Environmental Engineering Reference
In-Depth Information
moose can have important localized effects on riparian vegetation by eat-
ing woody species such as aspen, willows, and cottonwoods (Berger et al.
2001). Indeed, they can be particularly damaging to the vegetation because
they can reach high densities in areas where predators are absent.The grad-
ual build-up of moose populations and corresponding decline in riparian
vegetation has now resulted in the decline of numerous migratory songbird
species, including species that depend wholly on riparian habitat for their
existence (Berger et al. 2001). The sobering point here is that the early-
twentieth-century government policy of carnivore extirpation has created
a legacy of ecosystemwide effects that are only fully realized by the grand-
children and great-grandchildren of those originally engaged in the pred-
ator control efforts.
These examples illustrate that
species diversity is represented not by
members of a single trophic level but
by food webs in which species in dif-
ferent trophic levels of the system are
directly and indirectly connected to
each other. This diversity is main-
tained because carnivores control the
abundance of herbivores.This in turn
prevents herbivores from overeating
plants, allowing the entire system to
remain intact. Removing the preda-
tors leads to loss of species diversity. Such effects can, however, be reversed
through careful management aimed at restoring direct and indirect effects.
The sobering point is that the
early-twentieth-century government
policy of carnivore extirpation
has created a legacy of ecosys-
temwide effects that are only fully
realized by the grandchildren and
great-grandchildren of those origi-
nally engaged in the predator
control efforts.
Restoring Interconnections in Ecosystems
Prior to the late 1990s, wolves had been absent from Yellowstone National
Park for the better part of seventy years (Ripple and Beschta 2003). Dur-
ing that time, there was a herbivore-caused decline in the riparian vegeta-
tion, especially cottonwoods (
Populus sp.
) and associated woody plants, along
the Soda Butte Creek and the Lamar River in the northeastern part of the
park, similar in nature to that chronicled in the Berger et al. (2001) study
described above. In this case, however, the dominant herbivore is the North
American elk (
Cervus elaphus
) rather than the moose.Wolves were reintro-
duced to the park in the winter of 1995-1996 (Ripple and Beschta 2003)
and within seven years, there were pronounced differences in elk browsing
