Читать книгу Ecology - Michael Begon - Страница 126

Figures 5.12a–c reiterate the fact that as density increases, the per capita birth rate eventually falls and the per capita death rate eventually rises. There must, therefore, be a density at which these curves cross. At densities below this point, the birth rate exceeds the death rate and the population increases in size. At densities above the crossover point, the death rate exceeds the birth rate and the population declines. At the crossover density itself, the two rates are equal and there is no net change in population size. This density therefore represents a stable equilibrium, in that all other densities will tend to approach it. In other words, intraspecific competition, by acting on birth rates and death rates, can regulate populations at a stable density at which the birth rate equals the death rate. This density is known as the carrying capacity of the population and is usually denoted by K (Figure 5.12). It is called a carrying capacity because it represents the population size that the resources of the environment can just maintain (‘carry’) without a tendency to either increase or decrease.

real populations lack simple carrying capacities

However, while hypothetical populations caricatured by line drawings like Figures 5.12a–c can be characterised by a simple carrying capacity, this is not true of any natural population. There are unpredictable environmental fluctuations, individuals are affected by a whole wealth of factors of which intraspecific competition is only one, and resources not only affect density but respond to density as well. Hence, the situation is likely to be closer to that depicted in Figure 5.12d. Intraspecific competition does not hold natural populations to a predictable and unchanging level (the carrying capacity), but it may act upon a very wide range of starting densities and bring them to a much narrower range of final densities, and it therefore tends to keep density within certain limits. It is in this sense that intraspecific competition may be said typically to be capable of regulating population size.

In fact, the concept of a population settling at a stable carrying capacity, even in caricatured populations, is relevant only to situations in which density dependence is not strongly overcompensating. Where there is overcompensation, cycles or even chaotic changes in population size may be the result. We return to this point later (see Section 5.6.5).