Читать книгу Ecology - Michael Begon - Страница 123

Of course, the intensity of intraspecific competition experienced by an individual is not really determined by the density of the population as a whole. The effect on an individual is determined, rather, by the extent to which it is crowded or inhibited by its immediate neighbours. Even in a population of mobile animals, individuals are unlikely to move around enough to interact with every other member of the population.

three meanings of density

One way of emphasising this is by noting that there’s more than one meaning of ‘density’ (see Lewontin & Levins, 1989 , where details of calculations and terms can be found). Consider a population of insects, distributed over a population of plants (patches of resource) on which they feed. The density would usually be calculated simply as the number of insects (let us say 1000) divided by the number of plants (say 100), i.e. 10 insects per plant. However, this is actually the ‘resource‐weighted density’, but it only gives an accurate measure of the intensity of competition suffered by the insects (the extent to which they are crowded) if there are exactly 10 insects on every plant.

Suppose, instead, that 10 of the plants support 91 insects each, and the remaining 90 support just one insect. The resource‐weighted density would still be 10 insects per plant. But to determine average density experienced by the insects, we note that 910 of them experienced a density of 91 insects per plant and 90 experienced just one per plant, giving us an overall average of 82.9 insects per plant (910 × 91, plus 90 × 1, divided by 1000) This is the ‘organism‐weighted density’, and it clearly gives a much more satisfactory measure of the intensity of competition the insects are likely to suffer. Clearly, the normal practice of calculating the resource‐weighted density and calling it ‘the density’ can be misleading.

neighbours

The difficulties of relying on density to characterise the potential intensity of intraspecific competition are particularly acute with sessile, modular organisms, because, being sessile, they compete almost entirely only with their immediate neighbours, and being modular, competition is directed most at the modules that are closest to those neighbours. We can see this, for example, when seedlings of three species of fir tree, Douglas fir (Pseudotsuga menziesii), grand fir (Abies grandis) and noble fir (A. procera), were grown together in conspecific pairs or grown alone. In all three, after two years, the diameter growth of branches facing their competitor was significantly reduced in trees grown in pairs compared with the growth of branches of trees grown alone (Figure 5.10a). But these effects of competition were expressed almost entirely by those branches facing their neighbour (Figure 5.10b). Those on the other side of the tree had growth rates similar to those of the isolated plants. The individual plants, and their component modules, were affected by the proximity of their competitors rather than density per se.

Figure 5.10 When modular organisms compete, the modules closest to neighbours are the most affected. (a) The growth of ‘facing’ branches of three species of fir tree after two years of growth either in pairs (where the branches faced the competitor) or in isolation (where ‘facing’ branches grew in the same direction but with no competitor). (b) The ratio of the growth of facing branches (from (a)) to ‘opposing’ branches on the opposite side of the tree seedlings. In isolation, the ratios were not significantly different from 1; but in competition, the ratios indicate that the growth of opposing branches was similar to that of all branches in the isolated plants. Bars are 95% CIs.

Source: From data in Devine & Harrington (2011).

density: a convenient expression of crowding

Thus, whether mobile or sessile, different individuals meet or suffer from different numbers of competitors. Density, especially resource‐weighted density, is an abstraction that applies to the population as a whole but need not apply to any of the individuals within it. Nonetheless, density may often be the most convenient way of expressing the degree to which individuals are crowded – and it is certainly the way it has usually been expressed.