In freshwater systems predation is one of the major factors influencing macroinvertebrate population dynamics and community structure. Although the impact of predators on community structure, e.g. species diversity and composition, is well studied little is known about how changes in community structure translate into changes in ecosystem processing. Besides the direct lethal effect, predators can also induce behavioural effects. Simply the risk of predation is often enough to change prey behaviour such as foraging rates. In freshwater systems decomposition and grazing are two important ecosystem processes. The organisms that drive these processes include macroinvertebrate detritivores and grazers. Whereas periphyton only serves as a food source, leaves may serve both as a food source and a refuge from fish predators to the macroinvertebrates. Hence, leaf decomposition may be considered a “safe behaviour” in contrast to grazing behaviour that is more risky (increased exposure to predators on top of stones). In a laboratory experiment we studied the effect of species diversity and composition, on two ecosystem processes, grazing and decomposition. We also studied the effect of fish cue on these two processes. We had four macroinvertebrate species, representing a gradient from grazer to detritivores; the snail Lymnaea peregra, the mayfly larvae Cloeon dipterum, the isopod Asellus aquaticus and the amphipod Gammarus pulex.

Results/Conclusions

We found that not only species diversity but more important, species composition affect ecosystem processes. Furthermore, fish cue increased the decomposition rate at the expense of grazing. Another interesting finding was that L. peregra, mainly considered a grazer, were found to be as efficient detritivore as grazer. Furthermore, in our study, L. peregra had the highest decomposition- and grazing rate of all species indicating it might represent a keystone species in some systems. As always when concerning a keystone species, this can have large implications on the ecosystem would they for some reason decrease or disappear. However, in our study they did not seem to be affected by the other species and the ability to change food source would make them quite resilient. Our results illustrate the complexity of ecosystems. How species interact with each other cascade further in the ecosystem and the outcome is dependent on which species are involved and in what combinations.