Two interpretations governing the vertical distribution of plants and animals in the western mountain regions have been developed in the past years. Both are based on the premise that definite plants and animals (known as zone indicators) have maximum and minimum altitudes, above and below which they are unable to survive. The net effect is to group these plants and animals into belts or zones on mountain slopes, which vary but little in elevation above sea level throughout the western United States. The reasons why increases or decreases in mountain elevation so markedly affect the distribution of plant life, and to a much lesser degree the animal life, are closely correlated with the differences of temperature, available moisture, wind velocity, exposure of area to sunlight, soil, and topographic variations existing between these zones. Temperature in particular, being an easily measurable difference, has been used by Merriam in his classification of life zones. He computed the mean annual temperatures and made temperature summations for each clearly recognized zone of plant and animal life; he found that for each 1,000 foot rise in elevation there was a corresponding decrease in temperature of 3° F. Based on these temperature differences, definite geographical belts were formed and given names—arctic-alpine, hudsonian, canadian, transition and sonoran zones. Although in current use throughout the west, these zones are not clearly separable in the north-central Rocky Mountain region of Colorado, and therefore are not used here.

Weaver and Clements, following the same general idea, but considering all of the various factors mentioned above, devised a classification of zones which is applicable to the Park mountains and will be mentioned below. Actually, the trees and smaller plants fit very well into these zones, but animals, because of their mobility and wide adaptibility, can hardly be classed in any definite zones. Most animals range at various times of the year through all three zones mentioned, but because a few do inhabit certain areas a large part of the time, they are considered to be typical of these zones. Probably the real limiting factor for animal localization is the degree of severe winter conditions they can endure; the more adaptable they are to low temperatures, the higher they may be found in the mountains throughout the year. Of course, the distribution of herbivorous (plant-eating) animals largely determines the range of the predatory animals feeding on them.