0.11 Population diversity

A population is a group of individuals of the same species that share aspects of theirgenetics or demography more closely with each other than with other groups of individuals ofthat 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 differentpopulations. The diversity may also be measured in terms of the populations' demographics, such as numbers of individualspresent, and the proportional representation of different age classes and sexes. However, it can be difficult to measuredemography and genetics ( e.g. , allele frequencies) for all species. Therefore, a more practical way ofdefining 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 theecological boundaries that are important to the population (for example, a particular region and type of vegetation for apopulation 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 anindication of likelihood of movement of organisms between populations and subsequent genetic and demographicinterchange. Similarly, an estimate of the overall population size provides a measure of the potential genetic diversitywithin the population; large populations usually represent larger gene pools and hence greater potential diversity (see Genetic 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. Thegenetic 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 andthe genetic diversity they hold. Loss of isolated populations along with their unique component of genetic variation isconsidered 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 distinctpopulations of a single species may be referred to as subspecies according to some (but not all) species concepts. Populationsthat show less genetic divergence might be recognized as variants or races. However, the distinctions between subspecies and other categories can besomewhat 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, forspecies that live in wetlands, alpine zones on mountaintops, particular soil types or forest types, springs, and many othercomparable situations. Individual organisms may periodically disperse from one population to another, facilitating geneticexchange between the populations. This group of different but interlinked populations, with each different population locatedin 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, alarge 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 orcreate 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 netimmigration in order to sustain itself acts as a sink . The extent of genetic exchange between source and sink populations depends, therefore, on the size ofthe populations, the carrying capacity of the habitats where the populations are found, and the ability of individuals to movebetween habitats. Consequently, understanding how the patches and their constituent populations are arranged within themetapopulation, and the ease with which individuals are able to move among them is key to describing the population diversityand conserving the species. For more discussion, see the moduleon Metapopulations.