When prey aggregation is too risky

Posted on October 1, 2014

As studies on the behavioural ecology of predator-prey interactions have progressed, a deep rift has opened up between two apparently contradictory traditions. One tradition is based on the assumption that by aggregating, prey animals reduce their individual predation risk, while the other tradition is based on the expectation that predators should feed in patches with the highest density of prey. The rift has developed because most studies have focussed either on prey defences or on predator foraging, with few attempts to reconcile the two perspectives. Polish researchers examined the interactive behaviour of a typical plankton-feeding fish predator (roach, Rutilus rutilus) and microcrustaciean prey (Daphnia hyaline galeata) in a system of interconnected 1 m3 tanks that allowed fish to move between areas with different prey densities. Light levels imitated lake conditions at dusk and dawn, the main feeding times for planktivorous fish. Fish movements and changes in Daphnia density were followed for 2-6 days. Individual risk for Daphnia in a given tank was calculated as the prey capture rate divided by the Daphnia density. Contrary to the idea that swarming is always a good antipredator strategy, individual risk increased rather than decreased with prey densities of up to 20-30 individuals / litre. In other words, up to this density, the protective value of a swarm was outweighed by its tendency to attract predators. This result indicates that in the wild, prey aggregation may not be adaptive when prey populations are low and predators are common, and it helps to explain the uniform horizontal distribution of zooplankton that’s often seen in lakes and oceans. In contrast, swarming can be adaptive when prey densities are relatively high, especially in small lakes in the middle of the day, where predator populations are lower and fish are less willing to leave their daytime refuges.

Reference: Gliwicz, Z.M. , Maszczyk, P., Jabłonski, J. & Wrzosek , D. 2013. Patch exploitation by planktivorous fish and the concept of aggregation as an antipredation defense in zooplankton. Limnology & Oceanography 58(5), 1621–1639. http://aslo.org/lo/toc/vol_58/issue_5/1621.pdf