Foraging in a patchy environment

laden with berries is a patch for a fruit-eating bird; a leaf covered with aphids is a patch for a predatory ladybird. Alternatively, a 'patch' may only exist as an arbitrarily defined area in an apparently uniform environment; for a wading bird feeding on a sandy beach, different 10 m2 areas may be thought of as patches that contain different densities of worms. In all cases though, a patch must be defined with a particular consumer in mind. One leaf is an appropriate patch for a ladybird, but for a larger and more active insectivorous bird, 1 m2 of canopy or even a whole tree may represent a more appropriate patch.

Ecologists have been particularly interested in patch preferences of consumers where patches vary in the density of food or prey items they contain. There are many examples where predators show an 'aggregative response', spending more time in patches containing high densities (because these are the most profitable patches) (Figure 9.20a-d), although such direct density dependence is not always the case (Figure 9.20e). We deal with aggregative responses in more detail in Chapter 10 where their importance in population dynamics will be our focus, and particularly their potential to lend stability to predator-prey dynamics. For now, we concentrate on the behavior that leads to predator aggregation (Section 9.6.1), the optimal foraging approach to patch use (Section 9.6.2) and the distribution patterns that are likely to result when the opposing tendencies of predators to aggregate and to interfere with each other's foraging are both taken into account (Section 9.6.3).

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