In nature, areas of land and volumes of water contain assemblages of different species, in different proportions and doing different things. These communities of organisms have properties that are the sum of the properties of the individual denizens plus their interactions. The interactions are what make the community more than the sum of its parts. Just as it is a reasonable aim for a physiologist to study the behavior of different sorts of cells and tissues and then attempt to use a knowledge of their interactions to explain the behavior of a whole organism, so ecologists may use their knowledge of interactions between organisms in an attempt to explain the behavior and structure of a whole community. Community ecology, then, is the study of patterns in the structure and behavior of multispecies assemblages. Ecosystem ecology, on the other hand, is concerned with the structure and behavior of the same systems but with a focus on the flux of energy and matter.

We consider first the nature of the community. Community ecologists are interested in how groupings of species are distributed, and the ways these groupings can be influenced by both abiotic and biotic environmental factors. In Chapter 16 we start by explaining how the structure of communities can be measured and described, before focusing on patterns in community structure in space, in time and finally in a more complex, but more realistic spatiotemporal setting.

Communities, like all biological entities, require matter for their construction and energy for their activities. We examine the ways in which arrays of feeders and their food bind the inhabitants of a community into a web of interacting elements, through which energy (Chapter 17) and matter (Chapter 18) are moved. This ecosystem approach involves primary producers, decomposers and detritivores, a pool of dead organic matter, herbivores, carnivores and parasites plus the physicochemical environment that provides living conditions and acts both as a source and a sink for energy and matter. In Chapter 17, we deal with large-scale patterns in primary productivity before turning to the factors that limit productivity, and its fate, in terrestrial and aquatic settings. In Chapter 18, we consider the ways in which the biota accumulates, transforms and moves matter between the various components of the ecosystem.

In Chapter 19 we return to some key population interactions dealt with earlier in the book, and consider the ways that competition, predation and parasitism can shape communities. Then in Chapter 20 we recognize that the influence of a particular species often ramifies beyond a particular competitor, prey or host population, through the whole food web. The study of food webs lies at the interface of community and ecosystem ecology and we focus both on the population dynamics of interacting species in the community and on the consequences for ecosystem processes such as productivity and nutrient flux.

In Chapter 21 we attempt an overall synthesis of the factors, both abiotic and biotic, that determine species richness. Why the number of species varies from place to place, and from time to time, are interesting questions in their own right as well as being questions of practical importance. We will see that a full understanding of patterns in species richness has to draw on an understanding of all the ecological topics dealt with in earlier chapters of the book.

Finally, in the last of our trilogy of chapters dealing with the application of ecological theory, we consider in Chapter 22 the application of theory related to succession, food web ecology, ecosystem functioning and biodiversity. We conclude by recognizing that the application of ecological theory never proceeds in isolation - the sustainable use of natural resources requires that we also incorporate economic and sociopolitical perspectives.

To pursue an analogy we introduced earlier, the study of ecology at the community/ecosystem level is a little like making a study of watches and clocks. A collection can be made and the contents of each timepiece classified. We can recognize characteristics that they have in common in the way they are constructed and patterns in the way they behave. But to understand how they work, they must be taken to pieces, studied and put back together again. We will have understood the nature of natural communities when we know how to recreate those that we have, often inadvertently, taken to pieces.

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