Disturbance, predation and competition interact in a flood-prone stream

We investigated interactive effects of patchy physical disturbance (a simulated bed-moving flood), competition between invertebrates and fish predation on benthic organisms in a flood-prone stream. Three disturbance treatments (local scour, fill and undisturbed controls) were crossed with two predation treatments (exclusion/no exclusion of insectivorous bullies and trout) and two competition regimes in 48 plots (size 0.5 m²). Competition treatments consisted of twice-weekly removals or no removal of the abundant Potamopyrgus antipodarum, a snail known to displace other invertebrates when occurring at high densities. Fish exclusion was accomplished with electrified fences. Benthic algae and invertebrates were sampled six times during eight weeks post-disturbance. Disturbance frequently influenced the benthic community, especially during the first month after disturbance, with invertebrate densities and algal biomass generally highest in stable patches but invertebrate community evenness highest in scour patches. Fish exclusion and Potamopyrgus removal rarely influenced invertebrate distributions by themselves. However, both manipulations interacted frequently with disturbance and/or with each other. For example, fish exclusion resulted in higher Potamopyrgus densities in stable patches but not in disturbed patches on day 43. Our results indicate that the interplay between disturbance and biotic interactions can play a role in shaping benthic communities even in flood-prone streams.