Individual species can influence the composition of whole communities in a variety of ways. Every species provides resources for others that prey upon or parasitize them, but some species (trees, for example) provide a wide range of resources that are used by a large number of consumer species (discussed in Chapter 3). Thus, oak trees can be very influential in determining the composition and diversity of the communities of which they are part by providing acorns, leaves, stemwood and roots for their specialist herbivores as well as a similar range of dead organic materials that are exploited by detritivores and decomposers (see Chapter 11). Species may also help determine community composition and diversity by influencing conditions (see Chapter 2). Thus, large plants create microhabitats that encompass the niche requirements of many smaller plants and animals, whilst large animals provide a range of conditions on and in their bodies that are exploited by a variety of parasites (see Chapter 12). During succession we have also seen that some early colonizers facilitate the entry of later species by changing conditions in a way that favors the latter (see Chapter 16). We will not dwell further on these processes.

The current chapter pays particular attention to the way that competition, predation and parasitism can shape communities. The ideas we present reflect a debate that has been central to ecology for the last four decades. As we explain below, there are sound theoretical reasons for expecting interspecific competition to be important in shaping communities by determining which, and how many, species can coexist. Indeed, the prevalent view amongst ecologists in the 1970s was that competition was of overriding importance (MacArthur, 1972; Cody, 1975). Subsequently, the conventional wisdom has moved away from this monolithic view to one giving more prominence to nonequilibrial and stochastic factors, such as physical disturbance and inconstancy in conditions (see Chapter 16), and to an important role for predation and parasitism (e.g. Diamond & Case, 1986; Gee & Giller, 1987; Hudson & Greenman, 1998). We first consider the role of interspecific competition in theory and practice before proceeding to the other population interactions that in some communities and for some organisms make competition much less influential.

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