Consumers affect the distribution and abundance of the things they consume and vice versa, and these effects are of central importance in ecology. Yet, it is never an easy task to determine what the effects are, how they vary and why they vary. These topics will be dealt with in this and the next few chapters. We begin here by asking 'What is the nature of predation?', 'What are the effects of predation on the predators themselves and on their prey?' and 'What determines where predators feed and what they feed on?' In Chapter 10, we turn to the consequences of predation for the dynamics of predator and prey populations.
Predation, put simply, is consumption of one organism (the prey) by another organism (the predator), in which the prey is alive when the predator first attacks it. This excludes detritivory, the consumption of dead organic matter, which is discussed in its own right in Chapter 11. Nevertheless, it is a definition that encompasses a wide variety of interactions and a wide variety of 'predators'.
There are two main ways in which predators can be classified. Neither is perfect, but both can be useful. The most obvious classification is 'taxo-nomic': carnivores consume animals, herbivores consume plants and omnivores consume both (or, more correctly, prey from more than one trophic level - plants and herbivores, or herbivores and carnivores). An alternative, however, is a 'functional' classification of the type already outlined in Chapter 3. Here, there are four main types of predator: true predators, grazers, parasitoids and parasites (the last is divisible further into microparasites and macroparasites as explained in Chapter 12).
True predators kill their prey more or less immediately after attacking them; during their lifetime they kill several or many different prey individuals, often consuming prey in their entirety. Most of the more obvious carnivores like tigers, eagles, coccinellid beetles and carnivorous plants are true predators, but so too are seed-eating rodents and ants, plankton-consuming whales, and so on.
Grazers also attack large numbers of prey during their lifetime, but they grazers remove only part of each prey individual rather than the whole. Their effect on a prey individual, although typically harmful, is rarely lethal in the short term, and certainly never predictably lethal (in which case they would be true predators). Amongst the more obvious examples are the large vertebrate herbivores like sheep and cattle, but the flies that bite a succession of vertebrate prey, and leeches that suck their blood, are also undoubtedly grazers by this definition.
Parasites, like grazers, consume parts of their prey (their 'host'), rather than parasites the whole, and are typically harmful but rarely lethal in the short term. Unlike grazers, however, their attacks are concentrated on one or a very few individuals during their life. There is, therefore, an intimacy of association between parasites and their hosts that is not seen in true predators and grazers. Tapeworms, liver flukes, the measles virus, the tuberculosis bacterium and the flies and wasps that form mines and galls on plants are all obvious examples of parasites. There are also many plants, fungi and microorganisms that are parasitic on plants (often called 'plant pathogens'), including the tobacco mosaic virus, the rusts and smuts and the mistletoes. Moreover, many herbivores may readily be thought of as parasites. For example, aphids extract sap from one or a very few individual plants with which they enter into intimate contact. Even caterpillars often rely on a single plant for their development. Plant pathogens, and animals parasitic on animals, will be dealt with together in Chapter 12. 'Parasitic' herbivores, like aphids and caterpillars, are dealt with here and in the next chapter, where we group them definition of predation taxonomic and functional classifications of predators true predators together with true predators, grazers and parasitoids under the umbrella term 'predator'.
The parasitoids are a group of parasitoids insects that belong mainly to the order
Hymenoptera, but also include many Diptera. They are free-living as adults, but lay their eggs in, on or near other insects (or, more rarely, in spiders or woodlice). The larval parasitoid then develops inside or on its host. Initially, it does little apparent harm, but eventually it almost totally consumes the host and therefore kills it. An adult parasitoid emerges from what is apparently a developing host. Often, just one parasitoid develops from each host, but in some cases several or many individuals share a host. Thus, parasitoids are intimately associated with a single host individual (like parasites), they do not cause immediate death of the host (like parasites and grazers), but their eventual lethality is inevitable (like predators). For parasitoids, and also for the many herbivorous insects that feed as larvae on plants, the rate of 'predation' is determined very largely by the rate at which the adult females lay eggs. Each egg is an 'attack' on the prey or host, even though it is the larva that hatches from the egg that does the eating.
Parasitoids might seem to be an unusual group of limited general importance. However, it has been estimated that they account for 10% or more of the world's species (Godfray, 1994). This is not surprising given that there are so many species of insects, that most of these are attacked by at least one parasitoid, and that parasitoids may in turn be attacked by parasitoids. A number of parasitoid species have been intensively studied by ecologists, and they have provided a wealth of information relevant to predation generally.
In the remainder of this chapter, we examine the nature of predation. We will look at the effects of predation on the prey individual (Section 9.2), the effects on the prey population as a whole (Section 9.3) and the effects on the predator itself (Section 9.4). In the cases of attacks by true predators and parasitoids, the effects on prey individuals are very straightforward: the prey is killed. Attention will therefore be placed in Section 9.2 on prey subject to grazing and parasitic attack, and herbivory will be the principal focus. Apart from being important in its own right, her-bivory serves as a useful vehicle for discussing the subtleties and variations in the effects that predators can have on their prey.
Later in the chapter we turn our attention to the behavior of predators and discuss the factors that determine diet (Section 9.5) and where and when predators forage (Section 9.6). These topics are of particular interest in two broad contexts. First, foraging is an aspect of animal behavior that is subject to the scrutiny of evolutionary biologists, within the general field of 'behavioral ecology'. The aim, put simply, is to try to understand how natural selection has favored particular patterns of behavior in particular circumstances (how, behaviorally, organisms match their environment). Second, the various aspects of predatory behavior can be seen as components that combine to influence the population dynamics of both the predator itself and its prey. The population ecology of predation is dealt with much more fully in the next chapter.
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