Although it is not as obvious as competition for scarce resources, organisms also compete with each other to avoid being eaten by predators or infected by pathogens. This is sometimes called competition for "enemy-free-space". One way of looking at this is to consider enemies as negative resources, in that the more of them there are, the worse off is the individual. In contrast, the more positive resources like food and space there are, the better off is the individual.
Intraspecific competition to avoid enemies can evoke -feedback if the risk of death or debilitation increases with population density. For example, when pathogenic microorganisms are spread by contact between infected and uninfected hosts, the probability of infection usually increases with host density. Another example is that of generalist predators that switch to more abundant prey species, or aggregate in areas where prey are dense. Under these conditions, the prey death rate can increase with prey density, at least up to the point where the behavioral response of predators begins to saturate. Switching/aggregation is common in intelligent generalist predators such as birds and mammals (Holling 1959) but also seen in some arthropods (Murdoch and Oaten 1975, Gould et al. 1990). Switching/aggregation can produce sigmoid behavioral responses of the predator to prey density which may create two equilibrium points, a low-density stabilizing equilibrium (J) and an unstable escape threshold (E) at a higher density, and a so-called "predator pit" (see figure). The presence of predator-regulated equilibria, and the degree of separation between stable and unstable equilibria (E-J), is critically dependent on the density of predators. If the predator population is reduced, say by habitat destruction, then the "pit" becomes shallower, and the separation smaller, until the equilibrium points eventually disappear altogether. When this happens, the prey population can grow to very high densities; e.g., an outbreak of pests occurs. It is also evident that the larger the predator population, the deeper the "pit", the greater the separation between stable and unstable equilibrium, and the lower the likelihood of prey escape.
Morris (1963) and Holling (1965) were some of the first to recognize that predators with sigmoid behavioral responses can create a "predator pit" in the prey's R-function, and that this can result in prey being regulated at very sparse densities by their predators. They also realized that prey can escape predator regulation if changes in the environment cause the stable equilibrium point to vanish, or if the prey population rises above the population threshold (because of immigration or increased reproduction). This concept was used by Holling and his colleagues (Holling 1978) to model the dynamics of spruce budworm populations in eastern Canada, and was further expanded by Takahashi (1964), Southwood and Commins (1976) and Berryman (1978). Research on aggressive bark beetles also lead to the realization that plant resistance can also create a "resistance pit" with similar effects to the "predator pit" (Berryman 1978).
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© 1997 Alan A. Berryman