How big is a food patch?

Posted on June 19, 2017

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Most studies of food patch choice by foraging animals have assumed that the patches in question are separate and well-defined. However, in many cases, predators feed in continuous habitats without obvious, discrete patches, and researchers are forced to choose an arbitrary spatial scale when calculating prey densities and interpreting the impacts of predators. An important consequence of this is that the relationship between prey density and prey mortality depends on the patch scale chosen by the experimenter. But how does this scale compare to the patch scale perceived by the predator? A 2014 study found that guppies preferred prey clustered at a scale of less than 10 cm to those clustered at a scale of 15 cm, but they didn’t discriminate between prey clusters below 10 cm. In a follow-up experiment, with guppies feeding on different spatial arrays of bloodworm prey, researchers at the University of North Carolina used this information to show that the predator foraging scale determines the pattern of prey density dependence. If, as expected, a predator searches randomly at scales below its characteristic foraging scale, then prey mortality should show an inverse relationship with prey density because the per capita risk is lower in a larger group. The results of the experiment supported this prediction: when prey were clustered below the predator foraging scale, predators ignored variations in prey density and prey mortality and density were inversely related. This relationship didn’t hold when prey were grouped at a scale at or above the predator foraging scale, because in that case predators fed non-randomly by targeting high-density patches. The findings suggest an approach for predicting how predators and prey interact in continuous habitats. They also imply that aggregation by prey will only be selectively beneficial when it occurs at a spatial scale smaller than the foraging scale of their predators – which in turn opens up future lines of research on the co-evolution of predator-prey behaviour.

References:

McCarthy, E.K. & White, J.W. 2016.  Density‑dependent prey mortality is determined by the spatial scale of predator foraging. Oecologia 180, 305–311. https://link.springer.com/article/10.1007/s00442-015-3374-7

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