Introduction: the types of predators

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. We begin here by asking 'What is the nature of predation?' and 'What are the effects of predation on the predators themselves and on their prey?'   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. 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. These classifications will not be pursued with the aim of constructing a neat and tidy catalogue, or of settling any issues in semantics. However, by distinguishing various types of predator, and establishing what characteristics they share and how they differ, we will be able to develop a fuller understanding of the precise nature of predation. The most obvious classification, perhaps, is 'taxonomic': carnivores consume animals, herbivores consume plants and omnivores consume both. An alternative, however, is a 'functional' classifica­tion (Thompson, 1982). Here, there are four main types of predator: true predators, grazers, parasitoids and parasites, (the last divisible further into microparasites and macroparasites).

True predators kill their prey more or less immediately after attacking them; during their lifetime they kill several or many different prey individuals. Often, they consume prey in their entirety, but some true predators consume only parts of their prey. 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, one after the other, during their lifetime, but they remove only a 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 herbivorous like sheep and cattle, but the flies that bite vertebrates, and leeches that suck blood are also undoubtedly grazers by this definition.

Parasites, like grazers, consume parts of their prey (their ‘host’) rather than the whole. Also like grazers, their attacks 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 the course of their life. There is, therefore, an intimacy of association between parasites and their hosts which is not seen in 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 considerable numbers of plants, fungi and microorganisms that are parasitic on plants (often called ‘plant pathogens’), for instance, the rusts and smuts and the mistletoes. Moreover, there are also herbivores that 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 (although the association here is not so intimate). Caterpillars may be grazers (where they take small parts of several plants) or even true predators (where they destroy several plants – perhaps young seedlings) – and many undoubtedly may be somewhere on a continuum between these categories.

The parasitoids are a group of insects that are classified as such as the basis of the egg-laying behavior of the adult female and the subsequent developmental pattern of the larva. They are free-living as adults, but they lay their eggs in, on or near other insects (or, more rarely, in spiders or woodlice). The larvae of parasitoid then develops inside its host individual, which is usually a pre-adult. Initially, it does little apparent harm to the host, but eventually it almost totally consumes the host and therefore kills it before or during the pupal stage. An adult parasitoid, rather than an adult host, emerges from what is apparently a host pupa. Often, just one parasitoid develops from each host, but in some cases several 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 adult females oviposit. Each egg laid by the female is an 'attack' on the prey or host, even though it is the larva that develops from the egg that actually does the eating.

Parasitoids may 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 when we consider that there are so many species of insects, that most of these are attacked by at least one parasitoid and that even parasitoids themselves may be attacked by parasitoids. Moreover, a number of parasitoid species have been intensively studied by ecologists, and they have provided a wealth of information relevant to predation generally.

The effects of herbivory on a plant depend on precisely which parts are affected, and on the timing of the attack relative to the plant's development. Leaf biting, sap sucking, mining, meristem consumption, flower and fruit damage and root pruning are all likely to differ in the effect they have on the plant. The consequences of defoliating a germinating seedling are unlikely to be the same as those of defoliating a plant that is setting its own seed. Moreover, because the plant usually remains alive in the short term, the effects of herbivory are crucially dependent on the response of the plant. Minerals or nutrients may be diverted from one part to another, the overall rate of metabolism may change, the relative rates of root growth, shoot growth and reproduction may alter and special protective chemicals or tissues may be produced. Overall, the effect of a herbivore may be more drastic than it appears, or less drastic. It is only rarely what it seems.

THE BEHAVIOUR OF PREDATORS

Introduction

In this article we discuss the behaviour of predators. We examine where they feed what they feed on, how they are affected by other predators and how they are affected by the density of their prey. These topics are of interest in their own right, but they are also relevant in two other, broader contexts. First, foraging is an аspect of animal behaviour that is subject to the scrutiny of evolutionary biologists, within the general field of 'behavioural ecology'. The aim, put simply, is to try to understand how natural selection has favoured particular patterns of behaviour in particular circumstances (how, behaviourally, organisms match their environment).

Second, the various aspects of predatory behaviour can be seen as components that combine to influence the population dynamics of both the predator itself and its prey. It will be useful at various points to suggest the ways in which individual behaviour may affect population dynamics. If predation is such that one or other of the population is subjected to density dependent increases in mortality (or decreases in birth rate), then this will tend to regulate the size of that population within certain limits, i.e. it will tend to stabilize the dynamics of the population. This in turn will tend to stabilize the population dynamics of the interaction as a whole (the populations will be persistent and show relatively little variation in abundance). Conversely, if there is 'inverse density dependence', such that mortality decreases (or birth rate increase) with increasing density, then this will tend to destabilize the dynamics of the interaction. Predatory behaviour can clearly have a significance beyond its effect on the individuals concerned.