Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 408

short growing season (see Chapter 7). Thus, there is abundant food during the growing
season, and a large variety of animals spend their summers in high mountains, mainly
to reproduce. The lack of nutrients is an almost insurmountable problem to year-round
occupancy of alpine tundra and, in this respect, limits the utilization and efficiency of
the ecosystem. However, in the vicinity of glaciers there can be a surfeit of nutrients,
as glaciers grind rock into fertile silt and dust, which tend to accumulate downstream
and downwind of the glacier. Thus silt flats and loess fields, formed respectively from
rock dust transported by water and wind, are areas of high nutrient availability, some of
which crystallize out as salts and are used by animals as mineral licks (Geist 1978).
Lack of Habitat Diversity
The fundamental characteristic of high mountain landscapes is the openness of the hab-
itat—the low-lying alpine vegetation and bare rocks. The tundra is essentially a two-di-
mensional habitat. The absence of trees eliminates the vertical component and reduces
the number and diversity of ecological niches. Species that depend on the forest for
food or shelter are eliminated unless they can adapt to the open country. Birds must
build their nests on the ground and hover in the air to give their mating calls. Vegetation
above timberline consists of grasses and herbaceous species with a simple community
structure. Rocks usually abound, and these become increasingly important as habitats
(Hoffmann 1974). Some creatures are found almost exclusively in rocky areas—for ex-
ample, picas ( Ochotona spp.), marmots ( Marmota spp.), and mountain goats ( Oreamnos
americanus ) (Fig. 8.3). Snowcover is also a prime consideration: A number of species
depend on it for insulation and protection during winter. Patterns of population distribu-
tion for many creatures, large and small, are directly related to the distribution of snow
(Pruitt 1960, 1970; Sleeper et al. 1976; Stoecker 1976; Geist 1971).
The most important source of habitat diversity in the alpine zone is the lay of the
land itself. Topography largely controls the distribution of rocks and snow. Rocky crags,
cliffs, valleys, and differences in slope gradient and exposure provide a variety of sur-
faces for the interplay of climatic, geomorphic, edaphic, and biologic factors, resulting
in a mosaic of microhabitats. In general, areas of the least topographic diversity (e.g.,
smooth slopes) have the fewest ecological niches and least species diversity, where-
as areas of maximum topographic diversity support the greatest number of ecologic-
al niches and maximum species diversity. Minimum complexity is found on small and
simple mountains that slope directly to the summit (e.g., volcanic cones), whereas max-
imum complexity is found on large and diverse folded mountains, such as the Alps or
Himalaya. Topographic diversity is a major reason why there are more species in alpine
environments than in the arctic tundra (Hoffmann 1974). Nevertheless, habitat diversity
is considerably less in alpine than in lowland environments.
Youthfulness of the Ecosystem
As discussed in Chapter 5, most mountains are young landscapes, both because they are
relatively recent geological creations and because many have experienced severe glaci-
ations. The Tertiary, which extended over 65 million years, was a period of major moun-