Of Wolves and Trees ■ 371A Community Restored
Today, the return of the wolf is having a clear and
significant impact on the ecological community
of Yellowstone, from the rebirth of aspens and
cottonwoods to growing beaver populations.
These are signs of succession, the process by
which the species in a community change over
time. “We’re on a very important upward trend,”
says Beschta.
All ecological communities change over
time, sometimes because of human interven-
tion, as in Yellowstone, but also because of
natural changes in species composition—the
number of individuals in a population often
changes as the seasons change, for example—
and natural disturbances such as fires, floods,
and windstorms. In addition, communitiescollected and saved before 1995, prior to the
wolf return. Over a 19-year period, they found
that the percentage of berries in the grizzly diet
went up as elk populations went down. This is
an example of the competitive exclusion prin-
ciple, which predicts that different species that
use the same resource can coexist only if one of
the species adapts to using other resources. In
this case, grizzlies adapted to the presence of
elk by finding food sources other than berries,
and in the absence of elk they increased their
consumption of berries.
Organisms also compete through inter-
ference competition, in which one organism
directly excludes another from the use of a
resource. In Yellowstone, for example, bears
and wolves often fight over the carcass of an
animal (Figure 20.14). In interference compe-
tition, one individual physically interferes with
another individual that is trying to obtain a
resource.
Figure 20.14
Interference competition can be dramatic