An indication of the way in which competition can exaggerate underlying inequalities in a population comes from observations on a natural, crowded population of the woodland annual Impatiens pallida in southeastern Pennsylvania. Over an 8-week period, growth was very much faster in large than in small plants - in fact, small plants did not grow at all (Figure 5.26a). This increased significantly the size inequality within the population (Figure 5.26b). Thus, the smaller a plant was initially, the more it was affected by neighbors. Plants that established early preempted or 'captured' space, and subsequently were little affected by intraspecific competition. Plants that emerged later entered a universe in which most of the available space had already been preempted; they were therefore greatly affected by intraspecific competition. Competition was asymmetric: there was a hierarchy. Some individuals were affected far more than others, and small initial differences were transformed by competition into much larger differences 8 weeks later.
the inadequacy of the average
If competition is asymmetric because superior competitors preempt resources, then competition is most likely to be asymmetric when it occurs for resources that are most liable to be preempted. Specifically, competition amongst plants for light, in which a superior competitor can overtop and shade an inferior, might be expected to lend itself far more readily to preemptive resource capture than competition for soil nutrients or water, where the roots of even a very inferior competitor will have more immediate access to at least some of the available resources than
Figure 5.25 (right) Values of skewness (in the frequency distribution of lengths) and density (a) and of skewness and mean length (b) are expressed as standard deviations from mean values for the years 1957-94 for cod (Gadus morhua) from the Skagerrak, off the coast of Norway. Despite marked fluctuations from year to year, much of it the result of variations in weather, skewness was clearly greatest at high densities (r = 0.58, P < 0.01) when lengths were smallest (r = -0.45, P < 0.05), that is, when competition was most intense. (After Lekve et al., 2002.)
Figure 5.26 Asymmetric competition in a natural population of Impatiens pallida. (a) The increase in mass of survivors of different sizes over an 8-week period, and the distribution of initial sizes of those individuals that died over the same period. The horizontal axis is the same in each case. (b) The distribution of individual weights at the beginning (Gini coefficient, a measure of inequality, 0.39) and the end of this period (Gini coefficient, 0.48). (After Thomas & Weiner, 1989.)
Plants that died
the roots of its superiors. This expectation is borne out by the results of an experiment in which morning glory vines (Ipomoea tricolor) were grown as single plants in pots ('no competition'), as several plants rooted in their own pots but with their stems intertwined on a single stake ('shoots competing'), as several plants rooted in the same pot, but with their stems growing up their own stakes ('roots competing') and as several plants rooted in the same pot with their stems intertwined on one stake ('shoots and roots competing') (Figure 5.27). Despite the fact that root competition was more intense than shoot competition, in the sense that it led to a far greater decrease in the mean weight of individual plants, it was shoot competition for light that led to a much greater increase in size inequality.
Skewed distributions are one possible manifestation of hierarchical, asymmetric competition, but there are many others. For instance, Ziemba and Collins (1999) studied competition amongst larval salamanders (Ambystoma tigrinum nebulosum) that were either isolated or grouped together with competitors. The size of the largest surviving larvae was unaffected by competition (P = 0.42) but the smallest larvae were much smaller (P < 0.0001). This emphasizes that intra-specific competition is not only capable of exaggerating individual differences, it is also greatly affected by individual differences.
Asymmetric competition was observed on a much longer timescale in a population of the herbaceous perennial Anemone hepatica in Sweden (Figure 5.28) (Tamm, 1956). Despite the crops of seedlings that entered the population between 1943 and 1952, it is quite clear that the most important factor determining which individuals survived to 1956 was whether or not they were established in 1943. Of the 30 individuals that had reached
Figure 5.27 When morning glory vines competed, root competition was most effective in reducing mean plant weight (treatments significantly different, P <0.01, for all comparisons except (c) with (d)), but shoot competition was most effective in increasing the degree of size inequality, as measured by the coefficient of variation in weight (significant differences between treatments (a) and (b), P < 0.05, and (a) and (d), P < 0.01). (After Weiner, 1986.)
large or intermediate size by 1943, 28 survived until 1956, and some of these had branched. By contrast, of the 112 plants that were either small in 1943 or appeared as seedlings subsequently, only 26 survived to 1956, and not one of these was sufficiently well established to have flowered. Similar patterns can be observed in tree populations. The survival rates, the birth rates and thus the fitnesses of the few established adults are high; those of the many seedlings and saplings are comparatively low.
These considerations illustrate a final, important general point: asymmetries tend to reinforce the regulatory powers of intraspecific competition. Tamm's established plants were successful competitors year after year, but his small plants and seedlings were repeatedly unsuccessful. This guaranteed a near constancy in the number of established plants between 1943 and 1956. Each year there was a near-constant number of 'winners', accompanied by a variable number of 'losers' that not only failed to grow, but usually, in due course, died.
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