Because of the potential consequences for population dynamics, ecologists have been particularly interested in patch preferences where patches vary in the density of the food or prey items they contain (see Section 9.6). At one time it appeared, and was widely believed, that: (i) predators generally spent most time in patches containing high densities of prey (because these were the most profitable patches); (ii) most predators were therefore to be found in such patches; and (iii) prey in those patches were therefore most vulnerable to predation, whereas those in low-density patches were relatively protected and most likely to survive. Examples certainly exist to support the first two of these propositions (see Figure 9.20a-d), demonstrating an 'aggregative response' by the predators that is directly density dependent (predators spending most time in patches with high densities of prey such that prey and predator densities are positively correlated). However, this is not always the case. Furthermore, contrary to the third proposition, reviews of host-parasitoid interactions (e.g. Pacala & Hassell, 1991) have shown that prey (hosts) in high-density patches are not necessarily the most vulnerable to attack (direct density dependence): percentage parasitism may also be inversely density dependent or density independent between patches (see Figure 9.20e). Indeed, the reviews suggest that only around 50% of the studies examined show evidence of density dependence, and in only around 50% of these is the density dependence direct, as opposed to inverse. None the less, despite this variation in pattern, it remains true that the risk of predation often varies greatly between patches, and hence between individual prey.
Many herbivores also display a marked tendency to aggregate, and many plants show marked variation in their risk of being attacked. The cabbage aphid (Brevicoryne brassicae) forms aggregates at two separate levels (Way & Cammell, 1970). Nymphs quickly form large groups when isolated on the surface of a single leaf, and populations on a single plant tend to be restricted to particular leaves. When aphids attack only one leaf do predators aggregate in high-density prey patches?
plants may be protected by the aggregative responses of herbivores of a four-leaved cabbage plant (as they do naturally), the other three leaves survive; but if the same number of aphids are evenly spread over the four leaves, then all four leaves are destroyed (Way & Cammell, 1970). The aggregative behavior of the herbivores affords protection to the plant overall. But how might such heterogeneities influence the dynamics of predator-prey interactions?
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