Population

Population

A population is a geographically distinct group of individuals of the same species that is co-occurring in time and space, and can potentially interbreed with each other. Populations are an important element of evolutionary biology, and in the ecology of species and communities.

Evolution is sometimes defined as a change in the genetic information of populations over time. Individual organisms can be more or less successful in their reproduction, which means that they vary in "fitness." Because their fitness varies, individuals are the targets of natural selection. Individual organisms eventually die, and can not evolve. Populations and the species of which they are components are the units of evolution.

Populations of all species change in size over time in response to environmental factors that affect four population-related (or demographic) variables: birth rate (BR), immigration rate (IR), death rate (DR), and emigration rate (ER). The change in population size (P) during a unit of time (for example, one year) is described by the following equation: P = BR - DR + IR - ER. This demographic relationship is true of all populations, including humans. Population ecologists have developed mathematical models of population dynamics that account for the important influences of such factors as the intrinsic rate of population increase, the carrying capacity of habitats, the effects of predators and disease, and the effects of unpredictable events of disturbance.

In some cases, isolated (or closed) populations do not add individuals through immigration, and do not lose individuals to emigration. Under such conditions, P is calculated as BR - DR, which is sometimes known as the natural rate of population change. P is expressed as a percentage change by dividing its value by the initial population size. For example, a population of 1000 individuals that increases by 100 in one year has a 10% per year growth rate. If the percentage change in any population is constant, there will be an accelerating rate of population increase or decrease, known as exponential change.

Imagine a circumstance in which a fertile pair of individuals discovers a "new" habitat, which is suitable for use but has not been previously utilized by their species. Under such conditions the founder population can increase over time. Initially, resources are abundant and do not constrain population growth, so the percentage rate of population increase is constant, being limited only by how quickly progeny can be produced and become fertile, and only countered by the death rate of individuals. This maximum rate of population growth, which is limited only by the biology of the species in the given environment and not by competition for resources, is referred to as the intrinsic rate of population increase. Any population growing for a number of generations at the intrinsic rate of population increase (or indeed at any fixed percentage rate) will quickly explode in abundance.

Eventually, however, the carrying capacity of the habitat is approached, and space and resources become limiting. (Carrying capacity is the abundance of the species that can be sustained without degrading the resources.) Once this happens, growth and reproduction become constrained by the availability of resources, and individuals in the population compete with each other. Intense competition results in physiological stress, which generally results in decreases in birth rates and increases in death rates. The rate of population increase may then decrease to zero. In other words, the birth rate equals the death rate, which is referred to as zero population growth or ZPG. If ZPG is maintained, the population size eventually levels off, perhaps to the carrying capacity of the habitat. However, if the earlier population growth results in an abundance exceeding what the habitat can support, the over-population causes environmental degradation by over-exploitation, resulting in a decrease in carrying capacity. The population would then decrease because of a rapid increase in the rate of mortality, or perhaps by emigration in search of new habitats. These factors could result in a rapid and uncontrolled "crash" in the size of the population, usually to a level below the carrying capacity, creating a circumstance for renewed population growth. In small habitats, however, the population crash could be massive enough to render a local population extinct.