Wildlife corridor
A wildlife corridor, habitat corridor, or green corridor is an area of habitat connecting wildlife populations separated by human activities or structures (such as roads, development, or logging). This allows an exchange of individuals between populations, which may help prevent the negative effects of inbreeding, and reduced genetic diversity (via genetic drift) that often occur within isolated populations. Corridors may also help facilitate the re-establishment of populations that have been reduced or eliminated due to random events (such as fires or disease). This may potentially moderate some of the worst effects of habitat fragmentation, wherein urbanization can split up habitat areas, causing animals to lose both their natural habitat and the ability to move between regions to access resources. Habitat fragmentation due to human development is an ever-increasing threat to biodiversity, and habitat corridors serve to manage its effects.

Purpose
Habitat corridors can be considered a management tool in places where the destruction of a natural area has greatly affected its native species, whether as a result of human development or natural disasters. When areas of land are broken up, populations can become unstable or fragmented. Corridors can reconnect fragmented populations and reduce population fluctuations by contributing to three factors that can help to stabilize a population:  
 Colonization—animals are able to move and occupy new areas when food sources or other natural resources are lacking in their core habitat.
 Migration—species that relocate seasonally can do so more safely and effectively when it does not interfere with human development barriers.
 Interbreeding—animals can find new mates in neighboring regions, increasing genetic diversity.

Rosenberg et al. were among the first to define what constitutes a wildlife corridor, developing a conceptual model that emphasized the role of a wildlife corridor as a facilitator of movement that is not restricted by requirements of native vegetation or intermediate target patches of habitat.

Wildlife corridors additionally, have strong indirect effects on plant populations by increasing pollen and seed dispersal from animals, facilitating movement of disparate taxa between isolated patches. Corridors must be large enough to support minimum critical populations, reduce migration barriers, and maximize connectivity between populations.

Wildlife corridors may also encompass aquatic habitats (often called riparian ribbons) and usually come in the form of rivers and streams. Terrestrial corridors can come in the form of wooded strips connecting woodland areas or an urban hedge.

Users 
Species can generally be categorized in one of two groups; passage users and corridor dwellers.

Passage users occupy corridors for brief periods of time. These animals use corridors for such events as seasonal migration, dispersal of a juvenile, or moving between parts of a large home range. Animals such as large herbivores, medium to large carnivores, and migratory species are passage users.

Corridor dwellers can occupy the passage anywhere from several days to several years. Species such as plants, reptiles, amphibians, birds, insects, and small mammals can spend their entire lives in linear habitats. In this case, the corridor must provide sufficient resources to support such species.

Types 
Habitat corridors can be categorized according to their width, with wider corridors generally encouraging more use. However, overall corridor quality depends more on design when creating an effective corridor. The following are three divisions in corridor widths: 
 Regional – (> wide); connect major ecological gradients such as migratory pathways.
 Sub-regional – (> wide); connect larger vegetated landscape features such as ridge lines and valley floors.
 Local – (some <); connect remnant patches of gullies, wetlands, ridge lines, etc.

Habitat corridors can also be divided according to their continuity. Continuous corridors are strips that are not broken up, while "stepping stone" corridors are small patches of suitable habitat. However, stepping stone corridors may be more susceptible to edge effects.

Corridors can also take the form of wildlife crossings, underpasses or overpasses used for crossing man-made feature such as roads, reducing human-wildlife conflict such as roadkill. Observations have shown that underpasses are actually more successful than overpasses because many times animals are too timid to cross over a bridge in front of traffic and would prefer to be more hidden.

Monitoring use 

Researchers can use mark-recapture techniques and hair snares in order to evaluate genetic flow to observe how a corridor is being used. Marking and recapturing animals is more useful when keeping a close eye on individual movement. However, tagging does not give any insight into whether the migrating individuals are successfully breeding with other populations.

Genetic techniques can be more effective when evaluating migration and mating patterns. By looking at a population's gene flow, researchers can understand the genetic consequences of corridors via information about the migration patterns of a population over time.

Design
Wildlife corridors are most effective when they are designed considering the ecology of their target species, in addition to factors like seasonal movement, avoidance behavior, dispersal, and habitat requirements.

Generally, corridors are best built with a certain degree of randomness or asymmetry, and oriented perpendicular to patches. Wildlife corridors are susceptible to edge effects; habitat quality along the edge of a habitat fragment is often much lower than in core habitat areas. Habitat corridors are important for large species requiring significant sized ranges; however, they are also vital as connection corridors for smaller animals and plants as well as ecological connectors to provide a ‟rescue effect’’.

Examples 

In Alberta, Canada, an overpass was constructed to keep animals off the busy highway which crosses a national park. The top of the bridge is planted with native grasses, and fences were put on either side to help guide animals.

In Southern California, 15 underpasses and drainage culverts were observed to see how many animals used them as corridors. They proved to be especially effective on wide-ranging species such as carnivores, mule deer, small mammals, and reptiles, even though the corridors were not intended specifically for animals. Researchers also learned that factors such as surrounding habitat, underpass dimensions, and human activity also played a role in the frequency of usage.

