Populations and Communities

A population is a group of the same species that lives in an area at the same time. For example, all the moose on Isle Royale make up a population, as do all the spruce trees. Ecologists determine and analyze the number and growth of populations and the relationships between each species and the environmental conditions.

Factors that control populations. The size of any population depends upon the interaction of two basic forces. One is the rate at which the population would grow under ideal conditions. The second is the combined effect of all the less-than-ideal environmental factors that limit growth. Such limiting factors may include low food supply, predators, competition with organisms of the same or different species, climate, and disease.

The largest size of a particular population that can be supported by a particular environment has been called the environment's carrying capacity for that species. Real populations normally are much smaller than their environment's carrying capacity for them because of the effects of adverse weather, a poor breeding season, hunting by predators, or other factors.

Factors that change populations. Population levels of a species can change considerably over time. Sometimes these changes result from natural events. For example, a change in rainfall may cause some populations to increase and others to decrease. Or the introduction of a new disease can severely decrease the population of a plant or animal species. In other cases, changes may result from human activities. For example, power plants and automobiles release acidic gases into the atmosphere, where they may mix with clouds and fall to earth as acid rain. In some regions that receive large amounts of acid rain, fish populations have declined dramatically.

Communities

A community is a group of animal and plant populations living together in the same environment. Wolves, moose, beavers, and spruce and birch trees are some of the populations that make up the forest community of Isle Royale. Ecologists study the roles different species play in their communities. They also study the different types of communities, and how they change. Some communities, such as an isolated forest or meadow, can be identified easily. Others are more difficult to define.

A community of plants and animals that covers a large geographical area is called a biome. The boundaries of different biomes are determined mainly by climate. The major biomes include deserts, forests, grasslands, tundra, and several types of aquatic biomes. 

The role of a species in its community is called its ecological niche. A niche consists of all the ways that a species interacts with its environment. It includes such factors as what the species eats or uses for energy; what predators it has; the amounts of heat, light, or moisture it needs; and the conditions under which it reproduces. Ecologists have long noted that many species occupy a highly specialized niche in a given community. Various explanations have been proposed for this. Some ecologists feel that it results from competition-that if two species try to "fill" the same "niche," then competition for limited resources will force one of the species out. Other ecologists maintain that a species that occupies a highly specialized niche does so because of the rigid physiological demands of that particular role in the community. In other words, only one species occupies the niche not because it has out-competed other species, but because it is the only member of the community physiologically capable of playing that role.

Changes in communities occur over time in a process called ecological succession. This process occurs as a series of slow, generally predictable changes in the number and kinds of organisms in an area take place. Differences in the intensity of sunlight, protection from wind, and changes in the soil may alter the kinds of organisms that live in an area. These changes may also alter the number of populations that make up the community. Then, as the number and kinds of species change, the physical and chemical characteristics of the area undergo further changes. The area may reach a relatively stable condition called the climax community, which may last hundreds or even thousands of years. 

Ecologists distinguish two types of succession-primary and secondary. In primary succession, organisms begin to inhabit an area that had no life, such as a new island formed by a volcanic eruption. Secondary succession takes place after an existing community suffers a major disruption-for example, after a climax forest community is destroyed by fire. In this example, a meadow community of wildflowers and grasses will grow first, followed by a community of shrubs. Finally trees will reappear, and the area will eventually become a forest once more, until it is disturbed again. Thus, the forces of nature ultimately cause even climax communities to change. Increasingly, ecologists view fires and other large natural disturbances as acceptable and even desirable.