Structuring power of competition

Even when competition is potentially intense, the species concerned may nevertheless coexist. This has been highlighted in theoretical studies of model communities in which species compete for patchy and ephemeral resources, and in which species themselves have aggregated distributions, with each species distributed independently of the others (e.g. Atkinson & Shorrocks, 1981; Shorrocks & Rosewell, 1987; see Chapter 8). The species exhibited 'current competition' (like that in Schoener's and Connell's surveys), in

Parts of plants -see diagram

Feeding methods

Rachls Pinna Costa Costule

Feeding methods

Rachls Pinna Costa Costule

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Suck

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1

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Pinna, 'leaf' blade

-Rachis, stem

Pinna, 'leaf' blade

-Rachis, stem

Costa, stalk of pinna

Costule, lateral veins of pinna, joining costa

the strength of competition is likely to vary from community to community

Atkinson and

Shorrock's simulations

Figure 19.2 Feeding sites and feeding methods of herbivorous insects attacking bracken (Pteridium aquilinum) on three continents. (a) Skipwith Common in northern England; the data are derived from both a woodland and a more open site. (b) Hombrom Bluff, a savanna woodland in Papua New Guinea. (c) Sierra Blanca in the Sacramento Mountains of New Mexico and Arizona, USA; as at Skipwith, the data here are derived from both an open and a wooded site. Each bracken insect exploits the frond in a characteristic way. Chewers live externally and bite large pieces out of the plant; suckers puncture individual cells or the vascular system; miners live inside tissues; and gall-formers do likewise but induce galls. Feeding sites are indicated on the diagram of the bracken frond. Feeding sites of species exploiting more than one part of the frond are joined by lines. •, open and woodland sites; o, open sites only. (After Lawton, 1984.)

that the removal of one species led to an increase in the abundance of others. But, despite the fact that competition coefficients were high enough to lead to competitive exclusion in a uniform environment, the patchy nature of the environment and the aggregative behaviors of individuals of the species made coexistence possible without any niche differentiation. Thus, even if interspecific competition is actually affecting the abundance of populations, it need not determine the species composition of the community. In a field study of 60 insect taxa (Diptera and Hymenoptera) that exploit the patchy resources provided by 66 mushroom taxa, Wertheim et al. (2000) found insect coexistence could be explained by intraspecific aggregation in the manner described above, while resource partitioning was judged not to contribute detectably to biodiversity.

On the other hand, even when interspecific competition is absent or difficult to detect, this does not necessarily mean that it is unimportant as a structuring force. Species may not compete at present because selection in the past favored an avoidance of competition, and thus a differentiation of niches (Connell's 'ghost of competition past' - see Chapter 8). Alternatively, unsuccessful competitors may already have been driven to extinction; the present, observed species may then simply be those that are able to exist because they compete very little or not at all with other species. Furthermore, species may compete only rarely (perhaps during population outbreaks), or only in localized patches of especially high density, but the results of such competition may be crucial to their continued existence at a particular location. In all of these cases, interspecific competition must be seen as a powerful influence on community structure, affecting which species coexist and the precise nature of those species. Yet, this influence will not be reflected in the level of current competition. It is clear that the intensity of current competition may sometimes be linked only weakly to the structuring power of competition within the community.

This weak link has led a number of community ecologists to carry out studies on competition that do not rely on the existence of current competition. The approach has been first to predict what a community should look like if interspecific competition was shaping it or had shaped it in the past, and then to examine real communities to see whether they conform to these predictions.

The predictions themselves emerge readily from conventional competition theory (see Chapter 8).

1 Potential competitors that coexist in a community should, at the very least, exhibit niche differentiation (see Section 19.2.3).

2 This niche differentiation will often manifest itself as morphological differentiation (see Section 19.2.4).

3 Within any one community, potential competitors with little or no niche differentiation would be unlikely to coexist. Their distributions in space should therefore be negatively associated: each should tend to occur only where the other is absent (see Section 19.2.5).

In the following sections, we will discuss studies that deal with the documentation of patterns consistent with a role for competition in structuring communities.

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Responses

  • rudi
    What are the patterns we should look for to determine if competition is structuring a community?
    11 months ago

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