Shoaling behaviour enhances risk of predation from multiple predator guilds in a marine fish
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Predicting the consequences of predator biodiversity loss on prey requires an understanding of multiple predator interactions. Predators are often assumed to have independent and additive effects on shared prey survival; however, multiple predator effects can be non-additive if predators foraging together reduce prey survival (risk enhancement) or increase prey survival through interference (risk reduction). In marine communities, juvenile reef fish experience very high mortality from two predator guilds with very different hunting modes and foraging domains—benthic and pelagic predator guilds. The few previous predator manipulation studies have found or assumed that mortality is independent and additive. We tested whether interacting predator guilds result in non-additive prey mortality and whether the detection of such effects change over time as prey are depleted. To do so, we examined the roles of benthic and pelagic predators on the survival of a juvenile shoaling zooplanktivorous temperate reef fish, Trachinops caudimaculatus, on artificial patch reefs over 2 months in Port Phillip Bay, Australia. We observed risk enhancement in the first 7 days, as shoaling behaviour placed prey between predator foraging domains with no effective refuge. At day 14 we observed additive mortality, and risk enhancement was no longer detectable. By days 28 and 62, pelagic predators were no longer significant sources of mortality and additivity was trivial. We hypothesize that declines in prey density led to reduced shoaling behaviour that brought prey more often into the domain of benthic predators, resulting in limited mortality from pelagic predators. Furthermore, pelagic predators may have spent less time patrolling reefs in response to declines in prey numbers. Our observation of the changing interaction between predators and prey has important implications for assessing the role of predation in regulating populations in complex communities.
KeywordsNon-additive mortality Predator–prey theory Shoaling Ecosystem stability Multiple predator effects
We would like to thank Christian Jung, Dean Chamberlain, Matt Le Feuvre, Madhavi Colton, Mal Lindsay and Jessica Smith for the back-breaking construction of artificial reefs, these again plus Evan Hallein, Simon Pahor, Dan Corrie and Seann Chia for setups and surveys, and John Ahern for boat and technical support. We thank Craig Osenberg for his helpful guidance on determining non-additivity in the system. Operational costs were covered by grants from the Holsworth Wildlife Research Endowment to J. R. F. and the Australia and Pacific Science Foundation to S. E. S. Animal ethics approval was granted under Melbourne University AEC0810932. The authors declare they have no conflict of interest.
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