Uncoupling strong predator-prey interactions in streams: the role of marginal macrophytes

Predation refugia can facilitate the coexistence of predators and prey within an ecosystem by weakening trophic links between the two. The marginal macrophytes of shallow lakes are used facultatively by zooplankton to escape fish predation, leading to the stabilisation of lentic food webs. Little is known about such a role for the marginal macrophytes of lotic systems. In this paper, we examine whether the marginal macrophytes of chalk streams help buffer the interaction between the freshwater shrimp, Gammarus pulex and a benthic fish predator, Cottus gobio, both characteristically abundant in such systems. Quantitative surveys were taken of Gammarus and bullheads in winter and summer in a southern English chalk stream. These indicated that Gammarus seasonally switched their habitat preference, from marginal macrophytes in summer, to midchannel habitats in winter. Bullheads exhibited an opposite trend, preferring midchannel habitats in summer and all habitats, particularly marginal macrophytes, in winter. Large Gammarus and precopula pairs were found almost exclusively in the margins in summer. This spatial separation between Gammarus and bullheads in summer, though not in winter, was reflected in bullhead diets, as determined by gut analysis. In field manipulation experiments, bullhead presence was the strongest factor explaining the between-habitat distribution of Gammarus in both summer and winter, indicating that the habitat shift of Gammarus was driven by the distribution of fish. Other abundant invertebrate taxa, including Asellus, mayflies and chironomids, exhibited little avoidance of habitat patches of high bullhead density. We conclude that marginal macrophytes in chalk streams can potentially facilitate the coexistence of high densities of both Gammarus and bullheads by spatially separating predator and prey in summer. They may further allow large populations of Gammarus to persist in the presence of high bullhead density at stretch-wide spatial scales, by reducing the predation by bullheads of large breeding and newly born individuals of Gammarus in summer.