Linking distance with community composition: long-term trends in distance decay in freshwaters

Over the past decades, environmental conditions in freshwater ecosystems have largely improved in the global north as consequence of the implementation of environmental regulations, resulting in a higher ecological status of streams and rivers. Although a recovery of the aquatic communities has been recently reported, it still remains unclear whether the improvement of these conditions has also restored the underlying biodiversity patterns. In natural systems, biodiversity is shaped by spatial and environmental gradients, but human pressures tend to disrupt these relationships. Typically, the composition of biological communities decrease in similarity with increasing geographical distance due to environmental differences between sites and the dispersal limitations of organisms; a biogeographical pattern known as distance decay. Anthropogenic impacts can further alter this pattern and temporal fluctuations in environmental conditions can also lead to varying rates of distance-decay in community similarity. Here, we aim at understanding the variation of continental scale community structure and composition over the last decades by analyzing the decay in similarity between communities with increasing geographical distance. We hypothesize that communities have become more homogeneous, both taxonomically and functionally, through time due to the improvement of environmental conditions, leading to a shallower decay in similarity over time. For this purpose, we collected publicly available data of fish monitoring surveys from five different European countries for the last 20 years. For each year, we calculated taxonomic and functional distance between communities, as well as geographic distance between sampling sites. We additionally obtained environmental information such as flow connection, climatic region and human footprint index for each site. We used a hierarchical Bayesian model approach to determine how community similarity was associated to the variables of interest and how distance decay changed with time. Preliminary results suggest that the relationship between community similarity and geographic distance changes over time, although the effects of human impact are still obvious.