COS 28-3
Streams as dendritic metacommunities: Community composition responds differently to local habitat manipulations at different locations in a dispersal network
Factors that operate on a local scale such as environmental conditions and regional factors such as dispersal affect community composition. Surveys and experiments suggest that the network structure of dispersal pathways connecting local habitat patches can affect dispersal. Stream networks are dendritically organized and have relatively isolated headwater streams that flow into well-connected mainstem streams. We tested the hypothesis that isolated headwater stream communities would primarily be structured by local forces whereas well connected mainstem stream communities would be structured by a combination of local and regional forces (i.e., dispersal). We tested this hypothesis by manipulating substrate heterogeneity (a local factor) in headwater and mainstem streams in 4 different watersheds. To evaluate the prediction that beta diversity should be higher between treatments in headwater streams when compared to mainstem streams we calculated beta diversity for replicates. Additionally, we used repeated measures ANOVA to test the prediction that richness would be higher in high complexity substrates than low complexity substrates in headwaters but not in mainstem sites. We also tested the prediction that substrate complexity should be a stronger filter in headwater streams than mainstem streams by calculating dissimilarity indices for both within and between treatments for all 4 watersheds.
Results/Conclusions
Headwater streams had higher within stream beta diversity and lower richness than mainstem streams. Beta diversity was lower in high complexity substrates but only in headwater streams. Richness of experimental replicates was significantly higher in mainstem streams than in headwater streams. Low complexity substrates had lower richness than high complexity substrates but only in headwater streams. In contrast, after the first two sampling dates richness did not differ between high and low complexity substrates in mainstem streams. Headwater stream dissimilarities within and between replicates for a given stream/network location/substrate complexity were higher than mainstem streams. These findings illustrate that headwater and mainstem stream macroinvertebrate communities are structured differently as a result of their differing locations within a dispersal network. Mainstem macroinvertebrate communities appear to be heavily influenced by organisms dispersing in the water column. The increased beta diversity in headwater streams is likely due to their increased isolation. Similarly, the difference in richness between substrate complexity treatments in headwaters but not in mainstems is indicative of local forces overriding regional forces in headwaters but the reverse happening in mainstem streams. This implies that stream restorations of mainstem streams may fail if upstream habitats are not also restored.