fractions of biodiversity, such as insects. We analyze the temporal changes (between 1998 and 2015) of leaf beetle communities in an altitudinal gradient in the Sierra de Ancares (NW Spain). Our results show temporal changes in the composition of local communities, with a tendency to assemblage thermophilization, as well as a homogenization of the spatial turnover pattern, mostly driven by an increased similarity between communities at the lower and intermediate altitudes. These temporal changes in community composition and in the spatial structure of biodiversity were associated with upward shifts of the upper altitudinal limit of warm-adapted species and with downward shifts of the lower altitudinal limit of cold-adapted species. While this upward shift is consistent with expectations of climate change effects, the observed downward shift suggests a land-use change effect. Our results point to the joint effect of multiple factors (climate and land-use change) behind temporal changes of these leaf beetle communities, which result in compositional reorganization and biotic homogenization, rather than a mere coherent displacement toward higher altitudes. More generally, we show that understanding temporal change of biodiversity requires assessing multiple community-level metrics (e.g., variation in assemblage composition and/or changes in spatial turnover) for the detection of tendencies among the species-specific signals (e.g., altitudinal range shifts).