Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 402

the decrease in numbers of species, it is not the ultimate reason. The ultimate limiting
factors are, rather, large environmental oscillations, lack of nutrients, lack of habitat di-
versity, and youthfulness of the ecosystem (Dunbar 1968). Alpine environments have all
of these problems, plus three other limiting characteristics of their own: the disjunct,
island-like distribution of mountain areas; reduced oxygen with altitude; and the need
to deal with gravity and broken landscapes, ascending or descending. Whether or not
temperature is the ultimate limiting factor, each of the above-mentioned factors takes
its toll and helps to establish the environmental framework in which mountain life ex-
ists.
Temperature
Although temperature has been singled out as being fundamental in controlling the dis-
tribution of organisms in mountains, all climatic factors are interrelated. The alteration
of one factor alters all the others, and the impact of any one of these may, in specific
cases, exceed the effect of temperature. This is implicit, but there is something com-
pelling about narrowing the explanation for any given phenomenon to a single cause. A
classic example is C. Hart Merriam's life-zone concept, in which he attempted to show
that the flora and fauna of North America are distributed into distinct life zones based
on temperature (Merriam 1890, 1894, 1898). This concept had a profound impact on
North American ecology, but has fallen into disrepute specifically because of its singu-
lar dependence on temperature (Daubenmire 1968; Kendeigh 1932, 1954). Such an ap-
proach may be useful in a small area where the mesh of factors is kept fairly constant,
but when the fundamental relationships change, so will the relative importance of each.
Alexander von Humboldt's climatic zones tierra caliente, tierra templada, and tierra
fria (see Chapter 11) work reasonably well in the humid tropics, but not elsewhere.
Likewise, Merriam's life zones (e.g., arctic-alpine, Hudsonian, Canadian) can be applied
fairly usefully in the region of his original study, the San Francisco Mountains, Arizona
(although the assumed convertibility of latitude and altitude is questionable), but not
necessarily in other areas. These were pioneering attempts that, in modern times, blos-
somed into the use of satellite imagery, often incorporated in geographic information
systems (GIS) (Aranoff 1989), as well as their application via ecosystems management
(Boyce and Haney 1997).
Another major problem with temperature as a limiting factor is in ascertaining its ef-
fect on organisms. Humboldt's and Merriam's life zones, and those of many other stud-
ies, were based on correlations between temperature and the distribution of plants and
animals. A certain species will reach a particular elevation but go no higher: The tem-
peratures at that elevation are presumed to be responsible for this point of maximum
distribution, but it is hard to identify the specific processes involved. For example, as
discussed in Chapter 7, the upper timberline coincides closely with the 10°C (50°F) iso-
therm for the warmest month, though the reason for this relationship remains elusive.
A number of animals reach their altitudinal limits at timber-line, but it would be dan-
gerous to say that these creatures are limited by temperature, when in fact they may
simply be limited by the absence of trees. In this case, then, temperature may be more
or less directly responsible for the location of timberline, but only indirectly responsible
for the distribution of the associated animal species.