POPULATION ECOLOGY
Population ecology is the science that measures changes in population size and composition and identifies the causes of these fluctuations. Population ecology is not concerned solely with the human population. In ecological terms, a population consists of the individuals of one species that simultaneously occupy the same general area, rely on the same resources, and are affected by similar environmental factors. The characteristics of a population are shaped by its size and by the interactions among individuals and between individuals and their environment.
Population size is a balance between factors that increase numbers and factors that decrease numbers. Some factors that increase populations are favorable light and temperature, adequate food supply, suitable habitat, ability to compete for resources, and ability to adapt to environmental change. Factors that decrease populations are insufficient or excessive light and temperature, inadequate food supply, unsuitable or destroyed habitat, too many competitors for resources, and inability to adapt to environmental change.
An important characteristic of any population is its density. (1) Population density is the number of individuals per unit, such as the number of maple trees per square kilometer in a county. (2) Ecologists can rarely determine population size by actually counting all individuals within geographical boundaries. (3) Instead, they often use a variety of sampling techniques to estimate densities and total population sizes. (4) In some cases, they estimate population size through indirect indicators, such as the number of nests or burrows, or signs such as tracks or droppings.
Another important population characteristic, dispersion, is the pattern of spacing among individuals within the population’s geographical boundaries. Various species are distributed in their habitats in different ways to take better advantage of food supplies and shelter, and to avoid predators or find prey. Within a population’s range, densities may vary greatly because not all areas provide equally suitable habitat, and also because individuals space themselves in relation to other members of the population.
Three possible patterns of dispersion are clumped, uniform and random. A clumped dispersion pattern means that individuals are gathered in patches throughout their habitat. Clumping often results from the irregular distribution of resources needed for survival and reproduction. For example, fallen trees keep the forest floor moist, and many forest insects are clumped under logs where the humidity is to their liking. Clumping may also be associated with mating, safety, or other social behavior. Crane flies, for example, swarm in great numbers, a behavior that increases mating chances, and some fish swim in large schools so they are less likely to be eaten by predators.
A uniform or evenly spaced distribution results from direct interactions among individuals in the population. For example, regular spacing of plants may result from shading and competition for water. In animal populations, uniform distribution is usually caused by competition for some resource or by social interactions that set up individual territories for feeding, breeding, or resting.
Random spacing occurs in the absence of strong attraction or repulsion among individuals in a population. Overall, random patterns are rare in nature, with most populations showing a tendency toward either clumped or uniform distribution.
Populations change in size, structure, and distribution as they respond to changes in environmental conditions. Four main variables—births, deaths, immigration, and emigration— determine the rate of change in the size of the population over time. A change in the birth rate or death rate is the major way that most populations respond to changes in resource availability. Members of some animal species can avoid or reduce the effects of environmental stress by emigrating from one area and immigrating to another with more favorable environmental conditions, thus altering the population’s dispersion.