Behavioural interactions in stream food webs: the case of drift‐feeding fish, predatory invertebrates and grazing mayflies

Abstract

Summary 1. We examined direct and indirect behavioural responses of grazing mayfly nymphs (Baetis rhodani) to predation risk from a drift‐feeding fish (European minnow; odour manipulated), and two types of invertebrate predators, Diura bicaudata (stonefly) and Rhyacophila nubila (caddis larva). We also assessed the direct responses of the invertebrate predators to fish chemicals. 2. Based on diel gut content periodicity, D. bicaudata nymphs were strongly nocturnal foragers. R. nubila was also nocturnal, but only in a fish stream; in a stream lacking fish, their gut contents did not vary significantly on a diel basis. In the laboratory, Diura was nocturnal even in the absence of fish and almost ceased moving when fish was present. Rhyacophila shifted from aperiodic to nocturnal foraging in the presence of fish. 3. The contrasting behaviours of the two predators may be explained by their respective field distributions: D. bicaudata always co‐occurs with fish, whereas R. nubila is also found in streams without fish. Therefore, a capacity for plastic anti‐fish responses is beneficial for R. nubila, but not for D. bicaudata. 4. Drift of large Baetis nymphs was aperiodic when Rhyacophila was present, but nocturnal in all other treatments. Drift rate was highest when both Diura and fish were present and lowest in treatments with Rhyacophila. Predatory stoneflies, but not the caddis larva, induced a night‐time peak in the drift of both Baetis size‐classes. In the absence of predators, small Baetis drifted aperiodically. Interaction terms in three‐way anovas testing for the indirect effects of fish and invertebrate predators on mayfly drift periodicity and drift rate were all non‐significant, indicating that the response of Baetis to one type of predator was not modified by the other predator. 5. Our results indicate that the drift periodicity of lotic mayflies may be fine‐tuned to variations in the multi‐predator environment and that prey responses are size‐specific. We conclude that the effects of invertebrate predators on prey behaviour vary in relation to predator's foraging strategy and generalizations based on studies with only one type of predator should be avoided.