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A useful way to look at corridors focuses on the ecological functions that a corridor serves for a given species. In the broad view we see corridors as pathways for the flow of genes over time. In a closer view we see corridors as routes for individual animals moving between populations. On the finest scale we find corridors that animals need to survive and reproduce on a yearly or even daily basis.

Terms used in the table are defined in the text that follows. Highlighted text in the table may be selected to bring up further information or you can scroll through the text to find the same information.

Gene flow corridors

In the context of corridors, gene flow can be considered to occur at three different rates in the table above.

Gene flow 1 occurs within a single generation. This generally occurs when a dispersing individual (often a subadult male) moves from its natal home range to a more distant area where gene frequencies in the local population are somewhat different than its own. The immigration of a single successfully-breeding individual per generation is sufficient to maintain the same alleles in both populations; although the relative frequencies of those alleles may be quite different. The temporal and geographic scales at which gene flow occurs depend upon the ecology and life history strategy of the species in question: in general, larger-bodied animals travel over greater distances. For grizzly bears gene flow 1 can occur through regional corridors; for pikas it will occur through local corridors.

Gene flow 2 represents the movement of alleles from one population to another over several generations. Alleles are passed on from one individual who disperses, to its offspring who also disperse, to their offspring who also disperse. Some of the original alleles are lost through segregation along the way, and populations seperated at this scale maintain alleles that are locally unique. Again, the temporal and geographic scales at which this occurs depends upon the ecology and life history strategy of the species in question. For example, gene flow 2 occurs in grizzly bear populations on a continental scale, but in marten or fisher on a regional scale.

Gene flow 3 can be conceived of as the movement of alleles over even longer genetic distances: it requires many generations for an allele to be introduced from one population to another. Populations connected by gene flow at this rate are quite distinct from one another; they have unique alleles and probably unique, co-evolved gene complexes. In a genetic sense such populations may be regarded as subspecies with local adaptations. Gene flow 3 occurs over evolutionary, rather than ecological, time. Once again, the temporal and geographic scales at which this occurs depends upon the ecology and life history strategy of the species in question.

Ecological Corridors

Dispersal: [subadults in search of suitable habitat]

Many species of terrestrial vertebrates, particularly mammals, have evolved life history strategies where one or both sexes disperse away from their parents as they approach breeding age (after weaning in mammals). If their habitat is fully occupied (at carrying capacity) they may have to travel long distances to find a place to live. Alternatively, they may live marginally, in the interstices between occupied territories or home ranges, until suitable habitat is vacated by death. Those animals that do disperse may need to cross expanses of unsuitable habitat, or they may utilize a corridor which has enough resources to sustain them in transit but does not have all the resources necessary to maintain a breeding pair throughout their lifetimes. Dispersal corridors function to maintain gene flow at level 1. Dispersal corridors may require quite different physical attributes for different species of animal.

Grizzly bears, for instance, are generalist omnivores that can utilize a very broad spectrum of habitat types. They are also capable of travelling distances of several hundred kilometers in a few days. Prehistorically, the entire Rocky Mountain region and riparian areas of the Great Plains were grizzly bear habitat. The only barriers to movement were large arid regions with sparse vegetation and little water. Currently, human development has fragmented grizzly habitat into core areas of protected wilderness. Human presence is the major barrier to movement and bear-human interractions increase the probability of mortality for the bear. Consequently, dispersal corridors are defined primarily by lack of human development; mainly remote, roadless areas with food and cover.

Mountain lions are generalist predators that also disperse over long distances. They are more tolerant of human activities than grizzlies, and less apt to utilize human-provided sources of food. They have been known to utilize long, narrow corridors for dispersal and even to pass through bottlenecks such as highway underpasses. When subadult males leave their natal areas and disperse to find a suitable place to live, they range widely, utilizing a wide variety of habitat, much of which is unsuitable for full-time occupancy. After a month or more of travel in this "dispersal mode" they key in on habitat that will support them, and establish a home range. They can disperse over long distances of marginal habitat; the only absolute requirement seems to be some degree of cover during daylight hours.
In cooperation with the Craighead Environmental Research Institute and the Montana Department of Fish, Wildlife, and Parks, American Wildlands is helping to follow the movements of dispersing juvenile cougars in the Big Belt Mountains east of Helena.

