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Thursday, November 13, 2008

Habitat fragmentation

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 such as land conversion, which can alter the environment on a much faster time scale. The former is suspected of being one of the major causes of speciation. The latter is causative in extinctions of many species.
Fragmentation and destruction of Great Ape habitat in Central Africa, from the GLOBIO and GRASP projects.

Habitat fragmentation is frequently caused by humans when native vegetation is cleared for human activities such as agriculture, rural development or urbanization. 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 the wheatbelt of central western New South Wales, Australia 90% of the native vegetation has been cleared and over 99% of the tallgrass prairie of North America has been cleared, resulting in extreme habitat fragmentation.

The term habitat fragmentation includes six discrete phenomena:

* Reduction in the total area of the habitat
* Increase in the amount of edge
* Decrease in the amount of interior habitat
* Isolation of one habitat fragment from other areas of habitat
* Breaking up of one patch of habitat into several smaller patches
* Decrease in the average size of each patch of habitat

Habitat destruction

One of the major ways that habitat fragmentation affects biodiversity is by reduction in the amount of available habitat for plants and animals like rainforests, forests, and oceans etc. Habitat fragmentation invariably involves some amount of habitat destruction. Plants and other sessile organisms in these areas are usually directly destroyed. Mobile animals (especially birds and mammals) retreat into remnant patches of habitat. This can flead crowding effects and increased competition.

The remaining habitat fragments are smaller than the original habitat. Species that can move between fragments may use more than one fragment. Species which cannot move between fragments must make do with what is available in the single fragment in which they ended up. Since one of the major causes of habitat destruction is agricultural development, habitat fragments are rarely representative samples of the initial landscape.

Reduced viability

Area is the primary determinant of the number of species in a fragment. The size of the fragment will influence the number of species which are present when the fragment was initially created, and will influence the ability of these species to persist in the fragment. Small fragments of habitat can only support small populations of plants and animals and small populations are more vulnerable to extinction. Minor fluctuations in climate, resources, or other factors that would be unremarkable and quickly corrected in large populations can be catastrophic in small, isolated populations. Thus fragmentation of habitat is an important cause of species extinction. Population dynamics of subdivided populations tend to vary asynchronously. In an unfragmented landscape a declining population can be "rescued" by immigration from a nearby expanding population. In fragmented landscapes, the distance between fragments may prevent this from happening. 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. Even small species such as the Columbia spotted frog are reliant on the rescue effect. Studies showed 25% of juveniles travel a distance over 200m compared to 4% of adults. Of these, 95% remain in their new locale, demonstrating that this journey is necessary for survival. High dispersal in other amphibians indicate that it is crucial to the endangerement of rare species in the class.

Additionally, habitat fragmentation leads to edge effects. Microclimatic changes in light, temperature, and wind can alter the ecology around the fragment, and in the interior and exterior portions of the fragment. Fires become more likely in the area as humidity drops and temperature and wind levels rise. Exotic and pest species can establish themselves more easily in such disturbed environments, and the proximity of domestic animals often upsets the natural ecology. Also, habitat along the edge of a fragment has a different climate and favours different species from the interior habitat. Small fragments are therefore unfavourable for species which require interior habitat.

Conservation implications

Habitat fragmentation is often a cause of species becoming threatened or endangered. 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 solution to the problem of habitat fragmentation is to link the fragments by planting corridors of native vegetation. This has the potential to mitigate 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 mitigation measure is the enlargement of 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.

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