Intraspecific competition and densitydependent mortality and fecundity
Figure 5.3 shows the pattern of mortality in the flour beetle Tribolium confusum when cohorts were reared at a range of densities. Known numbers of eggs were placed in glass tubes with 0.5 g of a flouryeast mixture, and the number of individuals that survived to become adults in each tube was noted. The same data have been expressed in three ways, and in each case the resultant curve has been divided into three regions. Figure 5.3a describes the relationship between density and the per capita mortality rate  literally, the mortality rate 'per head', i.e. the probability of an individual dying or the proportion that died between the egg and adult stages. Figure 5.3b describes how the number that died prior to the adult stage changed with density; and Figure 5.3c describes the relationship between density and the numbers that survived.
Figure 5.2 Those red deer that are smallest when born are the least likely to survive over winter when, at higher densities, survival declines. (After CluttonBrock et al., 1987.)
Throughout region 1 (low density) the mortality rate remained constant as density was increased (Figure 5.3a). The numbers dying and the numbers surviving both rose (Figure 5.3b, c) (not surprising, given that the numbers 'available' to die and survive increased), but the proportion dying remained the same, which accounts for the straight lines in region 1 of these figures. Mortality in this region is said to be density independent. Individuals died, but the chance of an individual surviving to become an adult was not changed by the initial density. Judged by this, there was no intraspecific competition between the beetles at these densities. Such densityindependent deaths affect the population at all densities. They represent a baseline, which any densitydependent mortality will exceed.
In region 2, the mortality rate increased with density (Figure 5.3a): there was densitydependent mortality. The numbers dying continued to rise with density, but unlike region 1 they did so more than proportionately (Figure 5.3b). The numbers surviving also continued to rise, but this time less than proportionately (Figure 5.3c). Thus, over this range, increases in egg density continued to lead to increases in the total number of surviving adults. The mortality rate had increased, but it 'undercompensated' for increases in density.
In region 3, intraspecific competition was even more intense. The increasing mortality rate 'overcompensated' for any increase in density, i.e. over this range, the more eggs there were present, the fewer adults survived: an increase in the initial number of eggs led to an even undercompensating density dependence overcompensating density dependence
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Figure 5.3 Densitydependent mortality in the flour beetle Tribolium confusum: (a) as it affects mortality rate, (b) as it affects the numbers dying, and (c) as it affects the numbers surviving. In region 1 mortality is density independent; in region 2 there is undercompensating densitydependent mortality; in region 3 there is overcompensating densitydependent mortality. (After Bellows, 1981.)
greater proportional increase in the mortality rate. Indeed, if the range of densities had been extended, there would have been tubes with no survivors: the developing beetles would have eaten all the available food before any of them reached the adult stage.
A slightly different situation is exactly compensating shown in Figure 5.4. This illustrates density dependence the relationship between density and mortality in young trout. At the lower densities there was undercompensating density dependence, but at higher densities mortality never overcompensated. Rather, it compensated exactly for any increase in density: any rise in the number of fry was matched by an exactly equivalent rise in the
mortality rate. The number of survivors therefore approached and maintained a constant level, irrespective of initial density.
The patterns of densitydependent fecundity that result from intraspecific competition are, in a sense, a mirrorimage of those for mortality (Figure 5.5). Here, though, the per capita birth rate falls as intraspecific competition intensifies. At low enough densities, the birth rate may be density independent (Figure 5.5a, lower densities). But as density increases, and the effects of intraspecific competition become apparent, birth rate initially shows undercompensating density dependence (Figure 5.5a, higher densities), and may then show exactly compensating density dependence (Figure 5.5b, throughout; Figure 5.5 c, lower densities) or overcompensating density dependence (Figure 5.5 c, higher densities).
Thus, to summarize, irrespective of variations in over and undercompensation, the essential point is a simple one: at appropriate densities, intraspecific competition can lead to densitydependent mortality and/or fecundity, which means that the death rate increases and/or the birth rate decreases as density increases. Thus, whenever there is intraspecific competition, its effect, whether on survival, fecundity or a combination of the two, is density dependent. However, as subsequent chapters will show, there are processes other than intraspecific competition that also have densitydependent effects.
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