Gray wolves also disperse over long distances and can cross marginal habitat. A wolf that was radio-tagged near Glacier Park ended up just East of Missoula where she mated and the pair raised three young. Her dispersal route went through downtown BigFork, Montana at one point. Wolf movement corridors in Banff National Park, Canada, have been carefully analyzed by Dr. Paul Paquet with World Wildlife Fund, Canada.

Elk also disperse to some extent from the natal herd. Animals may leave their herd during summer and join another herd, then follow that herd to a different winter range area. In other cases, several herds coalesce on the same winter range and individuals follow a new herd to a different summer range the following season. American Wildlands is conducting a pilot study on elk habitat and migration corridors using GIS in the Madison Range west of Yellowstone Park.

Home range corridors

In contrast to dispersal corridors where an individual leaves one place to settle in another place, and probably never returns; animals often need to move within their home ranges from one type of habitat to another. These movements can be regular and predictable depending upon the species and the season, or they can be stochastic in nature; depending upon varying climatic conditions and availability of food or other resources.

For many ungulates, such as elk, pronghorn antelope, and bighorn sheep, the greatest movements and utilization of corridors occurs during annual migrations between summer and winter ranges. As mentioned above, dispersal and consequent gene flow in these species occurs when herds come into contact on seasonal ranges or when individuals wander within the seasonal range. Elk that summer in Yellowsone Park migrate up to 100 km south into Jackson Hole in winter; or migrate east, west, or north depending upon the herd they follow. Even deer have been known to make long seasonal movements. A female mule deer was radiocollared on her winter range in the Bridger Mountains, near Bozeman, Montana. In two successive summers, she moved to an area near Madison Junction in Yellowstone Park (about 100 miles away) and returned to the Bridgers each fall. To do this, she needed to cross Interstate 90 near Bozeman twice a year.

Grizzly bears utilize a wide variety of foods, most of which vary in abundance from year to year, and season to season. During years with good berry crops, grizzlies will move to those areas in the fall to forage. During years with few berries but good whitebark pine cone production, grizzlies will concentrate in the fall in areas of whitebark pine. Seasonal movements also vary depending upon sex and age class. For example, if good berry or whitebark pine habitat is dominated by aggressive, adult males, females and subadult males will avoid those areas and forage in less desirable habitat.

After winters of heavy snowfall, grazing animals may be restricted in their movements and foraging by late-melting snowpacks. They may travel to other areas where food is available earlier or utilize more patchy food sources.

Within territories

Animals that defend discrete territories, such as wolves, will utilize different habitats within those territories at different times of the year. Depending upon the availability of prey they may move fairly long distances, through corridors with few resources, to seasonal use areas. In the spring they will move to a den site and remain there until the pups can be moved to a better location if needed.

Corridors within seasonal range- utilizing different foods

On the summer range elk may move from lower elevations to higher elevations as food becomes available, and then back to lower elevations as high elevations become snow covered.

Daily or weekly use corridors

An animal's use of habitat varies throughout the day and from day to day. Bighorn sheep, for example, will generally move down from foraging habitat and escape terrain in order to drink from streams or water holes once a day. In many cases, regular corridors of movement are used. In a similar fashion, bighorn sheep, mountain goats and other species utilize mineral licks on a daily or several-day basis.

Many species move regularly between resting and foraging habitat. If they feed in meadows in the early morning they may move to more secure, perhaps shaded, habitat during mid-day to rest, and return to foraging areas in the evening. These local movement corridors are particularly important where they are constrained by human development. Deer, for example, may winter on open hillsides near a town, but need to move through developed areas in order to drink. Conversely, they may feed in developed areas such as lawns and golf courses, and move to more remote areas to rest.

A corridor need not consist of contiguous habitat. For many volant species such as butterflies, passerine birds, waterfowl, or raptors, a corridor may be composed of stepping stones of habitat connecting breeding areas with wintering areas. Waterfowl, for instance, that breed in the far north require stepping stones of open water habitat in order to migrate to and from their wintering areas in the south. Some stepping stones may be utilized as brief stops to rest, whereas others that are adjacent to good foraging habitat may be used for several days.

Demographic rescue

Dr. Michael Soule, with the Wildlands Project, introduced the term 'conservation corridor' which "facilitates the biologically effective transport of animals between larger patches of habitat dedicated to conservation functions". This definition focuses on the movements of individual animals, but implies the genetic and demographic processes that such movements maintain. Conservation corridors are geographic features which allow the persistence of populations within and between larger islands of habitat; over both ecological and evolutionary time frames.

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