The activity of any organism changes the environment in which it lives. It may alter conditions, as when the transpiration of a tree cools the atmosphere, or it may add or subtract resources from the environment that might have been available to other organisms, as when that tree shades the plants beneath it. In addition, though, organisms interact when individuals enter into the lives of others. In the following chapters (8-15) we consider the variety of these interactions between individuals of different species. We distinguish five main categories: competition, predation, parasitism, mutualism and detritivory, although like most biological categories, these five are not perfect pigeon-holes.

In very broad terms, 'competition' is an interaction in which one organism consumes a resource that would have been available to, and might have been consumed by, another. One organism deprives another, and, as a consequence, the other organism grows more slowly, leaves fewer progeny or is at greater risk of death. The act of deprivation can occur between two members of the same species or between individuals of different species. We have already examined intraspecific competition in Chapter 5. We turn to interspecific competition in Chapter 8.

Chapters 9 and 10 deal with various aspects of 'predation', though we have defined predation broadly. We have combined those situations in which one organism eats another and kills it (such as an owl preying on mice), and those in which the consumer takes only part of its prey, which may then regrow to provide another bite another day (grazing). We have also combined herbivory (animals eating plants) and carnivory (animals eating animals). In Chapter 9 we examine the nature of predation, i.e. what happens to the predator and what happens to the prey, paying particular attention to herbivory because of the subtleties that characterize the response of a plant to attack. We also discuss the behavior of predators. Then, in Chapter 10, we examine the 'consequences of consumption' in terms of the dynamics of predator and prey populations. This is the part of ecology that has the most obvious relevance to those concerned with the management of natural resources: the efficiency of harvesting (whether of fish, whales, grasslands or prairies) and the biological and chemical control of pests and weeds - themes that we take up in Chapter 15.

Most of the processes in this section involve genuine interactions between organisms of different species. However, when dead organisms (or dead parts of organisms) are consumed -decomposition and detritivory - the affair is far more one-sided. None the less, as we describe in Chapter 11, these processes themselves incorporate competition, parasitism, predation and mutualism: microcosms of all the major ecological processes (except photosynthesis).

Chapter 12, 'Parasitism and Disease', deals with a subject that in the past was often neglected by ecologists - and by ecology texts. Yet more than half of all species are parasites, and recent years have seen much of that past neglect rectified. Parasitism itself has blurred edges, particularly where it merges into predation. But whereas a predator usually takes all or part of many individual prey, a parasite normally takes its resources from one or a very few hosts, and (like many grazing predators) it rarely kills its hosts immediately, if at all.

Whereas the earlier chapters of this section deal largely with conflict between species, Chapter 13 is concerned with mutualistic interactions, in which both organisms experience a net benefit. None the less, as we shall see, conflict often lies at the heart of mutualistic interactions too: each participant exploiting the other, such that the net benefit arises only because, overall, gains exceed losses. Like parasitism, the ecology of mutualism has often been neglected. Again, though, this neglect has been unwarranted: the greater part of the world's biomass is composed of mutualists.

Ecologists have often summarized interactions between organisms by a simple code that represents each one of the pair of interacting organisms by a '+', a '—' or a '0', depending on how it is affected by the interaction. Thus, a predator-prey (including a herbivore-plant) interaction, in which the predator benefits and the prey is harmed, is denoted by +—, and a parasite-host interaction is also clearly +—. Another straightforward case is mutualism, which, overall, is obviously + +; whereas if organisms do not interact at all, we can denote this by 0 0 (sometimes called 'neutralism'). Detritivory must be denoted by + 0, since the detritivore itself benefits, while its food (dead already) is unaffected. The general term applied to + 0 interactions is 'commensalism', but paradoxically this term is not usually used for detritivores. Instead, it is reserved for cases, allied to parasitism, in which one organism (the 'host') provides resources or a home for another organism, but in which the host itself suffers no tangible ill effects. Competition is usually described as a--interaction, but it is often impossible to establish that both organisms are harmed. Such asymmetric interactions may then approximate to a - 0 classification, generally referred to as 'amensalism'. True cases of amensalism may occur when one organism produces its ill effect (for instance a toxin) whether or not the potentially affected organism is present.

Although the earlier chapters in this section deal with these various interactions largely in isolation, members of a population are subject simultaneously to many such interactions, often of all conceivable types. Thus, the abundance of a population is determined by this range of interactions (and indeed environmental conditions and the availability of resources) all acting in concert. Attempts to understand variations in abundance therefore demand an equally wide ranging perspective. We adopt this approach in Chapter 14.

Finally in this section, we discuss in Chapter 15 applications of the principles elaborated in the preceding chapters. Our focus is on pest control and the management of natural resources. With the former, the pest species is either a competitor or a predator of desirable species (for example food crops), and we are either predators of the pest ourselves or we manipulate its natural predators to our advantage (biological control). With the latter, again, we are predators of a living, natural resource (harvestable trees in a forest, fish in the sea), but the challenge for us is to establish a stable and sustainable relationship with the prey, guaranteeing further valuable harvests for generations to come.

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