The scent of danger - predator-mediated shifts in benthic invertebrate community composition and genotypic diversity in multiply-stressed riverine ecosystems
Predation is a major selective force structuring natural biological communities and causing the evolution of many defence or avoidance mechanisms in prey organisms. Predators in riverine ecosystems are primarily fish but also several macroinvertebrate taxa. Beside the direct predation effect, indirect interactions between predator and prey (e.g., drift or hiding behaviour) have been shown to be mediated by chemical cues (kairomones) released by predators. However, single and multiple stressors might significantly interfere with this evolved interspecific interaction as anthropogenic stressors may inhibit or modify the signal transfer. Therefore, the effects of predation in stream ecosystems affected by stressors might differ substantially from near-natural conditions with consequences for the structure of biological communities. If the expression of the evolved defensive traits against predators are affected by stressors, the effect of predation on macroinvertebrate taxa might increase and shifts and losses in biodiversity of the stream habitat on several trophic levels might be a consequence. It has been shown, however, that rapid evolution within short ecological periods can occur. Predation pressure in multiply stressed habitats might favour those genotypes that still can sense the predators.
Therefore, we will take advantage of the field experiments (ExStream), which are planned in the Emscher catchment (degraded environment, nearly no fishes due to pollution history) and in the Kinzig catchment (near-natural environment, normal fish fauna) to compare the general response to fish cues and the interaction with stressors and additionally the genotypic signatures of prey species relative to their predator avoidance behaviour. We will use the stressors temperature increase and salinity as well as reduced flow individually and in combination and compare them to control conditions without stressor application. All treatments will additionally be combined with chemical cues from fish (provided by header tanks containing fish) as indirect predation signals. The ExStream system offers the unique advantage of allowing a high number of treatments and replicates to be run simultaneously. We test the effect of predation on the asymmetric response concept (ARC) in stressor increase and release experiments. Taking predation into account is pivotal to the generation of a composite hypothesis describing the effects of multiple stressors.