Summary

Predation is the consumption of one organism by another, in which the prey is alive when the predator first attacks it. There are two main ways in which predators can be classified. The first is 'taxonomic' - carnivores consume animals, herbivores consume plants, etc.- and the second is 'functional', in which true predators, grazers, parasitoids and parasites are distinguished.

The effects of herbivory on a plant depend on which herbivores are involved, which plant parts are affected, and the timing of attack relative to the plant's development. Leaf-biting, sap-sucking, mining, flower and fruit damage and root pruning can be expected to differ in the effect they have on the plant. Because the plant usually remains alive in the short term, the effects of herbivory are also crucially dependent on the response of the plant. The evolutionary selection pressure exerted by herbivores has led to a variety of plant physical and chemical defenses that learning and migration

Figure 9.30 The effect of a cost to migration in predators distributing themselves across prey patches in a simulation model. The interference coefficient, m, is 0.3 and would lead to direct density dependence in the absence of a migration cost. (a) Low migration cost: direct density dependence is maintained. (b) Intermediate cost: a 'domed' relationship. (c) High cost: inverse density dependence. (After Bernstein et al., 1991.)

resist attack. These may be present and effective continuously (constitutive defense) or increased production may be induced by attack (inducible defense). It is not straightforward to determine whether the supposed 'defenses' actually have measurable, negative effects on the herbivore and positive consequences for the plant, especially after the costs of mounting the response have been taken into account. We discuss the difficulties of revealing such effects and review the relationships between herbivory and plant survival and fecundity.

More generally, the immediate effect of predation on a population of prey is not always predictably harmful, first because the individuals that are killed are not always a random sample (and may be those with the lowest potential to contribute to the population's future) and second because of compensatory changes in the growth, survival or reproduction of the surviving prey (especially via reduced competition for a limiting resource). From the predator's point of view, an increase in the amount of food consumed can be expected to lead to increased rates of growth, development and birth, and decreased rates of mortality. However, there are a number of factors that complicate this simple relationship between consumption rate and consumer benefit.

Consumers can be classified on a continuum from monophagy (feeding on a single prey type) to polyphagy (many prey types). The preferences of many consumers are fixed - they are main tained irrespective of the relative availabilities of alternative food types. But many others switch their preference, such that food items are eaten disproportionately often when they are common. A mixed diet may be favored first because each food type contains a different undesirable toxic chemical. More generally, a generalist strategy would be favored if a consumer has more to gain than lose in accepting low-quality items, once encountered, rather than ignoring them and continuing to search. We discuss this in the context of optimal diet theory, the aim of which is to predict the foraging strategy to be expected under specified conditions.

Food is generally distributed patchily and ecologists have been particularly interested in patch preferences of consumers where patches vary in the density of food or prey items they contain. We describe the behaviors that lead to aggregated distributions and the nature of the distribution patterns that result. The advantages to a consumer of spending more time in higher profitability patches are easy to see. However, the detailed allocation of time to different patches is a subtle problem, depending on the precise differentials in profitability, the average profitability of the environment as a whole, the distance between patches, and so on. This is the domain of the theory of optimal patch use. The predictions of both optimal foraging and optimal patch use theory have to be modified when there is a simultaneous risk of a consumer being preyed upon.

Chapter 10

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The Population Dynamics

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of Predation

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