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.