geographic range

geographic range, in ecology, the collective area in which all members of a particular species are found during their lifetime. The term geographic range has often referred to the natural extent of a species’ distribution; however, it also includes areas where a species was introduced by human beings, on purpose or by accident, or by other forces (see also invasive species). The geographic range of each form of life is unique, with some ranges spanning vast regions and others limited to small patches within tiny habitats. The geographic ranges of some species may be relatively static and unchanging, whereas others may vary across short periods of time. Historic range is a related term often used to describe the geographic range of a species at some point in the past.

Use as an ecological tool

Geographic range, along with population size, is a useful measure for ecologists because it can provide clues to how a species interacts with its environment. A species’ geographic range is often limited by abiotic (nonliving) factors, such as mountain ranges, oceans and large waterways, and other geographic barriers, and it may be closely associated with the species’ tolerance limits (that is, the range of environmental conditions within which a species can survive), such as those that apply to oxygen levels, as well as temperature, humidity, and other factors imposed by climate. For example, terrestrial and aquatic animals obtain oxygen in different ways—and thus the geographic ranges of most air-breathing animals do not coincide with large marine or freshwater environments. Similarly, the geographic ranges of the overwhelming majority of fishes do not include terrestrial environments, although there are exceptions. Biotic (living) considerations—which relate to a species’ body size and role in its habitat, as well as how it interacts with others, such as how it obtains food, how well it competes with other species, or how successfully it can avoid predators—may also limit the size and shape of a species’ geographic range.

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conservation: Range size

In addition, monitoring changes in a species’ geographic range as it adapts to changes in the environment can help ecologists and wildlife managers understand the likelihood of that species becoming extinct. Along with its population size, a species’ geographic range is one of the key considerations used by the International Union for Conservation of Nature (IUCN) in its assessment of a species’ conservation status. All things being equal, plants, animals, and other forms of life with large, stable geographic ranges carry a low risk of extinction, whereas species that can be found only in tiny habitats and those experiencing rapid, precipitous declines in their spatial distribution are often viewed as particular causes for concern.

Although there is some season-to-season or year-to-year fluctuation along the fringes of the geographic ranges of plants and animals in response to small or moderate variations in weather, climate, food availability, and other factors, ecologists are concerned that the pace of climate change (which is often accompanied by changes in temperature and precipitation patterns, along with an increase in the frequency of droughts, extended heat waves, flooding, and other extreme events) could surpass the ability of many plants and animals to adapt to the new conditions. Evidence shows that the geographic ranges of many species of trees, shrubs, and other plants are moving toward the poles and up mountain slopes in search of cooler environments, whereas many animals, especially those found in marine environments, are migrating toward new habitats in response to changing climate patterns. Although many species will be able to move and adapt in this fashion, predictive studies anticipate that continued climatic changes will push many other species, which are unable to migrate or might not otherwise be able to adapt to the changing conditions, into extinction.

Geographic range examples

Although no species is truly cosmopolitan in its distribution across the planet’s oceans and land areas, some species have very large geographic distributions. The blue whale (Balaenoptera musculus), Earth’s largest animal, has the world’s largest geographic range; it can be found in all of the world’s oceans except for the Arctic Ocean and selected seas and gulfs. Human beings (Homo sapiens) have made permanent settlements on all of Earth’s continents except Antarctica and have colonized most of the world’s islands. One of humanity’s more noticeable commensal species is the brown, or Norway, rat (Rattus norvegicus), which occurs in nearly every place human beings also occur (see also commensalism). In contrast, the Devil’s Hole pupfish (Cyprinodon diabolis), a critically endangered species, has one of the smallest naturally occurring geographic ranges, being confined to one side of a deep water-filled limestone cave only 20 square metres (215 square feet) in area.