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Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Habitat fragmentation can be caused by geological processes that slowly alter the layout of the physical environment (suspected of being one of the major causes of speciation), or by human activity such as land conversion, which can alter the environment much faster and causes extinctions of many species.
The term habitat fragmentation includes five discrete phenomena:
- Reduction in the total area of the habitat
- Decrease of the interior : edge ratio
- 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
"fragmentation ... not only causes loss of the amount of habitat, but by creating small, isolated patches it also changes the properties of the remaining habitat" (van den Berg et al. 2001). Habitat fragmentation is the landscape level of the phenomenon, and patch level process. Thus meaning, it covers; the patch areas, edge effects, and patch shape complexity.
Evidence of habitat destruction through natural processes such as volcanism, fire, and climate change is found in the fossil record. For example, habitat fragmentation of tropical rainforests in Euramerica 300 million years ago led to a great loss of amphibian diversity, but simultaneously the drier climate spurred on a burst of diversity among reptiles.
Habitat fragmentation is frequently caused by humans when native vegetation is cleared for human activities such as agriculture, rural development, urbanization and the creation of hydroelectric reservoirs. 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 cropland, pasture, pavement, or even barren land. The latter is often the result of slash and burn farming in tropical forests. In the wheat belt of central western New South Wales, Australia, 90% of the native vegetation has been cleared and over 99% of the tall grass prairie of North America has been cleared, resulting in extreme habitat fragmentation.
One of the major ways that habitat fragmentation affects biodiversity is by reducing the amount of suitable habitat available for organisms. Habitat fragmentation often involves both habitat destruction and the subdivision of previously continuous habitat. Plants and other sessile organisms are disproportionately affected by some types of habitat fragmentation because they cannot respond quickly to the altered spatial configuration of the habitat.
As the remaining habitat patches are smaller, they tend to support smaller populations of species. Small populations are at an increased risk of a variety of genetic consequences that influence their long-term survival. Remnant populations often contain only a subset of the genetic diversity found in the previously continuous habitat. Processes that act upon underlying genetic diversity such as adaptation have a smaller pool of fitness-maintaining alleles to survive in the face of environmental change.
Populations can maintain genetic diversity through migration. In continuous habitats, migrants have few barriers to establish themselves in suitable sites. In fragmented habitats however, the separation between suitable sites disrupts migration, and therefore gene flow, limiting a populations capacity to supplement the reduced genetic diversity of the remnant populations. With lower migration, inbreeding becomes a of increasing concern as the level of homozygosity increases, facilitating the expression of deleterious alleles that reduce the fitness of the population called inbreeding depression.
The percentage preservation of contiguous habitats is closely related to both genetic and species biodiversity preservation. Generally a 10% remnant contiguous habitat will result in a 50% biodiversity loss.
Area is the primary determinant of the number of species in a fragment. 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 immigrants 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.
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 may establish themselves 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.
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 preserving or planting corridors of native vegetation. This has the potential to mitigate the problem of isolation but not the loss of interior habitat. In rare cases a conservation reliant species may gain some measure of disease protection 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 island biogeography.
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- van den Berg LJL, Bullock JM, Clarke RT, Langsten RHW, Rose RJ. 2001. Territory selection by the Dartford warbler (Sylvia undata) in Dorset, England: the role of vegetation type, habitat fragmentation and population size. Biol. Conserv. 101:217-28
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- GLOBIO, an ongoing programme to map the past, current and future impacts of human activities on the natural environment, specifically highlighting larger wilderness areas and their fragmentation
- Monash Virtual Laboratory – Simulations of habitat fragmentation and population genetics online at Monash University's Virtual Laboratory.
- Defragmentation in Belgium (Flanders) – Connecting nature, connecting people. Accessed: Jan 22, 2009
- Wildlife passages – De-Fragmentation in the Netherlands – How to evaluate their effectiveness? Accessed: Jan 22, 2009
- Landscape Fragmentation in Europe The technical report from 2006 - the result of a collaboration between the Swiss Federal Office for the Environment (FOEN) and the European Environment Agency (EEA). Accessed: Feb 22, 2016