The magnitude and unpredictability of environmental variation strongly affects foraging strategies. The term "variance sensitivity" generally applies to decision making in the face of uncertainty (Stephens and Krebs 1986; Bateson and Kacelnik 1998). Unstable population dynamics in one species create a varying resource for any species that exploits it. In the next few paragraphs, we will discuss conceptual examples suggesting that temporal variability in the abundance of prey populations can change the relative fitnesses of alternative foraging strategies.
We will first consider the effect of temporal variation in the abundance of a preferred prey type on a forager's decision to be selective or opportunistic. Assume that a predator encounters two prey types sequentially. The predator feeds in accord with the classic diet model, so while it is handling an item of one prey type it cannot encounter any other prey. These assumptions lead to a prediction, described thoroughly in Stephens and Krebs (1986): the decision to be a generalist or specialist depends on the abundance ofthe higher-quality item (as measured by the b/h ratio). The model predicts an abrupt shift between specializing on the higher-quality item when it is abundant and eating both prey types at lower abundances of the preferred prey. Figure 11.1A shows this model graphically. A predator that consumes just the better prey, resource 1, has a type II functional response and a corresponding saturation curve (the solid line, Y(R)) describing the benefit it derives from foraging. The predator obtains a constant rate of return while consuming a single item of the less preferred species (the dashed line at bi/hi). If the benefit Y(R) (the solid curve) exceeds bi/hi, the consumer will specialize on resource 1; if it falls below bi/hi, the consumer should also take resource 2 whenever it is available. The switch between the behaviors occurs at the resource level R', where the solid and dashed lines cross.
How does temporal variation in resource availability affect this switch point? Assume that the preferred prey has a constant abundance, but the less preferred prey varies greatly and unpredictably in abundance. Such variation does not matter to inclusion of the better prey in the diet because its inclusion does not depend on its abundance. By contrast, temporal variation in the abundance of the preferred prey can influence the predator's decision regarding the poorer prey, and in particular, makes indiscriminate consumption more likely. Let R1(t) be a function of time that describes the dynamics of the preferred prey about an arithmetic mean abundance of R. Assume that predators can instantly and accurately assess resource abundance. Then, if R1(t) > R', the predator should specialize on resource 1; if R1(t) < R', it should generalize. Ifthe predator can assess average foraging returns only over some
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