A : 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.
B : 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 the southern part of the continent has deserts, so the bison could not spread there. For aquatic creatures, strong currents, differences in salinity, or land areas may serve as barriers.
C : Some barriers involve competition with other species. A dandelion seed may be carried by the wind to 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.
D : 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. Kirkland’s 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. Brazil’s 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.
E : 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 America’s Rocky Mountains, which stretch from Alaska to northern Mexico, are 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.
F : 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 route consists of several similar types.
G : The third type of natural pathway is called a sweepstakes route. This is dispersal caused by the chance combination of favorable conditions. Bird watchers are familiar with “accidentals,” which are birds that appear in places far from their native areas. Sometimes they may find a habitat with favorable conditions and “colonize” it. Gardeners are familiar with “volunteers,” cultivated plants that grow in their gardens although they never planted the seeds for these plants. Besides birds and plants, insects, fish, and mammals also colonize new areas. Sweepstakes routes are unlike either corridors or filter routes in that organisms that travel these routes would not be able to spend their entire lives in the habitats that they pass through.
H : Some organisms cross barriers with the intentional or unintentional help of humans, a process called invasion. An example is the New Zealand mud snail, which was accidentally brought to North America when trout from New Zealand were imported to a fish hatchery in the United States. It has caused extensive environmental damage in streams and rivers. In the invasive species’ native environments, there are typically predators, parasites, and competitors that keep their numbers down, but in their new habitat, natural checks are left behind, giving the invaders an advantage over native species. Invasive species may spread so quickly that they threaten commercial, agricultural, or recreational activities.
