Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 410

The immaturity of mountain ecosystems can perhaps best be seen by comparing
mountains with lowland tropical environments. The humid tropics are the oldest envir-
onments, containing the greatest number of species and organic complexity. The ma-
jor problem for life in the tropics is scarcity of energy and nutrients, and thus severe
competition within and between species. Consequently, organisms use their energies
primarily for specialization, the corollary of which is ecological diversification. The res-
ult is a complex network of species occupying a maze of ecological niches for a near-
complete utilization of available resources (Dobzhansky 1950). Just the opposite is true
in mountains, where the bulk of energy is devoted to coping with difficult physical con-
ditions and there is little diversification or specialization.
Island-like Distribution
As discussed in Chapter 7, mountains have been likened to islands, since they provide
habitats unlike those of surrounding lowlands and support different floras and faunas.
The analogy is only approximate, however. The surrounding lowlands present a less hos-
tile environment to terrestrial alpine organisms than does the sea. An alpine bird tiring
from flight between mountain areas can stop to rest in the lowlands, but one who tires
over the sea is almost certainly doomed. Another difference is that the zone of transition
between environments for oceanic islands takes place abruptly at the shoreline and al-
ways at the same elevation—that of sea level. The mountain equivalent of a shoreline
occurs at various absolute elevations, depending on the climate, and the transition from
the adjoining environment is more gradual.
Nevertheless, mountains and oceanic islands share a number of ecological charac-
teristics (Diamond 1972, 1973, 1975, 1976; MacArthur 1972; Carlquist 1974; Mayr and
Diamond 1976). In particular, the number of species and their diversity are closely re-
lated to the size of the island (mountain) and its distance from the mainland or other
islands (mountains). The larger the island, the greater its resource base, carrying capa-
city, and variety and availability of habitats. The greater the distance from other islands,
the fewer species are likely to be found. Both of these characteristics are demonstrated
by the size and distance between enclaves of alpine vegetation in the northern Andes
and the number of bird species they support.
Another characteristic of islands is their increasing taxonomic specificity with insu-
larity. The more isolated and remote an island, the more unique its flora and fauna, since
evolution has operated under local conditions to create endemic taxa. One has only to
think of the exotic species on islands such as New Zealand or the Galápagos for con-
firmation. For high mountains, the greatest insularity exists in the tropics. As with plant
species, high proportions of endemic animal species are found in the alpine areas of
East Africa (Salt 1954; Coe 1967; Coe and Foster 1972).
The opposite situation exists in arctic and subarctic mountains: Because there is free
interchange with lowland tundra species, virtually no endemic species occur (Brinck
1974). Midlatitude mountains are intermediate in this regard, their insularity being de-
termined by their age and the nature of the surrounding habitats. For example, the
alpine zone in the Rocky Mountains is surrounded by subalpine forests, with direct con-
nections in the north to the eastern boreal forest and the western forests of the Pacific
shores. The plants and animals thus have fairly easy access from arctic and subarctic