Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 405

one nesting a year, whereas birds at lower elevations may nest two or three times (Lack
1954). Insects at high altitudes frequently take two or three years to complete their life
cycle (Fig. 8.2). They may hibernate at different stages of development in succeeding
years—egg, larvae, pupae, and adult—until complete metamorphosis is achieved (Mani
1962). This lower level of productivity in turn reduces opportunities for diversification
and speciation.
Temperature also affects animal distributions indirectly through its influence on land
form, vegetation, soil development, habitat diversity, and food supply. Temperature con-
trols the distribution of permafrost, which limits root penetration or excavation by or-
ganisms. Warm temperatures ascend the slopes in spring, beginning on southern expos-
ures and expanding to north-facing slopes. Consequently, a band of sprouting, highly di-
gestible and nutritious vegetation moves uphill, and so do large herbivores. Therefore,
the taller the mountains, the longer the availability of high-quality food for mountain
herbivores. Similarly, vegetation sprouts at the edges of snowfields that are retreating
as temperatures rise. Cliff faces exposed to the winter sun shed their snow earliest and
become favorite places for mountain sheep and mountain goats in late winter. The body
heat of resting sheep and goats accelerates the sprouting of plants rooted at the edge
of such beds. Consequently, the first spots to green up in spring may be the habitual
resting places of wild sheep. Since these animals defecate and urinate after rising, the
beds tend to be well fertilized and humus-rich from millennia of feces deposition.
Daily temperature fluctuations can generate very hazardous snow conditions. In
spring, snowfields may thaw in the afternoon but freeze solid overnight, forming a sun-
crust, a steep ice surface on which no large animal can hold itself, sometimes leading
to fatal accidents. On more level areas, sun-crusts allow sheep and goats to disperse
before the snow melts at noon. Sun-crusts also limit the food supply for mountain ungu-
lates to the afternoon and early evening, as their hooves cannot penetrate the ice crust
when ambient temperatures are low. Consequently, wild sheep will rest until about noon
before feeding, instead of feeding at dawn as they normally do. In cirques, the shadow
effect may cause very cool summer temperatures, much sought out by caribou seeking
relief from biting flies. Caribou also avidly seek out cool temperatures on snowfields
and glaciers in summer to avoid insect pests, warble flies in particular. Slopes exposed
to the sun generate upward-moving air currents that are used for lift by eagles, vul-
tures, and condors, but may also discourage biting flies. Increasing daily temperatures
in the spring lead to predictable avalanche activity in the afternoon, as well as accel-
erated rock fall. Both are mortality factors for the unwary. The warm katabatic winds
(e.g., Chinooks or foehn ) generated by mountains (see Chapter 3) clear exposed ridges
of snow, thus exposing plants that mountain ungulates consume as food (Geist 1971,
2002). Very low temperatures along glacial fronts may maintain soft, powdery snow that
animals can remove by pawing, allowing access to forage, even when elsewhere thaw-
ing and freezing have formed ice crusts on the snow.
Environmental Oscillation
Fluctuation of environmental conditions increases with latitude and altitude, greatest at
the poles and least at the equator. Consequently, organisms at high latitudes and alti-
tudes must adapt to wide ranges of climatic, ecological, and foraging conditions. Latit-