Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 413

Another method responsible for the isolation of high-altitude species is the process
of mountain uplift (Fig. 8.5c). Some plants and animals appear to have originally been
lowland species but, as the land was uplifted, they were able to adapt to the slowly chan-
ging conditions. In this way, they became isolated from the lowland forms. One bird of
the genus Tinamous in the páramos of the tropical Andes lives near and on the snow,
breeding only above the timberline, while all other species of the genus live in the trop-
ical lowlands. The bird resembles a northern hemisphere ptarmigan ( Lagopus spp.) in
appearance and behavior, but is far removed in terms of structure and taxonomy, an ex-
ample of convergent evolution (Brown 1942).
A final method of colonization is through direct immigration during periods of cli-
matic change (Fig. 8.5d). This has happened chiefly in the middle and high latitudes,
where major migrations took place during the glacial and interglacial periods. Species
moved in advance of the ice and established themselves in belts or zones around the
ice in much the same way as happens today in arctic and alpine areas, that is, tundra
first, then coniferous forest. As the climate warmed and the ice melted, these plants
and animals began to reoccupy their former sites. Some moved northward, while others
moved upward into mountains where suitable habitats existed. In some cases, as in the
Rockies, the potential for exchange of species and colonization has continued, whereas
in other smaller, isolated ranges, the alpine communities were cut off and became rel-
ict populations. Examples are the alpine species in the Basin and Range mountains. For
this reason, they do not follow the equilibrium theory mentioned above; that is, there
have been extinctions but no new colonization (Brown 1971).
The “creep” of habitats latitudinally and altitudinally is possible during glacials and
interglacials if there are long stretches of unbroken land, as in Central Asia, with its
huge mountains and long valleys. “Habitat creep” is not possible where the landscape is
structured by many sharp, low mountain ranges with short distances between them, as
in Europe, where interglacial habitats go extinct with glaciations, and reassemble again
during interglacials. In Asia, this allows species to remain by merely following their hab-
itats during the glacial cycles. This conserves primitiveness. Where habitats go extinct
during glacial cycles, plastic generalists that can switch to different habitats are selec-
ted. Moreover, the huge amount of rock dust liberated by glacial grinding of bedrock
forms itself into long, circumpolar stretches of periglacial loess-steppe (Geist 1978),
which, due to its characteristic Ice Age fauna, has been labeled the “mammoth steppe”
(Guthrie 1990). Rock dust deposited by water (silt) and wind (loess) formed deep, fertile
soils close to glaciers along the shallow, braided streams, as well as shallow marches
along the edges of the huge proglacial lakes. Only tiny fragments of these habitats exist
during interglacials—that is, today. For the above reasons, Asia retained primitive or
specialized mammalian species, while Europe generated assertive generalists. This ex-
tends even to humans. Thus, while our primitive parent species Homo erectus survived
in Asia virtually from its inception some 1.6 million years ago, until displaced by “out of
Africa” modern man about 50,000 years ago, in Europe there evolved an advanced new
species, Neanderthal man, and later the modern Cro-Magnon of cave art fame. We see
a similar pattern in bears and deer (Geist 1999). The size of mountains and valleys thus
has a profound effect on speciation and survival during glacial cycles.