Читать книгу Ecology - Michael Begon - Страница 139

The processes by which these size inequalities may be generated, and also ultimately diminished, are illustrated by a study of competition in two separate species of large brown seaweed, Laminaria digitata and Fucus serratus, off the Isle of Man, UK, reared at a range of densities, from 650 to 5156 plants per m² (Figure 5.28). We can see, firstly, that for all populations there was, for a large part of the experiment, a marked tendency for size inequality to increase (Figure 5.28a). The Gini coefficient (a measure of inequality originally developed by economists to capture inequalities in wealth), applied to frond (leaf) lengths, increased steadily as the plants grew, that is, as mean mass increased and competition intensified. However, for many of the populations, as the plants grew further still, the Gini coefficient declined again.

Figure 5.28 Size inequalities first increase then decrease in competing populations of seaweeds. (a) The effect of density on the relationship between inequality of plant frond lengths (measured by Gini coefficients) and mean plant mass in populations of Laminaria digitata (above) and Fucus serratus (below) at the range of densities indicated. Data are means (n = 4). Curved lines are fitted by polynomial functions to indicate the general pattern of change between variables. Arrows indicate the direction of the time sequence. (b) The relative growth rates of L. digitata plants grown singly and in populations consisting of two or three sizes of plants over four selected time periods. Data are means of n = 6 singly grown plants and n = 3 population means for portions of populations. Bars are numbered to indicate initial plant size (in cm). Bars are SEs. (c) The frond length of L. digitata and F. serratus plants at the sample time before death, plotted against the population mean frond length at that time. Two densities for each species are plotted to demonstrate the generality of the relationships. The diagonal line represents the condition where the length of the dying plants equals the population mean plant length.

Source: After Creed et al. (1998).

The process driving the initial increases in inequality is illustrated in Figure 5.28b. This shows data on the relative growth rates of L. digitata plants of different initial lengths (5 cm, 10 cm and 15 cm). Relative growth rates measure changes in mass relative to initial mass, which makes sense since without this correction, large plants would almost inevitably grow faster than small ones. The plants were combined in populations of different size‐composition but at a high density (7619 plants m^–2) where competition would have been intense. Plants were also grown in isolation. When grown singly, the smallest plants grew fastest, at least initially. However, as soon as they were combined, either with two or three size‐classes together, the growth‐rate differentials were reversed, with the smallest plants growing least and the largest most. Indeed, in some cases, the growth of the larger plants was barely affected by competition. Size inequality increased, therefore, because plants that were smaller initially were more affected by their neighbours. Small initial differences were transformed by competition into much larger differences over time. Competition amongst the seaweeds was therefore asymmetric: there was a hierarchy. It seems that the larger plants pre‐empted or ‘captured’ space and resources, and subsequently were little affected by intraspecific competition, while the smaller plants were being starved of these resources. Berntson and Wayne (2000) provide a rare empirical confirmation of such size‐uptake relationships for developing stands of birch seedlings (Betula alleghaniensis).

On the other hand, later in the main experiment (Figure 5.28a) size inequality decreased in many cases. The reason is apparent in Figure 5.28c, which shows that the plants that died in the populations, ultimately, were the smallest ones. Thus, while size‐dependent growth‐differentials increased the size inequalities, size‐dependent survival‐differentials decreased them again by cutting off the tail of the size distribution. The modular nature of plants makes this separation of processes especially likely. There can be an extended period over which the smaller, weaker competitors are stunted in their growth but do not die, and only later, after the inequalities have been exaggerated, do differentials in survival reverse this. For most (unitary) animals, stunting is not an option and the weakest competitors die far sooner. As we have seen, however, patterns in animals like those seen in plants, while perhaps rarer, are certainly not unknown.

roots and shoots: the strength and asymmetry of competition

If competition is asymmetric because superior competitors pre‐empt resources, then competition is most likely to be asymmetric when it occurs for resources that are most liable to be pre‐empted. 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 pre‐emptive 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 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 5.29). 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. However, it was shoot competition for light that led to a much greater increase in size inequality and determined which individuals were dominant – a general conclusion drawn by Kiær et al. (2013) in their meta‐analysis of root and shoot competition.

Figure 5.29 Root and shoot competition can have contrasting effects on mean size and size inequalities. 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).

Source: After Weiner (1986).