We have chosen to start this book with chapters about organisms, then to consider the ways in which they interact with each other, and lastly to consider the properties of the communities that they form. One could call this a 'constructive' approach. We could though, quite sensibly, have treated the subject the other way round - starting with a discussion of the complex communities of both natural and manmade habitats, proceeding to ieconstruct them at ever finer scales, and ending with chapters on the characteristics of the individual organisms - a more analytical approach. Neither is 'correct'. Our approach avoids having to describe community patterns before discussing the populations that comprise them. But when we start with individual organisms, we have to accept that many of the environmental forces acting on them, especially the species with which they coexist, will only be dealt with fully later in the book.
This first section covers individual organisms and populations composed of just a single species. We consider initially the sorts of correspondences that we can detect between organisms and the environments in which they live. It would be facile to start with the view that every organism is in some way ideally fitted to live where it does. Rather, we emphasize in Chapter 1 that organisms frequently are as they are, and live where they do, because of the constraints imposed by their evolutionary history. All species are absent from almost everywhere, and we consider next, in Chapter 2, the ways in which environmental conditions vary from place to place and from time to time, and how these put limits on the distribution of particular species. Then, in Chapter 3, we look at the resources that different types of organisms consume, and the nature of their interactions with these resources.
The particular species present in a community, and their abundance, give that community much of its ecological interest. Abundance and distribution (variation in abundance from place to place) are determined by the balance between birth, death, immigration and emigration. In Chapter 4 we consider some of the variety in the schedules of birth and death, how these may be quantified, and the resultant patterns in 'life histories': lifetime profiles of growth, differentiation, storage and reproduction. In Chapter 5 we examine perhaps the most pervasive interaction acting within single-species populations: intraspecific competition for shared resources in short supply. In Chapter 6 we turn to movement: immigration and emigration. Every species of plant and animal has a characteristic ability to disperse. This determines the rate at which individuals escape from environments that are or become unfavorable, and the rate at which they discover sites that are ripe for colonization and exploitation. The abundance or rarity of a species may be determined by its ability to disperse (or migrate) to unoccupied patches, islands or continents. Finally in this section, in Chapter 7, we consider the application of the principles that have been discussed in the preceding chapters, including niche theory, life history theory, patterns of movement, and the dynamics of small populations, paying particular attention to restoration after environmental damage, biosecurity (resisting the invasion of alien species) and species conservation.
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