Abstract

Climate change intensifies extreme events, increasing the risk to freshwater ecosystems. The ways in which these events threaten freshwater diversity and contribute to biotic homogenization have not yet been fully explored. Our objective is to compare the spatiotemporal responses of two summer macroinvertebrate assemblages at the northern and southern limits of the Chilean Mediterranean ecoregion. We represent the extreme climatic event with a historical record of drought until 2022 and subsequent floods due to a single El Ni & ntilde;o year in 2023. We have noted greater taxonomic homogenization in community structure in the northern streams than in the southern streams. However, functional diversity increased in the northern streams in the summer after flooding. The beta-diversity components showed differences between taxonomic and functional diversity. These differences were observed between years and stream groups (clusters) where turnover due to hydrological fluctuations predominates. The highest richness was found in the southern Mediterranean sites, though proportional functional richness is higher in the north. The most significant physicochemical variables related to these results are water temperature and pH in the north, and oxygen variables in the south. Hellinger and Jaccard distances significantly decreased in northern Mediterranean streams in 2024, indicating a decrease in biological similarity between sampling sites. Community composition remained consistent in the micro-habitats of southern Mediterranean sites, suggesting a possible baseflow condition that makes them resilient. Our study demonstrates that ecological stress caused by extreme climatic events predicts patterns of macroinvertebrate diversity, with heterogenization and homogenization arising equivalently in Mediterranean regions.