How predators see prey patches

Posted on March 23, 2015

Optimal foraging theory has proved to be a very useful framework for researchers interested in predator behaviour. A basic prediction of the theory is that predators should choose to feed in habitat patches that provide the highest energy return. Usually, these are patches with the highest densities of prey. But how do predators interpret food patch distribution in the context of the habitat mosaic as a whole? In particular, in continuous habitats that aren’t broken up into a series of obvious, well-defined “hotspots”, how do predators define the scale of a prey patch? To help answer these questions, biologists at the University of North Carolina presented predatory guppies (Poecilia reticulata) with arrays of 12 test tubes containing plankton prey (Daphnia magna). Although the total number of prey in an array was always 24, the Daphnia were distributed between the tubes in six different ways, ranging from highly dispersed (two prey in each tube) to compact (24 prey in one tube). Therefore, the more compact arrangements had higher prey densities at smaller spatial scales. In each trial, the fish were offered a choice between two arrays with different prey distributions. All combinations of prey group arrangement were compared. Guppies were considered to favour an array when they were within 27 cm of it; this distance was half the width of the foraging arena and within the distance that Daphnia are visually detected by foraging fish. It was assumed that fish would prefer the array with the highest density at the scale at which they had defined a prey patch. The results suggested that guppies defined foraging patches at a scale of 8 to 12 test tubes, which corresponded to a distance of 10-15 cm, or 5-10 times their own (standard) body length. It’s likely that other species with the same sort of hunting behaviour have a similar perception of prey patchiness when foraging in continuous habitats. In principle, the defined patch size should also correspond to the spatial scale at which predators produce density-dependent mortality in their prey. And there are implications for prey behaviour too, since prey should aggregate within the scale defined by the predator if they are to take best advantage of avoidance strategies such as the confusion effect, risk dilution and information-sharing.

Reference: Birk, M.A. & White, J.W. 2014. Experimental determination of the spatial scale of a prey patch from the predator’s perspective. Oecologia 174, 723–729.