12: Population Diversity

A population is a group of individuals of the same species that share aspects of their genetics or demography more closely with each other than with other groups of individuals of that species (where demography is the statistical characteristic of the population such as size, density, birth and death rates, distribution, and movement of migration).

Population diversity may be measured in terms of the variation in genetic and morphological features that define the different populations. The diversity may also be measured in terms of the populations' demographics, such as numbers of individuals present, and the proportional representation of different age classes and sexes. However, it can be difficult to measure demography and genetics (e.g., allele frequencies) for all species. Therefore, a more practical way of defining a population, and measuring its diversity, is by the space it occupies. Accordingly, a population is a group of individuals of the same species occupying a defined area at the same time (Hunter, 2002: 144). The area occupied by a population is most effectively defined by the ecological boundaries that are important to the population (for example, a particular region and type of vegetation for a population of beetles, or a particular pond for a population of fish).

The geographic range and distribution of populations (i.e., their spatial structure) represent key factors in analyzing population diversity because they give an indication of likelihood of movement of organisms between populations and subsequent genetic and demographic interchange. Similarly, an estimate of the overall population size provides a measure of the potential genetic diversity within the population; large populations usually represent larger gene pools and hence greater potential diversity.

Isolated populations, with very low levels of interchange, show high levels of genetic divergence (Hunter, 2002: 145), and exhibit unique adaptations to the biotic and abiotic characteristics of their habitat. The genetic diversity of some groups that generally do not disperse well - such as amphibians, mollusks, and some herbaceous plants - may be mostly restricted to local populations (Avise, 1994). For this reason, range retractions of species can lead to loss of local populations and the genetic diversity they hold. Loss of isolated populations along with their unique component of genetic variation is considered by some scientists to be one of the greatest but most overlooked tragedies of the biodiversity crisis (Ehrlich & Raven 1969).

Populations can be categorized according to the level of divergence between them. Isolated and genetically distinct populations of a single species may be referred to as subspecies according to some (but not all) species concepts. Populations that show less genetic divergence might be recognized as variants or races. However, the distinctions between subspecies and other categories can be somewhat arbitrary (see Species diversity).

A species that is ecologically linked to a specialized, patchy habitat may likely assume the patchy distribution of the habitat itself, with several different populations distributed at different distances from each other. This is the case, for example, for species that live in wetlands, alpine zones on mountaintops, particular soil types or forest types, springs, and many other comparable situations. Individual organisms may periodically disperse from one population to another, facilitating genetic exchange between the populations. This group of different but interlinked populations, with each different population located in its own, discrete patch of habitat, is called a metapopulation.

There may be quite different levels of dispersal between the constituent populations of a metapopulation. For example, a large or overcrowded population patch is unlikely to be able to support much immigration from neighboring populations; it can, however, act as a source of dispersing individuals that will move away to join other populations or create new ones. In contrast, a small population is unlikely to have a high degree of emigration; instead, it can receive a high degree of immigration. A population that requires net immigration in order to sustain itself acts as a sink. The extent of genetic exchange between source and sink populations depends, therefore, on the size of the populations, the carrying capacity of the habitats where the populations are found, and the ability of individuals to move between habitats. Consequently, understanding how the patches and their constituent populations are arranged within the metapopulation, and the ease with which individuals are able to move among them is key to describing the population diversity and conserving the species. For more discussion, see the module on Metapopulations.