Net recruitment curves

An alternative general view of intraspecific competition is shown in Figure 5.8a, which deals with numbers rather than rates. The difference there between the two curves ("births minus deaths' or 'net recruitment') is the net number of additions expected in the population during the appropriate stage or over one interval of time. Because of the shapes of the birth and death curves, the net number of additions is small at the lowest densities, increases as density rises, declines again as the carrying capacity is approached and is then negative (deaths exceed births) when the initial density exceeds K (Figure 5.8b). Thus, total recruitment into a population is small when there are few individuals available to give birth, and small when intraspecific competition is intense. It reaches a peak, i.e. the population increases in size most rapidly, at some intermediate density.

The precise nature of the relationship between a population's net rate of recruitment and its density varies with the detailed biology of the species concerned (e.g. the trout, clover plants, herring and whales in Figure 5.10a-d). Moreover, because recruitment is affected by a whole multiplicity of factors, the data points rarely fall exactly on any single curve. Yet, in each case in Figure 5.10, a domed curve is apparent. This reflects the general nature of density-dependent birth and death whenever there is intraspecific competition. Note also that one of these (Figure 5.10b) is modular: it describes the relationship between the leaf area index (LAI) of a plant population (the total leaf area being borne per unit area of ground) and the population's growth rate (modular birth minus modular death). The growth rate is low when there are few leaves, peaks at an intermediate LAI, and is then low again at a high LAI, where there is much mutual shading and competition and many leaves may be consuming more in respiration than they contribute through photosynthesis.

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