Ecology

C

Population Growth Rates

Populations have a birth rate (the number of young produced per unit of population per unit of time), a death rate (the number of deaths per unit of time), and a growth rate. The major agent of population growth is births, and the major agent of population loss is deaths. When births exceed deaths, a population increases; and when deaths exceed additions to a population, it decreases. When births equal deaths in a given population, its size remains the same, and it is said to have zero population growth.

When introduced into a favorable environment with an abundance of resources, a small population may undergo geometric, or exponential growth, in the manner of compound interest. Many populations experience exponential growth in the early stages of colonizing a habitat because they take over an underexploited niche or drive other populations out of a profitable one. Those populations that continue to grow exponentially, however, eventually reach the upper limits of the resources; they then decline sharply because of some catastrophic event such as starvation, disease, or competition from other species. In a general way, populations of plants and animals that characteristically experience cycles of exponential growth are species that produce numerous young, provide little in the way of parental care, or produce an abundance of seeds having little food reserves. These species, usually short-lived, disperse rapidly and are able to colonize harsh or disturbed environments. Such organisms are often called opportunistic species.

Other populations tend to grow exponentially at first, and then logistically—that is, their growth slows as the population increases, then levels off as the limits of their environment or carrying capacity are reached. Through various regulatory mechanisms, such populations maintain something of an equilibrium between their numbers and available resources. Animals exhibiting such population growth tend to produce fewer young but do provide them with parental care; the plants produce large seeds with considerable food reserves. These organisms are long-lived, have low dispersal rates, and are poor colonizers of disturbed habitats. They tend to respond to changes in population density (the number of organisms per unit area) through changes in birth and death rates rather than through dispersal. As the population approaches the limit of resources, birth rates decline, and mortality of young and adults increases.

D

Community Interactions

Major influences on population growth involve various population interactions that tie the community together. These include competition, both within a species and among species; predation, including parasitism; and coevolution, or adaptation.

---

---

D

1

Competition

When a shared resource is in short supply, organisms compete, and those that are more successful survive. Within some plant and animal populations, all individuals may share the resources in such a way that none obtains sufficient quantities to survive as adults or to reproduce. Among other plant and animal populations, dominant individuals claim access to the scarce resources and others are excluded. Individual plants tend to claim and hold onto a site until they lose vigor or die. These prevent other individuals from surviving by controlling light, moisture, and nutrients in their immediate areas.

Many animals have a highly developed social organization through which resources such as space, food, and mates are apportioned among dominant members of the population. Such competitive interactions may involve social dominance, in which the dominant individuals exclude subdominant individuals from the resource; or they may involve territoriality, in which the dominant individuals divide space into exclusive areas, which they defend. Subdominant or excluded individuals are forced to live in poorer habitats, do without the resource, or leave the area. Many of these animals succumb to starvation, exposure, and predation.

Competition among members of different species results in the division of resources in a community. Certain plants, for example, have roots that grow to different depths in the soil. Some have shallow roots that permit them to use moisture and nutrients near the surface. Others growing in the same place have deep roots that are able to exploit moisture and nutrients not available to surface-rooted plants.

D

2

Predation

One of the fundamental interactions is predation, or the consumption of one living organism, plant or animal, by another. While it serves to move energy and nutrients through the ecosystem, predation may also regulate population and promote natural selection by weeding the unfit from a population. Thus, a rabbit is a predator on grass, just as the fox is a predator on the rabbit. Predation on plants involves defoliation by grazers and the consumption of seeds and fruits. The abundance of plant predators, or herbivores, directly influences the growth and survival of the carnivores. Thus, predator-prey interactions at one feeding level influence the predator-prey relations at the next feeding level. In some communities, predators may so reduce populations of prey species that a number of competing species can coexist in the same area because none is abundant enough to control the resource. When predators are reduced or removed, however, the dominant species tend to crowd out other competitors, thereby reducing species diversity.

D

3

Parasitism

Closely related to predation is parasitism, wherein two organisms live together, one drawing its nourishment at the expense of the other. Parasites, which are smaller than their hosts, include many viruses and bacteria. Because of this dependency relationship, parasites normally do not kill their hosts the way predators do. As a result, hosts and parasites generally coevolve a mutual tolerance, although parasites may regulate some host populations, lower their reproductive success, and modify behavior. See Parasite.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain