Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 411

to alpine and subalpine settings. Differences between the two tundras increase south-
ward: In Colorado, more than half the plant species are non-arctic in origin, and the
percentage for animals is even higher. But a few endemic species do exist. On the oth-
er hand, the Caucasus Mountains between the Black and Caspian Seas, separated from
the Arctic by vast expanses of semiarid steppe, are much more isolated. Consequently,
more than half the species found there are endemic (Zimina 1967, 1978). Another strik-
ing example is provided by the Basin and Range mountains of the American West (Fig.
8.4). These mountains exist as isolated alpine tracts surrounded by desert shrublands;
as such, they are extremely insular. The alpine populations are totally isolated, and each
peak has its own set of endemic species (Brown 1971; Johnson 1975).
FIGURE 8.4 Mountain “islands” in the Great Basin between the Sierra Nevada (left) and Rockies
(right). Each of these islands is at least 3,000 m high, surrounded by lower-lying country support-
ing desert shrubs. During the Pleistocene, there was relatively easy access from one mountain
area to the other, but as the climate became warmer and drier, the species became isolated on
individual mountain islands. The two top islands are unshaded because their faunas are poorly
known and were not used in the original investigation. (After Brown 1971: 468.)
Theoretically, the number of species an island supports is set by the establishment of
an equilibrium between rates of colonization of new species and extinction of other spe-
cies. This principle also operates in mountains (Vuilleumier 1970; Brinck 1974). While
true islands can only be colonized by immigration from the mainland or from other is-
lands, mountains may be colonized by several other methods. One of these is by species
from adjacent lowlands adapting to the habitats of higher elevations (Fig. 8.5a).
Speciation commonly occurs by stages: There may be a series of species up a moun-
tain slope, each belonging to the same genus and fundamentally related, but isolated
through morphology or behavior so that they may no longer interbreed. One good ex-
ample from the mountains of East and South Africa is the hyrax, a small, rabbit-like un-
gulate, related to the coney of the Bible. Several species live on the mountain slopes.