How Do Fishing and Environmental Effects Propagate Among and Within Functional Groups?

Competition and predation can play different roles in mediating the influence of external pressures, such as fishing or environmental variations, on marine communities. Pressure effects propagate through food webs along predation links. These predator-prey interactions may result in trophic cascades, but they can be buffered by competitive interactions. We investigated these mechanisms by taking a functional-group approach. Are functional groups affected by external pressures in a predictable way? Within functional groups, do all species respond in the same way, or does competitive release allow for compensation among species? We constructed a simple community model, with functional groups connected by predation links. Loop analysis was used to make qualitative predictions of the changes in functional groups that might result through either direct or indirect effects from changes in pressures. Actual changes in biomass and average weight in functional groups were then tracked from fish trawl-survey data; compensation within groups was examined with dynamic factor analysis. This approach was applied to the Georges Bank, Bay of Biscay, and North Sea fish communities, which have been subject to different fishing regimes and have undergone environmental changes over the last decades. All three communities changed substantially. Compensation did not prevent impacts from propagating through the three food webs; rather, antagonistic pressures did. Community responses to perturbation were mostly determined by community structure and by fisheries selectivity with respect to both functional groups and species.