Introduction

The two phenomena mentioned in the title of this chapter are closely related to interspecific competition, but also differ from it in important ways. Intraspecific com­petition involves effects of the abundance of a consumer species on its own population growth, while apparent competition involves effects of one species on the popula­tion growth of others that are transmitted by one or more shared consumer species rather than shared resources.

Intraspecific competition always accompanies interspecific competition and influ­ences its effect on population density. Apparent competition is not an absolutely necessary component of either intra- or interspecific competition, because it does not take place in systems having a single resource or in systems with abiotic resources. However, single-resource systems are extremely rare. Most real consumer species have two or more resources, and apparent competition occurs between the resources if they are living entities. In fact, apparent competition is a key determinant of the form of intraspecific competition in the single consumer, no matter how that form is measured. When intraspecific and apparent competition co-occur in a two-trophic- level system, the important distinguishing feature is the identity of the initiator and receiver of the effect. The two focal species occupy the lower trophic level in the case of apparent competition, and the focal species occupies the higher trophic level for intraspecific competition. ‘Real’ and apparent competition are closely connected with living resources because most consumers utilize two or more resources, and the nature of intraspecific competition in such species is always influenced by the indi­rect interaction of the resources via the consumer (apparent competition between the resources).

Intraspecific competition is closely related to the process of natural selection with­in a species.

Competition Theory in Ecology. Peter A. Abrams, Oxford University Press. © Peter A. Abrams (2022).

DOI: 10.1093∕oso∕9780192895523.003.0005 identical to those of a system with interspecific competition. The one difference is the possibility that one genotype gives rise to individuals of the other type via mutation.

While arguing that apparent competition should be viewed as distinct from ‘tra­ditional’ (resource-based) competition, I will also suggest that, like interspecific competition, it should be explored using a wider range of consumer-resource models than has generally been employed in the past. Models of apparent competition have always included the dynamics of the transmitting entity or entities (the consumer(s)). This makes the body of theory somewhat stronger than that for ‘normal’ competi­tion. However, most of the literature on apparent competition has been restricted to the simplest possible system—two dynamically independent resource species with a single shared consumer species, usually assuming a stable equilibrium. This limita­tion seems to have been even more prevalent than the corresponding focus of models of interspecific competition on systems having just two consumers and at most two resource types. Apparent competition was unnamed and virtually unstudied before Holt’s (1977) article, so it has had a shorter history. Perhaps as a consequence, models having some nonlinear functional components have been more common than in the literature on competition for resources.

5.2