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Curves of the type shown in Figure 5.13a and b may also be used to suggest the pattern by which a population might increase from an initially very small size (e.g. when a species colonises a previously unoccupied area). This is illustrated in Figure 5.13c. Imagine a small population, well below the carrying capacity of its environment (point A). Because the population is small, it increases in size only slightly during one time interval, and only reaches point B. Now, however, being larger, it increases in size more rapidly during the next time interval (to point C), and even more during the next (to point D). This process continues until the population passes beyond the peak of its net recruitment curve (Figure 5.13b). Thereafter, the population increases in size less and less with each time interval until the population reaches its carrying capacity (K) and ceases completely to increase in size. The population might therefore be expected to follow an S‐shaped or ‘sigmoidal’ curve as it rises from a low density to its carrying capacity. This is a consequence of the hump in its recruitment rate curve, which is itself a consequence of intraspecific competition.

Of course, Figure 5.13c, like the rest of Figure 5.13, is a gross simplification. It assumes, apart from anything else, that changes in population size are affected only by intraspecific competition. Nevertheless, something akin to sigmoidal population growth can be perceived in many natural and experimental situations (Figure 5.15).

Figure 5.15 Real examples of S‐shaped population increase. (a) The bacterium Lactobacillus sakei (measured as grams of ‘cell dry mass’ or CDM per litre) grown in nutrient broth. (b) The population of shoots of the annual plant Juncus gerardii in a salt marsh habitat on the west coast of France. (c) The population of the willow tree (Salix cinerea) in an area of land after myxomatosis had effectively prevented rabbit grazing.

Source: (a) After Leroy & de Vuyst (2001). (b) After Bouzille et al. (1997). (c) After Alliende & Harper (1989).

Intraspecific competition will be obvious in certain cases (such as overgrowth competition between sessile organisms on a rocky shore), but this will not be true of every population examined. Individuals are also affected by predators, parasites and prey, competitors from other species, and the many facets of their physical and chemical environment. Any of these may outweigh or obscure the effects of intraspecific competition; or the effect of these other factors at one stage may reduce the density to well below the carrying capacity for all subsequent stages. Nevertheless, intraspecific competition probably affects most populations at least sometimes during at least one stage of their life cycle.