The effect of predation and immigration on aquatic macroinvertebrate community structure

To assess relative influences of local and regional processes, I created different intensities of predation (local process) and immigration (regional process) in an enclosure/exclosure field experiment on aquatic macroinvertebrate communities in a stream ecosystem. Twenty-four enclosures were manipulated in a two level, full factorial, repeated measures design which created four treatments: high predation/high immigration high predation/low immigration low predation/high immigration and low predation/low immigration. I conducted Non-metric Multidimensional Scaling and Principal Coordinate Analysis on community data to determine differences in treatments. To look at changes in trophic structure, I quantified the percentage of each functional feeding group for each sampling unit and performed Two-way ANOVA repeated measures models to assess interactive effects of predation and immigration through time. I measured several different metrics of variability in communities to assess temporal dynamics and patterns: aggregate variability (AV), compositional variability (CV), and variability among the replicates. Simulated local (predation) and regional (immigration) processes both impacted local community structure, although I did not find an interactive effect of immigration and predation. Compared to low predation treatments high predation treatments displayed differences in functional feeding groups, greater Simpson's diversity, and increased CV. High immigration treatments altered community composition and more closely reflected the regional species' pool than low immigration treatments. High predation treatments influenced the relative abundance of species differently shown by the elevated CV and opposing responses from certain functional feeding groups. In high immigration treatments the difference in community composition indicated a regional effect by the input of macroinvertebrates from the regional species' pool. The most influential species in the high immigration treatments responded differently depending on their abiotic preferences and dispersal abilities. Overall my results support conclusions from other studies where both dispersal processes and local environmental conditions explained local patterns in aquatic macroinvertebrate communities.