Environmental Engineering Reference
In-Depth Information
be allocated to reproduction and that small parents tend to produce off-
spring with a similar body plan. Individuals with the smaller body plan, a
smaller
phenotype
, will then have higher fitness at the cooler margin than
their larger counterparts and over time come to dominate the population
at the margin. Suppose now that small individuals preferred to breed with
similar-size individuals (what evolutionary biologists call
assortative mating
,
[Mayr 1982]) and that larger individuals tended to avoid the cooler margin
whenever possible.The consequence is that eventually, smaller individuals
might only associate with other smaller individuals and thus create popula-
tions of their own that eventually do not interbreed with larger individuals.
Such reproductive isolation, over a long period of time, may eventually lead
to a new species.That is, adaptation via natural selection has led to a new
species that tolerates the cooler conditions much better than the large indi-
viduals of the other species from which it originated.
Climate-Space
One of the difficulties in forecasting the effects of climate on species is that
it is hard, in practice, to measure individual fitness under field conditions.
As a consequence, ecologists (e.g., Porter and Gates 1969; Gates 1980) have
used principles of energy physics to define the larger envelope of climatic
conditions that a species could tolerate.This now classic approach is called
a
climate-space
analysis (see Box 3.1).
Organisms tolerate temperature changes through day-to-day physiolog-
ical and behavioral adjustments and seasonal acclimatization. But, coping
with climate isn't simply about coping with temperature. Organisms also
must tolerate changes in incoming solar radiation and radiative energy ex-
changes with their surrounding environment. To illustrate this principle
consider an elk (
Cervus elaphus
) lying on a patch of grass on a hillside cov-
ered by trees and rocks.
The elk will lose heat through several processes. It will radiate heat to the
environment if its body temperature is warmer than the environment. It will
conduct body heat to the ground upon which it is lying.Wind will draw
heat away from the body surface exposed to the air through convection.
The stronger the wind, the faster heat will be lost. It will lose heat by pant-
ing.The moisture in the mouth leads to the same kind of evaporative cool-
ing as in humans when they sweat. Unlike humans, however, elk and many
other species pant because they do not have sweat glands.
The elk will also gain heat from several sources.The elk will absorb in-