In South Carolina, five remnant areas of land were monitored; one was put in the center and four were surrounding it. Then, a corridor was put between one of the remnants and the center. Butterflies that were placed in the center habitat were two to four times more likely to move to the connected remnant rather than the disconnected ones. Furthermore, male holly plants were placed in the center region, and female holly plants in the connected region increased by 70 percent in seed production compared to those plants in the disconnected region. The most impressive dispersal into the connected region, though, was through bird droppings. Far more plant seeds were dispersed through bird droppings in the corridor-connected patch of land.

There have also been positive effects on the rates of transfer and interbreeding in vole populations. A control population in which voles were confined to their core habitat with no corridor was compared to a treatment population in their core habitat with passages that they could use to move to other regions. Females typically stayed and mated within their founder population, but the rate of transfer through corridors in the males was very high.

In 2001, a wolf corridor was restored through a golf course in Jasper National Park, Alberta, which successfully altered wildlife behavior by being frequently used by the wolves.

Major wildlife corridors
 The Paséo Pantera (also known as the MesoAmerican Biological corridor or Paséo del Jaguar)
 The Eastern Himalayan Corridor
 China-Russia Tiger Corridor
 Tandai Tiger Corridor
 The European Green Belt
 The Siju-Rewak Corridor, located in the Garo Hills of India, protects an important population of elephants (thought to be approximately 20% of all the elephants that survive in the country). This corridor project links together the Siju Wildlife Sanctuary and the Rewak Reserve Forest in Meghalaya State, close to the India-Bangladesh border. This area lies within the meeting place of the Himalayan Mountain Range and the Indian Peninsula and contains at least 139 other species of mammal, including tiger, clouded leopard and the Himalayan black bear.
 The Ecologische Hoofdstructuur is a network of corridors and habitats created for wildlife in the Netherlands
 The  long Kanha-Pench elevated corridor on NH 44.
 Two elephant passes and two minor bridges on NH 54 in Assam’s Lumding Reserve Forest.
 Three elephant underpasses, each with  of vertical clearance on NH 72 and NH 58 in Uttarakhand, India.
 Terai Arc Landscapes, Lower Himalayan Region.

Evaluation 

The overall success of wildlife corridors has yet to be definitively demonstrated. Corridors are not a replacement for preserving land against fragmentation. However, they can help bolster populations against increasing urbanization and habitat destruction.

Some species are more likely to utilize habitat corridors depending on migration and mating patterns, making it essential that corridor design is targeted towards a specific species. Plants may use road verges as corridors, however some mammals may not cross roads.

There is usually a very limited amount of space available for corridors, so buffers are not usually added in. Without a buffer zone, corridors can become affected by disturbances from human land use change. There is a possibility that corridors could aid in the spread of invasive species, threatening multiple populations.

Unfortunately, another limiting factor to the implementation of corridors is that they are often extremely expensive to construct. With inconclusive data about the effectiveness of connecting land, it may be difficult to get proper funding.

Although wildlife corridors have been proposed as solutions to habitat and wildlife population fragmentation, there is little evidence that they are broadly useful as a conservation strategy for all biodiversity, as compared to protecting connectivity.

See also

 Aquatic organism passage
 Biolink zones
 Emerald network
 Habitat conservation
 Habitat destruction
 Landscape connectivity
 Marine Protected Area
 Natura 2000
 The Pollinator Pathway
 Roadkill
 Gary Tabor, wildlife corridor conservationist
 Tugay
 Wildlife crossing
 Yellowstone to Yukon Conservation Initiative

Further reading 

Bennett, A.F. 1999. Linkages in the Landscape: The Role of Corridors and Connectivity in Wildlife Conservation. The World Conservation Union, Gland, Switzerland.
De Chant, T. 2007. A Future of Conservation. Northfield Habitat Corridors Community Plan, Northfield, Minnesota.
Department of Environment and Conservation (DEC). 2004. Wildlife Corridors. DEC, New South Wales. 
Dole, J.W., Ng, S.J., Sauvajot, R.M. 2004. Use of Highway Undercrossings by Wildlife in Southern California. Biology Conservation, 115 (3):499-507. &Foreman, Dave. Rewilding North America: a Vision for Conservation in the 21st Century. Washington: Island, 2004.

M., S. 2002. Ecology: Insects, Pollen, Seeds, Travel Wildlife Corridors. Science News, 162 (10):269.

Roach, J. 2006. First Evidence that Wildlife Corridors Boost Biodiversity, Study Says. National Geographic Society, Washington, D.C.

References

External links

Defragmentation in Belgium (Flanders) - Connecting nature, connecting people. Accessed: 22 January 2009
Wildlife Corridors Project Regeneration
Wildlife passages - De-Fragmentation in the Netherlands - How to evaluate their effectiveness? Accessed: 22 January 2009
CorridorDesign.org - GIS tools for designing wildlife corridors Accessed: 9 March 2010
ConservationCorridor.org - information, tools and links to connect the science of landscape corridors to conservation in practice. Accessed: 14 September 2012

Conservation biology
Conservation projects
Ecological connectivity
Ecological restoration
Environmental conservation
Habitat
Habitats
Systems ecology
Urban studies and planning terminology
Wildlife conservation
Animal migration