Tropical forest snakes prefer it hot and wet

The ongoing biodiversity crisis increases the importance and urgency of studies addressing the role of environmental variation on the composition and evolutionary history of species assemblages, but especially the tropics and ectotherms remain understudied. In regions with rainy summers, coexistence of ectothermic species may be determined by the partitioning of the climatic niche, since ectotherms can rely on water availability and thermoregulatory behaviour to buffer constraints along their climatic niche. Conversely, tropical ectotherms facing dry summers would have fewer opportunities to climatic niche partitioning and other processes rather than environmental filtering would mediate species coexistence. Herein, we disentangle the relative contribution of thermal, water-related, and topographic conditions in structuring the compositional (CBD) and phylogenetic (PBD) beta-diversity of Atlantic Forest snake assemblages occuring under distinct climatological regimes: dry summers and rainy summers. Thermal conditions best explains CBD and PBD of snakes along the whole tropical forest ecoregion, whereas water-related factors best explain the structure of snake assemblages within a same climatological regime. CBD and PBD patterns show similar levels of spatial structure but snake assemblages facing dry summers are more affected by spatial processes operating at fine to intermediate spatial scale whereas those assemblages in regions with rainy summers have a stronger signature of coarser-scale processes. Snake assemblages experiencing rainy summers are more affected by environmental filtering relative to those under dry summers, and the synergism between thermal and water-related conditions is the key cause behind this difference. We conclude that differences in climatological regimes affect processes mediating species coexistence and demonstrate that broad-scale gradients are stronger on ectothermic assemblages experiencing rainy summers, where climatic niche partitioning is more likely. Overall, our findings highlight the potential stronger role of biotic interactions and neutral processes in structuring ectothermic assemblages facing changes towards warmer and dryer climates.