Population Ecology
Population ecology is a science that deals with measuring changes in population size, composition, and identifying the factors that cause these changes. A population is define as a group of one species of organisms occupying the same general area, using the same resources, and acted upon by the same environmental factors. Populations cannot grow indefinitely, many populations will become stable over a period of time while others will show sharp increases followed by similar decreases. Population characteristics that afford study are its density and the spacing of its individuals. Population density is the number of individuals per unit area or volume. Population dispersion is the pattern of spacing among the individuals within the parameters of the geographical boundaries of the population.
Measuring Density:
Due to the impracticality of capturing or counting each individual in a given area, ecologists use a variety of methods to determine the density of various populations of organisms. Some of the sampling techniques used are as follows: Counting the number of nests or burrows in a given area, examining the number of tracks, examination of solid waste products left behind, and an actual capture method called mark - recapture. In the mark-recapture method, animals are trapped within their boundaries, marked with a long lasting sign, and released. At a later time some of the animals will be recaptured along with other that are not marked. The data is then placed into the following formula to calculate the population's density
(number marked) x ( total catch the second time)
number of marked recaptures
This method assumes individuals have the same probability of being captured as unmarked individuals. This assumption is not always valid.
Patterns of Dispersion:
Local densities, within a population's range, may vary substantially due to differences in the limiting factors present. There are three general patterns of dispersion in relationship to other individuals: clumped, uniform, and random.
While the above patterns apply to individuals within a population, keep in mind that populations within a species show dispersal patterns. Biogeography is the study of factors that influence the distribution of a species over its range.
Demography is the study of the vital statistics affecting population size. This branch of science deals with the affects that immigration and emigration have on a given population. Other than new organisms leaving or entering the population birth and death rates are also studied.
Density-Dependent and Density-Independent Factors: