Community Assembly Processes Change With Location and Environmental Distinctiveness in River Networks

Abstract

Aim

Organisms disperse in space, thereby connecting local communities to metacommunities. Dispersal and environmental selection are two community assembly processes ultimately driving biodiversity and potentially scaling to ecosystem functioning. So far, these 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. For the first time, we aimed at 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.

Location

Three river networks: The Mara (Kenya), the Thur (Switzerland) and the Vjosa (Albania and Greece).

Time Period

We collected data during five field campaigns from January 2018 to October 2019.

Major Taxa Studied

Algae attached to streambed sediments (periphyton).

Methods

We fitted joint species distribution models to taxonomically highly resolved molecular biological datasets. We inferred the site-specific importance of dispersal and environmental selection from effects of spatial and environmental variables on community composition. We then used generalized additive models to reveal shifts in the relative importance of assembly processes along catchment area and environmental distinctiveness.

Results

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 in well-connected large rivers. We also found indication for dispersal limitation to affect spatially isolated headwaters in the Thur.

Main Conclusions

We revealed a systematic shift in assembly processes along river size and with environmental distinctiveness across all three river networks, advancing understanding and generalization of assembly processes within river networks. Sites with environmental selection result in communities that are closely linked to prevailing environmental conditions, which can enhance ecosystem functioning. The ability to identify such sites could improve biodiversity conservation and help maintain ecosystem functioning.