Interactions Among Organisms 121from the community, would the pines grow faster because
they were no longer competing for necessary resources?
Or would the increased presence of needle-eating insects
(caused by fewer omnivorous insects) inhibit pine growth?
Short-term experiments in which one competing
plant species is removed from a forest community have in
several instances demonstrated improved growth for the
remaining species. However, few studies have tested the
long-term effects on forest species of removing one com-
peting species. These long-term effects may be subtle, in-
direct, and difficult to assess. They may reduce or negate
the negative effects of competition for resources.Keystone Species
Certain species are more crucial to the maintenance
of their ecosystem than others. Such keystone species
are vital in determining an ecosystem’s species compo-
sition and how the ecosystem functions. The fact that
other species depend on or are greatly affected by the
keystone species is revealed when the keystone species
is removed. Keystone species are usually not the most
abundant species in the ecosystem. Although present
in relatively small numbers, keystone species exert a iÞÃÌiÊëiViÃÊUÊ}ÕÀiÊx°Óäprofound influence on the entire ecosystem because
they often affect the available amount of food, water, or
some other resource.
Identifying and protecting keystone species are cru-
cial goals of conservation biologists because if a keystone
species disappears from an ecosystem, other organisms
may become more common or more rare, or they may
even disappear. One example of a keystone species is a
top predator such as the gray wolf (}ÕÀiÊx°Óä>). Where
wolves were hunted to extinction, the populations of
deer, elk, and other herbivores increased explosively. As
these herbivores overgrazed the vegetation, plant species
that couldn’t tolerate such grazing pressure disappeared.
Smaller animals such as insects were lost from the ecosys-
tem because the plants they depended on for food had
become less abundant. Thus, the disappearance of the
wolf resulted in the ecosystem having considerably less
biological diversity.
The reverse can also be true for a keystone species: The
gray wolf affects its environment when it is reintroduced,
as it was in Yellowstone National Park beginning in 1995.
Following the wolf’s reintroduction, formerly soaring elk
populations in Yellowstone declined sharply, a trend attrib-
uted to predation by the wolves (}ÕÀiÊx°ÓäL).02,0004,0006,0008,00010,00012,00014,00016,00018,00020,0001992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004*Year0204060801001201992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004WolfElk*Data are missing for years when counting conditions were poor.Joel Sartore/NG Image Collection
Based on data from White, P. J., and R. A. Garrott. 2005. “Northern Yellowstone elk after wolf restoration.”
Wildlife Society Bulletin, 33: 942–955.
b. After wolf reintroduction in Yellowstone in 1995, elk populations declined
considerably.a. The gray wolf is considered a keystone
species in its ecosystem.