Habitat structural complexity mediates food web dynamics in a freshwater macrophyte community

A considerable amount of research has investigated the influence of habitat structure on pred- ator success, yet few studies have explored the implica- tions for community structure and food-web dynamics. The relative importance of macrophyte structure and fish predation on the composition of the macroinverte- brate and periphyton communities in a lowland river was investigated using a multifactorial caging experi- ment. We hypothesised that: (1) fish predators are less effective in a more structurally complex macrophyte analogue; (2) strong direct and indirect effects of fish predators (e.g. trophic cascades) are less likely to occur in a structurally complex habitat; and (3) the strength of these patterns is influenced by the composition of the prevailing community assemblage. We measured the abundance and composition of the macroinverte- brate and periphyton communities associated with three different-shaped macrophyte analogues, under different fish predator treatments and at different times. Macrophyte analogue architecture had strong, consistent effects on both the macroinvertebrate and periphyton communities; both were most abundant and diverse on the most structurally complex plant analogue. In contrast, the fish predators affected only a subset of the macroinvertebrate community and there was a suggestion of minor indirect effects on periphy- ton community composition. Contrary to expectations, the fish predators had their strongest effects in the most structurally complex macrophyte analogue. We con- clude that in this system, macrophyte shape strongly regulates the associated freshwater assemblage, result- ing in a diverse community structure less likely to exhibit strong effects of fish predation. © Springer-Verlag 2006.