The previous chapter dealt with the effects of competition on species. Here we examine effects in communities, although it should be noted that there is some overlap: effects on species and communities cannot always be clearly distinguished.

Isolationist (individualistic, non-interactive) and interactive communities

Wiens (1984) distinguished interactive communities (structured by interactive processes, mainly competition), and non-interactive communities (communities largely “structured” by individualistic responses of species). He points out that most studies deal with interactive systems. Holmes and Price (1986) applied this distinction to parasite communities, distinguishing interactive and isolationist (non-interactive) infracommunities of parasites. In the former, colonization probabilities of hosts are high, and communities are likely to be saturated and equilibrial. In isolationist communities, probabilities of colonization are low, resulting in unsaturated, nonequilibrial communities. Holmes and Price, in their synthesis, conclude that distinguishing between isolationist and interactive communities is “probably too crude to be of lasting utility.”

In the following section, I give some examples of interactive communities with evidence for interspecific competition, and of isolationist communities without such evidence.

Examples of competition in communities

Although evidence in many cases is poor, it seems nevertheless that competition is of some importance in many communities (for parasites see some contributions in Esch et al. 1990, and Lello et al. 2004).

Schoener (1983) reviewed evidence for interspecific competition in the past literature: in 90% of the studies and 76% of the species, some degree of competition was found.