Population biology is the study of the factors determining the size and distribution of a population, as well as the ways in which populations change over time. The discipline of population biology dates back to the 1960s, when researchers merged aspects of population ecology with aspects of population genetics. It employs a traditional empirical approach which consists of observation of the numbers of individuals in a population and the variation in those numbers over time and space, and the measurement of physical (abiotic) factors and the living (biotic) factors that may affect population numbers.
Given optimum conditions, the populations of most organisms grow at a constant rate of increase, doubling in size at regular intervals, which is known as exponential growth . Exponential population growth is explosive but it is usually opposed by factors that reduce numbers, such as disease, predation, or harsh climates. The result is logistic growth , where the rapidly growing population slows and reaches a stable but dynamic equilibrium at or near the carrying capacity of the environment . The populations of some species such as migratory locusts show boom and bust cycles: in these cases explosive growth produces vast numbers of individuals that eventually overwhelm the carrying capacity and a catastrophic dieback follows.
The rate of growth of a population is the net result of the gains and losses from a number of intrinsic factors operating within the population. These include natality, fecundity , life span, longevity, mortality , and immigration and emigration. Patterns of survivorship and age structure created by these interacting factors show how a population is growing and indicate what general role a species plays in the ecosystem . Density-dependent biotic factors that decrease natality or increase mortality include the numbers of competitors both interspecific and intraspecific, as well as predators, prey, parasites , and other interactive species. Stress and overcrowding are other density-dependent biotic factors that limit population size through excessive intraspecific competition for limited resources.
The local distribution pattern of populations of most species is limited by physical factors such as temperature, moisture, light, pH , soil quality, and salinity . Within their areas of distribution, animals occur in varying densities (either scattered thinly or crowded) and in varying dispersal patterns (either spaced evenly or clumped into herds). Population density and dispersion are often studied together, and are important in ecology and management. Density is measured by direct visual count, and by trapping , collecting fecal pellets, using pelt records, monitoring vocalization frequencies, and so on. Life tables constructed from these data show precisely how a population is age-structured.
Population genetics recognizes two important attributes of a population--its gene frequencies and its total gene pool . Population size exerts some influence on the genetic composition of its members, since the number of sexually interbreeding individuals influences the transfer of genes within a population. It also affects the kinds of genotypes that are available, and the survival and reproductive capacity of individuals with certain genes. The application of the principles of population genetics is vital to the success of programs to improve the breeds of animals and plants for agricultural use, and for the captive breeding of endangered species .
[Neil Cumberlidge Ph.D. ]
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