Population

The term population refers to all the members of the same species that live together in a particular place. This concept has proved very useful to the life sciences since it has only two basic requirements. Its individual members must belong to the same species (and be able to mate and produce young), and they must also live in the same area at the same time. Ecologists refer to all of the animal and plant populations that live together and interact in a given environment as a community. This is a larger category than population.

Every ecosystem (living organisms and their environment) is composed of populations or groups of the same type of organisms. In any particular natural place, such as a forest or a lake, there are populations of certain animals (deer, earthworms, trout) along with certain plant populations (maple trees, spruce trees, honeysuckle) that thrive in the same place. Ecologists have decided to categorize these same-species groups, such as all of trout or all of the maple trees, that live in the same place as populations.

Population size is influenced by the interaction of two major factors: the rate that a population grows under ideal conditions and the rate at which certain external or environmental factors limit population growth. An example of population growth under ideal conditions would be placing a pair of fertile animals in a "new" habitat with no predators, no diseases, more than enough food, and a perfect climate. Since there are no external factors hindering the animals in any way, their population growth would be limited only by "intrinsic" or internal factors as how quickly they can produce young, how quickly their young become fertile, and how long they naturally live. Under these conditions, the maximum rate of population growth is limited only by the biology of the species. Ecologists know that any population growing at this unnatural rate would eventually reach what is called "carrying capacity," when external or environmental factors come into play.

Carrying capacity is described as the largest size of a particular population that can be supported by a particular environment. Once a population has reached a certain critical size (which is technically past its carrying capacity), it begins to feel the limitations of its habitat in terms of space and resources. When this limit is surpassed, the rate of population growth can only decline since individuals now have to compete with one another for living space and resources like food and water. Intense competition by itself can result in stress, which results in lower birth rates and increases mortality.

In the real world, populations of animals and plants change in size all the time. Some factors that put limits on a population include: low food supply, predators, bad weather, disease, competition, and human interference. Ecologists have developed mathematical models, called population dynamics, which are used to study and predict the condition of a population in an ecosystem. Ecologists base a model on the four major environmental factors that affect populations: birth rate (BR), immigration rate (IR), death rate (DR), and emigration rate (ER). Such a model states that P = BR ? DR + IR ? ER. Therefore, the change in population size (P) is equal to the birth rate less the death rate, plus the immigration rate (how many new members join the habitat) minus the emigration rate (how many old members leave the habitat). This statistical model is true of all populations, including humans.

Population studies are important if we are to understand how human activities affect the natural world. Being able not only to measure or count a single group of the same species in a particular ecosystem, but to be able to understand the dynamics of what forces make its numbers grow or decline, is especially valuable. Understanding a population also gives ecologists insight into the larger and more complex concept of communities.

[See alsoPopulation Genetics; Population Growth and Control (Human) ]