Individual differences asymmetric competition

5.10.1 Size inequalities

Until now, we have focused on what happens to the whole population or the average individual within it. Different individuals, however, may respond to intraspecific competition in very different ways. Figure 5.24 shows the results of an experiment in which flax (Linum usitatissimum) was sown at three densities, and harvested at three stages of development, recording the weight of each plant individually. This made it possible to monitor the effects of increasing amounts of competition not only as a result of variations in sowing density, but also as a result of plant growth (between the first and the last harvests). When intraspecific competition was at its least intense (at the lowest sowing density after only 2 weeks' growth) the individual plant weights were distributed symmetrically about the mean. When competition was at its most intense, however, the distribution was strongly skewed to the left: there were many very small individuals and a few large ones. As the intensity of competition gradually increased, the degree of skewness increased as well. Decreased size - but increased skewness in size - is also seen to

First harvest (2 weeks from emergence)

Second harvest (6 weeks from emergence)

Low density 60 m-2

Medium density 1440 m-2

4 1016 28 40

Second harvest (6 weeks from emergence)

40

-

nt

la

30

_

CD

20

-

E

u z

10

-

0

High density 3600 m-2

4 10 16 28 40 Plant weight (mg)

80 240 160

Plant weight (mg)

80 240 160

Final harvest (maturity)

be associated with increased density (and presumably competition) in cod (Gadus morhua) living off the coast of Norway (Figure 5.25).

More generally, we may also say that increased competition increased the degree of size inequality within the population, i.e. the extent to which total biomass was unevenly distributed amongst the different individuals (Weiner, 1990). Rather similar results have been obtained from a number of other populations of animals (Uchmanski, 1985) and plants (Uchmanski, 1985; Weiner & Thomas, 1986). Typically, populations experiencing the most intense competition have the greatest size inequality and often have a size distribution in which there are many small and a few large individuals. Characterizing a population by an arbitrary 'average' individual can obviously be very misleading under such circumstances, and can divert attention from the fact that intra-specific competition is a force affecting individuals, even though its effects may often be detected in whole populations.

Figure 5.24 Competition and a skewed distribution of plant weights. Frequency distributions of individual plant weights in populations of flax (Linum usitatissimum), sown at three densities and harvested at three ages. The black bar is the mean weight. (After Obeid et al., 1967.)

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