Density Dependent Mortality Processes

The rate of mortality often depends on the population density, which results in a negative (or occasionally positive) feedback on population dynamics. High-density populations experience an increased mortality due to limitation of food and other resources, direct competition and cannibalism, and due to rapid spread of diseases (e.g., nuclear polyhedro-sis virus in some insects). If the instantaneous mortality rate is proportional to the population density, then the population grows according to a logistic model until its density reaches a stable level. The stronger the direct density dependence is, the more stable the population numbers are.

Mortality due to predation or parasitism is often density dependent; however, this relationship is indirect and mediated through the population dynamics of the natural enemy. Specialized parasitoids increase their numbers as the density of their host increases; however, the growth of parasitoid population is delayed compared with its host, and thus, the percent of parasitism does not increase until the host population reaches its carrying capacity and stops growing due to resource limitations or pathogen spread. Because parasitoid density continues to increase, the percent of parasitism increases rapidly which is followed by a decrease in the host population density. When hosts become sparse, the parasitoid population collapses as well, and then the cycle repeats. Thus, delayed density-dependent mortality may cause periodicity in the population dynamics. However, examples of successful biological control indicate that imported parasitoids suppressed population density without causing a cycle of outbreaks. Suppression and stabilization of population density are different effects which are not necessarily caused by the same mortality process. For example, parasitism may suppress host population density, whereas other processes (e.g., density-dependent predation or migration) may dampen density fluctuations.

Some mortality processes have an inverse density dependence within some range of densities, which results in a positive feedback in population dynamics. The numbers of vertebrate predators usually do not depend on the population density of one species of prey because they have many alternative food sources. For example, the density of white-footed mouse in North America depends mostly on the crop of acorns in the forest rather than on the density of the gypsy moth which is their alternative food. As the density of the gypsy moth increases, each mouse consumes more pupae, but because the numbers of mice do not change, the proportion of destroyed pupae decreases. Mortality in some herbivore insects caused by defense mechanisms of its host plant often decreases as the population density of insects increases. This kind of group effect is known for bark beetles, long-horn beetles, and larvae of pine sawflies. Inverse density dependence may result in an unstable equilibrium density (threshold) below which populations decline until extinction and above which they increase until some other processes will limit their growth.

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