Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 406

ude controls the length of day and the seasons; combined with latitude, these effects are
accentuated. For example, the growing season at any given latitude is generally short-
er in mountains. Even in the humid tropics, where there is no marked seasonality, the
range of daily environmental extremes increases with altitude—a species may have to
cope with intense sunlight every afternoon and freezing temperatures every night. The
number of species that can survive these conditions decreases with elevation, and each
loss brings about concurrent changes in community structure and environment. The
most striking change occurs at timberline, since the forest provides a buffer. Beyond
timberline, species must adapt to the open habitat or be eliminated. The environment
changes, and the soil becomes exposed to the full brunt of sun, rain, wind, and extremes
of temperature. This may lead to other processes such as increased frost creep and so-
lifluction (see Chapter 5), which in turn lead to greater erosion, stream infiltration, and
habitat instability. Consequently, with the loss of trees, community structure becomes
much less complex. There may be less food and cover, and greater environmental ex-
tremes.
Longer winters reduce the food supply, since the growing season is shorter and snow
and ice cover the vegetation. While there may be ample food during the summer, little is
available during the winter. Organisms must either leave the area in search of more fa-
vorable conditions (migrate), reduce their need for food physiologically (hibernate), or
circumvent the need for food in some other way. For example, the pocket gopher ( Tho-
momys spp.) stays active throughout the winter by burrowing and by harvesting plant
roots in soil protected from freezing by deep snow. The strategies of animals for dealing
with the lack of food also solve problems of dealing with great environmental extremes.
With increasing environmental oscillation, there is increasing fluctuation of nutrients
and of population numbers, and energy flow through the system is interrupted (Dunbar
1968). The opposite is true in humid tropical lowlands, where there is great environ-
mental complexity of species and a maximum flow of energy through the system.
While environmental oscillations can limit life, climatic oscillations generating sea-
sons also generate new and spectacular life forms. Because of the great discrepancies
between productivity in summer and winter, animals at high latitudes in summer escape
the limiting consequences of food competition and exploit the abundance in their sea-
sonal food supply. The seasonal overabundance of food creates a vacation from want,
allowing not only superior reproduction, but also superior body growth, the evolution of
luxury organs, massive fat deposition, and the evolution of novelty. By contrast, tropical
species may struggle with want year round, and their morphology and behavior reflect
the severe scarcity of resources for body growth, reproduction, and innovation. This
phenomenon has been explored in detail under the heading of net primary production
and is significant here as net primary production latitudinally is mirrored altitudinally
(Huston and Wolverton 2009, 2011; Wolverton et al. 2009).
Because the Ice Ages accentuated latitudinal habitat differentiation, they also gen-
erated sharply different adaptations with latitude and altitude. Moreover, because di-
versity of seasonal habitats generates new problems of adaptation with each season,
the brain responds by growing larger. We therefore observe, within the same family,
highly conservative, small-brained tropical species and highly evolved luxury species
with large brains at high latitudes. The latter, because of large size, showy hair coats,
large ornamental antlers and horns, fat deposits, and novel life strategies, may be called