Background/Question/Methods Human impacts on aquatic ecosystems often result in the addition or deletion of species, leading to changes in the natural community structure, and ultimately a modification of ecosystem functioning. However, we do not have a full understanding of the link between the structure of communities and the functioning of ecosystems. Here we report the results of a before/after study where we quantified the biomass of all the structural components of a stream ecosystem, and simultaneously measured ecosystem functioning in two ways. First, we measured the uptake of autochthonous algal basal resources by the community using pairs of colonisation tiles which were either exposed to grazers, or excluded grazers. Second, we monitored leaf breakdown rates (or uptake of allochtonous resources by the community) using mesh leaf packs which measured invertebrate leaf breakdown and microbial leaf breakdown, using the latter to correct the former. All data on functioning were temperature corrected. Our experimental design measured temporal and spatial variation. After one year, we manipulated the structure of the community by a controlled introduction of an “invasive” species near the top of the web (as part of an Atlantic salmon reintroduction programme in the River Frome, Dorset, UK). The consequences of the manipulation, both in terms of community structure and ecosystem functioning, were then followed for another year.

Results/Conclusions

The introduction of Atlantic salmon resulted in a doubling of salmonid density, which remained stable over the course of our experiment. We analysed invertebrate abundance, functional feeding group abundance and their relative proportions using a nested design with general mixed models, (PROC MIXED in SAS). We observed a significant decrease in the overall abundance of invertebrates, and the abundance of all functional feeding groups pre- and post-fish introduction. Stone scrapers and leaf shredders also showed a significant temporal variations in abundance, but only scrapers showed a significant change in spatial variation, or patchiness. However, the relative proportion of scrapers was significantly reduced after the introduction of salmon, indicating a directional change in community structure. We used a similar analysis to assess changes in our measures of ecosystem functioning. In line with decreases in invertebrate abundance, we observed a decrease in both grazing rate and leaf breakdown rate. Thus the introduction of fish, a common management practice, can have important consequences for community structure and this in turn can translate into changes to ecosystem processes.