Dispersal capacity and broad-scale landscape structure shape benthic invertebrate communities along stream networks

Dispersal is a fundamental trait influencing species' distribution patterns and metacommunity structure. Yet, for stream communities it remains unclear how communities differ in dispersal capacity. Due to the dendritic network structure of streams and the greater spatial variability in environmental conditions in headwaters than in mainstems, we asked three main questions: 1) Do benthic invertebrates inhabiting headwaters have lower community-wide dispersal capacities (DCc) on average than those living in mainstems? 2) In turn, does the degree of community dissimilarity among sites differ between the different locations in the river network? 3) Are these differences more pronounced in highland streams compared to lowland streams as a consequence of major landscape features (i.e. mountains)? To examine these questions, we compiled 1466 benthic invertebrate samples across the southern highland and northern lowland areas of Germany. Results showed that overall DCc increased with stream size in both highland and lowland streams. In highland streams, higher DCc in mainstems was associated with more homogeneous communities compared to headwater communities. However, this pattern did not occur in lowland streams. This suggests that both dispersal capacity and landscape structure interact to determine community structure in these networks. Our results therefore stress the importance of considering dispersal traits and landscape features, as well as habitat control (or environmental filtering) to better understand (meta-) community structure across various landscape types.