Appraisal of the role of competition

We can now draw a number of conclusions about the evidence for competition discussed in this section.

1 Interspecific competition is a possible and indeed a plausible explanation for many aspects of the organization of many communities - but it is not often a proven explanation.

2 One of the main reasons for this is that active, current competition has been studied and demonstrated in only a small number of communities. Its actual prevalence overall can be judged only imperfectly from the results and considerations discussed above.

3 As an alternative to current competition, the ghost of competition past can always be invoked to account for present-day patterns. But it can be invoked so easily because it is impossible to observe directly and therefore is difficult to disprove.

4 The communities chosen for study may not be typical. The ecologists observing them have usually been specifically interested in competition, and they may have selected appropriate, 'interesting' systems. Studies that fail to show niche differentiation may often have been considered 'unsuccessful' and are likely to have gone largely unreported.

5 The community patterns uncovered, even where they appear to support the competition hypothesis, often have alternative explanations. For example, species that have negatively associated distributions may have recently speciated allopatric-ally, and their distributions may still be expanding into one another's ranges.

6 The recurring alternative explanation to competition as the cause of community patterns is that these have arisen simply by chance. Niche differentiation may occur because the various species have evolved independently into specialists, and their specialized niches happen to be different. Even niches arranged along a resource dimension at random are bound to differ to some extent. Similarly, species may differ in their distribution because each has been able, independently, to colonize and establish itself in only a small proportion of the habitats that are suitable for it. Ten blue and 10 red balls thrown at random into 100 boxes are almost certain to end up with different distributions. Hence, competition cannot be inferred from mere 'differences' alone. But, what sorts of differences do allow the action of competition to be inferred? This is the domain of the null model approach.

7 The aim of the null model approach, whether applied to niche differentiation, morphological patterns or negatively associated distribution patterns, is undoubtedly worthy. We need to guard against the temptation to see competition in a community simply because we are looking for it. On the other hand, the approach is bound to be of limited use unless it is applied to groups (usually guilds) within which competition may be expected. In its favor, the null model approach concentrates the minds of investigators, and it can stop them from jumping to conclusions too readily. Ultimately, though, it can never take the place of a detailed understanding of the field ecology of the species in question, or of manipulative experiments designed to reveal competition by increasing or reducing species abundances (Law & Watkinson, 1989). It can only be part of the community ecologist's armory.

8 Interspecific competition is certain to vary in importance from community to community: it has no single, general role. For example, it appears frequently to be important in vertebrate communities, particularly those of stable, species-rich environments, and in communities dominated by sessile organisms such as plants and corals; whilst, for example, in some phytophagous insect communities it is less often important. A challenge for the future is to understand why some guilds show evidence for a role for competition, such as regularity in size ratios, whilst others do not (Hopf et al., 1993).

9 Finally, we should not lose sight of the fact that community organization in field studies is almost certain to be influenced by more than one kind of population interaction; for example the anenome fish (see Section 19.2.3.1) and ectomycorrhizal fungal cases (see Section 19.2.3.2) both involved mutualism as well as competition, and the mantids in Section 19.2.3.3 were intraguild predators as well as competitors. The interaction between predation and competition can be particularly influential, as we shall see in Section 19.4.

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