Local and regional processes determine plant species richness in a river-network metacommunity

Background/Question/Methods

The spatial organization of streams and rivers into branching networks has a large effect on the structure of riverine ecological communities. Riparian plants are a good example of such communities because they are affected by local habitat conditions as well as strong directional dispersal along the river network. Empirical studies integrating the role or regional, dispersal-related processes and local processes in determining local species composition in river networks have however been lacking. We studied riparian vegetation along a river network in boreal Sweden to investigate the importance of local conditions (i.e., disturbance regimes, soil properties, habitat heterogeneity) and regional processes (i.e., dispersal) for plant diversity and composition. We asked how (1) riparian plant species composition and diversity, and (2) riparian habitat conditions change with position in the river network, and (3) how species composition varies with habitat conditions. We further investigated (4) how plant dispersal along the network contributes to observed patterns. Sampling across the branches and main stem of the river network created a gradient of stream sizes which was used as a proxy for position along the river network.

Results/Conclusions

We found that riparian plant species richness was strongly determined by the position along the river network, linearly increasing with stream size. Plant species composition (e.g., plant growth forms, dispersal traits) was also closely related to the position in the river network. This was the result of combined effects of local and regional forcing. First, plant diversity and composition patterns were influenced by local habitat conditions such as flood disturbance, edaphic properties and habitat heterogeneity. Those conditions changed downstream in the network, resulting in larger habitat breadth, better soil quality and higher disturbance towards larger streams, favoring plant species richness. Second, plant species composition was nested implying that species accumulate downstream as more branches of the river network combine. Further, successively more species adapted to hydrochory (dispersal by water) were found downstream in the network. The strong role of dispersal along the network was also supported by increasing similarity in species composition with decreasing network distance between sites (distance-decay relationship). Our results provided the first clear empirical evidence that the riparian plant metacommunity is closely related to the position in the river network and such pattern is attributed to combination of local and regional drivers.