ABSTRACT
Competition between organisms within the habitats making up an ecosystem has been defined in many ways, but Keddy (1989) has provided a succinct and clear definition: see box.
Competition may occur between any pair of organisms, whether they are from populations of the same species (intraspecific competition) or drawn from populations of different species (interspecific competition). Competition occurs only when two populations compete for a resource in limited supply
which is necessary for the survival of each. In these circumstances there is a tendency for the more competitive population to exclude the less successful one. Early experimental work on yeast (Gause 1932) and beetles (Tribolium: Park 1954; see Figure 7.1) in limited-resource experimental systems suggested that competitive exclusion is a general principle in ecology. Pairs of very similar species (in terms of size and environmental requirements, i.e. having closely similar niches) find it difficult to coexist in the same ecosystem because competitive pressures between
them are too strong. Ecological differentiation (Hardin 1960) appears to be necessary for species to coexist in crowded, competitive ecosystems. In practice, coexistence seems to be what happens in real (i.e. not artificial experimental) ecosystems. Competitive exclusion is rarely seen to occur and ecologists have devoted enormous efforts, and much imagination, in trying to develop models which can successfully explain the coexistence of species. These are discussed further by Keddy (1989). With a few exceptions, major (i.e. broadly distributed, dominant, successful) species must be good competitors. The exceptions are those which have successfully colonised extensive stressed or disturbed environments, where interspecific competition pressures are low and possession of genetic traits for tolerance of stress-or disturbance-related pressures on survival are, instead, at a premium (see Chapters 5 and 6). Good examples are those mosses (e.g. woollyfringe moss, Racomitrium lanuginosum)
which have adapted to the cold conditions of high-latitude upland and tundra areas, and which are common and widely distributed plants in these cold-stressed habitats.
