Virtually all living things have some way of getting from here to there. Animals may walk, swim, or fly. Plants and their seeds drift on wind or water or are carried by animals. Therefore, it is reasonable to expect that, in time, all species might spread to every place on Earth where favorable conditions occur. Indeed, there are some cosmopolitan species. A good example is the housefly, found almost everywhere on Earth. However, such broad distribution is the rare exception. Just as barbed wire fences prevent cattle from leaving their pasture, biological barriers prevent the dispersal of many species.
What constitutes barriers depends on the species and its method of dispersal. Some are physical barriers. For land animals, bodies of water, chains of mountains, or deserts are effective. For example, the American bison spread throughout the open grasslands of North America, but in the southern part of the continent there are deserts, so the bison could not spread there. For aquatic creatures, strong currents, differences in salinity, or land areas may serve as barriers.
Some barriers involve competition with other species. A dandelion seed may be carried by the wind to bare ground, and, if environmental factors are right, it may germinate. There is not much chance, however, that any individual seedling will survive. Most places that are suitable for the growth of dandelions are already occupied by other types of plants that are well adapted to the area. The dandelion seedling must compete with these plants for space, water, light, and nutrients. Facing such stiff competition, the chances of survival are slim.
For animals, some barriers are behavioral. The blue spotted salamander lives only on mountain slopes in the southern Appalachian Highlands. Although these creatures could survive in the river valleys, they never venture there. Birds that fly long distances often remain in very limited areas. Kirklands warblers are found only in a few places in Michigan in the summer and fly to the Bahamas in winter. No physical barriers restrict the warblers to these two locations, yet they never spread beyond these boundaries. Brazils Amazon River serves as a northern or southern boundary for many species of birds. They could freely fly over the river, but they seldom do.
There are three types of natural pathways through which organisms can overcome barriers. One type is called a corridor. A corridor consists of a single type of habitat that passes through various other types of habitat. North Americas Rocky Mountains, which stretch from Alaska to northern Mexico, is an example. Various types of trees, such as the Engelmann spruce, can be found not only at the northern end of the corridor in Alaska but also at higher elevations along the entire length of this corridor.
A second type of natural pathway is known as a filter route. A filter route consists of a series of habitats that are different from one another but are similar enough to permit organisms to gradually adapt to new conditions as they spread from habitat to habitat. The greatest difference between a corridor and a filter route is that a corridor consists of one type of habitat, while a filter consists of several similar types.
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