Botkin Environmental Science Earth as Living Planet 8th txtbk

156 CHAPTER 8 Biological Diversity and Biological Invasions
Warm/Dry
Two species of Tribolium
A
B
Cool/Wet
A: Likes warm, dry conditions
B: Likes cool, wet conditions
Both: Like to eat wheat
In a uniform environment, one
will win out over the other. If the
environment is warm and dry, A
will win; if it is cool and wet, B
will win.
FIGURE 8.11 A classical experiment
with flour beetles. Two
species of flour beetles are placed
in small containers of flour. Each
container is kept at a specified temperature
and humidity. Periodically,
the flour is sifted, and the beetles are
counted and then returned to their
containers. Which species persists
is observed and recorded. (a) The
general process illustrating competitive
exclusion in these species; (b)
results of a specific, typical experiment
under warm, dry conditions.
In between
In a mixed environment, the
beetles will use separate parts of
the habitat.
In either case, the beetles do not
coexist.
(a)
(b)
Number of beetles
A
B
Years
Understanding the niche of a species is useful in assessing
the impact of land development or changes in land
use. Will the change remove an essential requirement for
some species' niche? A new highway that makes car travel
easier might eliminate your neighbor's bus route (an essential
part of his habitat) and thereby eliminate his profession
(or niche). Other things could also
eliminate this niche. Suppose a new school
were built and all the children could now
walk to school. A school bus driver would
not be needed; this niche would no longer
exist in your town. In the same way, cutting a
forest may drive away prey and eliminate the
wolf’s niche.
Measuring Niches
An ecological niche is often described and
measured as the set of all environmental
conditions under which a species can persist
and carry out its life functions. 19 It is illustrated
by the distribution of two species of
flatworm that live on the bottom of freshwater
streams. A study of two species of these
small worms in Great Britain found that
some streams contained one species, some the other, and
still others both. 17
The stream waters are cold at their source in the
mountains and become progressively warmer as they flow
downstream. Each species of flatworm occurs within a