Density-Dependent Factors

A limiting factor that depends on population size is called a density-dependent limiting factor. Density-dependent factors become limiting only when the population density—the number of organisms per unit area—reaches a certain level. These factors operate most strongly when a population is large and dense. They do not affect small, scattered populations as greatly.   Density-dependent limiting factors include competition, predation, parasitism, and disease.

Competition When populations become crowded, organisms compete with one another for food, water, space, sunlight, and other essentials. For example, puffins must compete for limited nesting sites. Competition among members of the same species is a density-dependent limiting factor. The more individuals living in an area, the sooner they use up the available resources. Likewise, the fewer the number of individuals, the more resources are available to them and the less they must compete with one another.

Competition can also occur between members of different species. This type of competition is a major force behind evolutionary change. When two species compete for the same resources, both species are under pressure to change in ways that decrease their competition. Over time, the species may evolve to occupy separate niches. That is because, as you may recall, no two species can occupy the same niche in the same place at the same time.

Predation Populations in nature are often controlled by predation. The regulation of a population by predation takes place within a predator-prey relationship, one of the best-known mechanisms of population control. The relationships between sea otters and sea urchins and between sea otters and killer whales are examples of predator-prey interactions that affect population growth.

A well-documented example of a predator-prey relationship is the interaction between wolves and moose on Isle Royale, an island in Lake Superior. The graph in the figure at right shows how periodic increases in the moose population—the prey—on Isle Royale are quickly followed by increases in the wolf population—the predators. As the wolves prey on the moose, the moose population falls. The decline in the moose population is followed, sooner or later, by a decline in the wolf population because there is less for the wolves to feed upon. A decline in the wolf population means that the moose have fewer enemies, so the moose population rises again. This cycle of predator and prey populations can be repeated indefinitely.

Wolf and Moose Population

Parasitism and Disease Parasites can also limit the growth of a population. Parasitic organisms range in size from microscopic, disease-causing bacteria to tapeworms 30 centimeters or more in length. These organisms are similar to predators in many ways. Like predators, parasites take nourishment at the expense of their hosts, often weakening them and causing disease or death. The wasp cocoons in the figure below, for example, can weaken or kill many caterpillars.

Wasp Cocoons This larval sphinx moth has been attacked by a parasitic wasp. The wasp inserted its eggs beneath the moth's skin. After hatching, the wasp larvae fed on their host internally until they appeared as white cocoons on its back.