Tage der Biodiversität 2027

Organisms disperse in space, thereby connecting local communities to metacommunities. Dispersal and environmental selection are two community assembly processes with different effects on biodiversity and ecosystem functioning. Environmental selection leads to communities that are closely linked to prevailing environmental conditions, which can enhance ecosystem functioning, whereas dispersal processes, particularly mass effects, may mask such community-environment relationships. The ability to identify sites with strong environmental selection could improve biodiversity conservation and help maintain ecosystem functioning.

So far, however, assembly processes have mainly been generalized to entire metacommunities, ignoring their variability among constituent sites. This limits our understanding of community assembly in river networks, where unidirectional water flow connects, but also isolates, environmentally heterogeneous sites across the network from headwaters to large rivers. We present a site-specific description of periphyton community assembly processes to investigate how the relative importance of dispersal and environmental selection at the local scale relates to spatial connection among sites and their environmental conditions.

We fitted joint species distribution models to taxonomically highly resolved molecular biological datasets derived from three river networks, the Mara (Kenya), the Thur (Switzerland) and the Vjosa (Albania and Greece). We inferred the site-specific importance of dispersal and environmental selection from effects of spatial and environmental variables on community composition.

We found a systematic shift in assembly processes along river size and environmental distinctiveness across all three river networks, advancing understanding and generalization of assembly processes and helping to prioritize locations for restoration. Environmental selection was particularly important in midsized streams and at sites with more distinct environmental conditions. Dispersal developed the greatest role in large rivers—possibly due to mass effects. We also found indication for dispersal limitation to affect spatially isolated headwaters.