Primary density-dependent factors include intraspecific and interspecific competition, for limited resources, and predation. The relative importance of these factors has been the topic of much debate. Malthus (1789) wrote the first theoretical treatise describing the increasing struggle for limited resources by growing populations. Effects of intraspecific competition on natality, mortality, and dispersal have been demonstrated widely (see Chapter 5). As competition for finite resources becomes intense, fewer individuals obtain sufficient resources to survive, reproduce, or disperse. Similarly, a rich literature on predator-prey interactions generally, and biocontrol agents in particular, has shown the important density-dependent effects of predators, parasitoids, parasites, and pathogens on prey populations (e.g., Carpenter et al. 1985, Marquis and Whelan 1994, Parry et al. 1997, Price 1997,Tinbergen 1960, van den Bosch et al. 1982, Van Dri-esche and Bellows 1996). Predation rates usually increase as prey abundance increases, up to a point at which predators become satiated. Predators respond both behaviorally and numerically to changes in prey density (see Chapter 8). Predators can be attracted to an area of high prey abundance, a behavioral response, and increase production of offspring as food supply increases, a numeric response.
Cooperative interactions among individuals lead to inverse density dependence. Mating success (and thus natality) increases as density increases. Some insects show increased ability to exploit resources as density increases. Examples include bark beetles that must aggregate to kill trees, a necessary prelude to successful reproduction (Berryman 1997, Coulson 1979), and social insects that increase thermoregulation and recruitment of nestmates to harvest suitable resources as colony size increases (Heinrich 1979, Matthews and Matthews 1978).
Factors affecting population size can operate over a range of time delays. For example, fire affects numbers immediately (no time lag) by killing exposed individuals, whereas predation requires some period of time (time lag) for predators to aggregate in an area of dense prey and to produce offspring. Hence, increased prey density is followed by increased predator density only after some time lag. Similarly, as prey abundance decreases, predators disperse or cease reproduction, but only after a time lag.
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