Calibrating the data logger…
McIntosh. A. R. and Peckarsky, B. L. 1999. Criteria determining behavioural responses to multiple predators by a stream mayfly - Oikos 85: 554-564. We investigated prey responses to multiple predators by observing drift and positioning of Buetis bicuudctrus (Ephemeroptera: Baetidae) nymphs in the presence of non-feeding brook trout (Salvelinusfontinalis chemical cues) and predatory stoneflies (Megtrrcj,~ signura with glued mouths). In experiments carried out in stream-side channels Baeti.~ antipredator behaviour differed markedly in response to each predator separately: they drifted away from stoneflies. and reduced exposure and drift levels in the presence of trout odour. When both predator cues were presented together the effect of stoneflies, which were observed more frequently at night. depended on the time of day and was modified by presence of fish odour. During the day Buetis responses to both predators were hierarchical as drift and the number on stone tops was reduced whenever fish odour was present regardless of the presence of stoneflies. At night when stoneflies were foraging, Baetis behaviour reflected a compromise between avoiding both predators, with responses to both predators being intermediate between that characteristic of stonefly avoidance (entering the drift) and that characteristic of fish avoidance (drift avoidance). The switch in antipredator behaviour from 'hierarchy' to 'compromise' between multiple predators reflects changes in the relative risk of predation during the day and night. During the day visually feeding trout present a higher predation risk. but at night when stoneflies are foraging and trout visual systems are less effective. the predation risks become more similar. We found no evidence for 'accumulative' avoidance where the magnitude of avoidance increases in response to the number of predators present. because avoiding stoneflies conflicted with avoiding trout. We predict that predator avoidance will take place in the form of a 'hierarchy' when one predator presents substantially higher predation risk than another; 'compromise' behaviour will be observed when the predation risk presented by two predators is more equivalent, but avoiding one predator conflicts with avoiding another; and 'accumulative' antipredatory responses will be observed only when responses to different predators do not conflict. We emphasise that investigation of the responses of prey to multiple predators is important to our understanding of natural communities. and is likely to reveal details about the mechanisms determining the outcome of interactions involving multiple predators.
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