Geography Reference
In-Depth Information
nah, and reduce the fuel load on the land, thus restricting the size and intensity of
lightning-set fires. In addition, the giants excavated drinking holes in dry creek and
river beds, and thus also provided water for others. With the extinction of the large
mammals and the widespread application of human-set fires, the landscape must have
changed to the disadvantage of many species that had coevolved with the megafauna.
Our current landscapes thus reflect not merely the forces of nature, but the severe dis-
tortions imposed deliberately and inadvertently by humans in prehistoric and historic
times (Komarek 1981; Delcourt and Delcourt 1997).
Today, the exact number and composition of species vary for different mountain re-
gions, depending on environmental conditions and the size, elevation, age, history, cli-
matic patterns, and relationship to other mountain areas. Generally, the larger, taller,
and thus more climatically diverse the mountains, the greater the number of species.
Thus, the largest number of mountain species exists in the Himalaya, with its enormous
canyons and its altitudinal and latitudinal striation of biotic communities from tropical
to glacial and from rainforest to alpine deserts. There are no accurate surveys for the
whole area, but species number in the hundreds (Hoffmann 1974). The opposite is also
true: Small, isolated mountains display the least diversity. A good example is the Big
Snowy Mountains, a small isolated range on the Great Plains of central Montana: Only
seven species of mammals and four species of birds are known to breed in this alpine
area (Hoffmann 1974). A number of other species visit in the summer, just as on the
Beartooth Plateau, but the contrast in numbers is considerable.
Although alpine and arctic zones have much in common, the island-like, patchy dis-
tribution of mountains and the large, nearly continuous circumpolar belt of the Arctic
constitute a fundamental difference. Arctic animals are able to maintain relatively large
populations, and there is a good degree of continuity and interchange, with the same
species occurring throughout. This is impossible in disjunct and widely scattered moun-
tains because of habitat patchiness and the difficulty in dispersal and colonization. Both
flora and fauna become increasingly diverse away from the polar regions, particularly
mammals, to the point that “not a single tundra species is common to both the alpine
[tundra] in temperate latitude mountains and the arctic tundra” (Hoffmann 1974). This
is true, however, for species that live only in arctic or alpine tundra; other widely ran-
ging species—that is, subarctic and subalpine—occasionally occupy both environments.
Birds, insects, and vegetation are less exclusive; a number of species are shared by arc-
tic and alpine tundra. Amphibians and reptiles are notably rare in both zones (Hock
1964a, 1964b).
Limiting Factors
In spite of differences in distribution, arctic and alpine animals have similar approaches
to life. One early observation in biogeography was that of convergent tendencies in the
environment, as expressed in the flora and fauna, with increase in latitude and altitude.
We now know that there are a great many exceptions to this simple rule, but at the
broadest level it is still useful.
Because it gets colder as altitude and latitude increase, it has also long been as-
sumed that low temperature is one of the major limiting factors to life. Dunbar (1968)
contends that, while low temperature may be the immediate or proximate cause for
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