Habitat fragmentation is a process of environmental change important in evolution and conservation biology. As the name implies, it describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat). Habitat fragmentation can be caused by geological processes that slowly alter the layout of the physical environment or by human activity which can alter the environment on a much, much faster time scale. The former is suspected of being one of the major causes of speciation. The latter has been implicated in the extinction of many species.

Human impact

Habitat fragmentation is frequently caused by humans when native vegetation is cleared for agriculture or development. Habitats which were once continuous become divided into separate fragments. After intensive clearing, the separate fragments tend to be very small islands isolated from each other by crop land, pasture, pavement, or even barren land. The latter is often the result of slash and burn farming in tropical forests. In some regions of Australia, such as the wheatbelt of central western New South Wales, 90% of the native vegetation has been cleared, resulting in extreme habitat fragmentation.

Reduced viability

Small fragments of habitat can only support small populations of fauna and small populations of fauna are more vulnerable to extinction. Small fluctuations in climate, resources, or other factors that would be unremarkable and quickly corrected in large populations can be catastrophic in small, isolated populations. Additionally, unoccupied fragments of habitat that are separated from a source of colonists by some barrier are less likely to be repopulated than adjoining fragments.

Furthermore, small fragments of habitat do not contain interior habitat. Habitat along the edge of a fragment has a different climate and favours different species than the interior. Small fragments are therefore unfavourable for those species which require interior habitat and may lead to the local extinction of those species.

Conservation implications

Habitat fragmentation is often implicated or at least associated with threatened or endangered species. The existence of viable habitat is critical to the survival of any species, and in many cases the fragmentation of any remaining habitat can lead to difficult decisions for conservation biologists. Given a limited amount of resources available for conservation is it preferable to protect the existing isolated patches of habitat or to buy back land to get the largest possible continuous piece of land. This ongoing debate is often referred to as SLoSS (Single Large or Several Small).

One popular solution to the problem of habitat fragmentation is to link the fragments by planting corridors of native vegetation. This has the potential to solve the problem of isolation but not the loss of interior habitat. In some cases a threatened species may gain some measure protection from disease by being distributed in isolated habitats.

Another solution is to enlarge small remnants in order to increase the amount of interior habitat. This may be impractical since developed land is often more expensive and could require significant time and effort to restore.

The best solution is generally dependent on the particular species or ecosystem that is being considered. More mobile species, like most birds, do not need connected habitat while some smaller animals, like rodents, may be more exposed to predation in open land. These questions generally fall under the headings of metapopulations or island biogeography.