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The Rocky Mountains represent the longest and in general the highest of the North American mountain ranges, extending for nearly two thousand miles from their origins in Alaska and northwestern Canada southward to their terminus in New Mexico, and forming the continental divide for this entire length. As such, these mountains have provided a convenient corridor for northward and southward movement of both plants and animal life, but on the other hand have produced important barriers to eastern and western plant and animal movements. These effects result nat only from their height and physical nature, but also from their manifold effects on such things as precipitation, humidity, temperature, and other important climatic factors affecting plant and animal life.
The bird life of the Rocky Mountains is surprisingly uniform, in spite of their great latitudinal spread and the equally wide altitudinal variations that occur in the region. Thus, a bird-watcher in Banff or Jasper national parks will encounter the vast majority of the same breeding species in the coniferous zones of those areas as one who is observing birds nearly a thousand miles to the south in Rocky Mountain National Park, although particular bird species would occur at considerably different altitudes. This is a result of the horizontal zonation patterns of organisms, which individually distribute themselves along the slopes of mountains within vertical bands that conform to their limits of physiological stress and their biological requirements for food, cover, reproduction sites, and the like. Because of these biotic interactions, complex plant and animal communities have evolved through time.