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NSF PR 98-25 - April 30, 1998
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Bison Grazing Increases Biodiversity in Grasslands
Grazing by herbivorous mammals like bison increases
biodiversity in North American grasslands, says National
Science Foundation (NSF) ecologist Scott Collins,
even during periods of frequent burning and other
stresses. In fact, loss of species diversity in these
grasslands due to frequent burning was reversed by
bison grazing, according to Collins.
"Thus, reestablishing grazing in grasslands stressed
by human activities enhances biodiversity," says the
scientist. Collins' research results are published
in this week's issue of the journal Science. His work
was conducted at NSF's Konza Prairie Long-Term Ecological
Research (LTER) site in northeastern Kansas, one of
a network of 20 such NSF sites in North America and
Antarctica.
Species diversity has declined in ecosystems worldwide
due to habitat fragmentation, eutrophication, and
land-use changes, scientists believe.
Collins and his colleagues conducted two long-term
field experiments in native grasslands to assess effects
of fire, addition of nitrogen, and grazing on plant
species diversity. In one experiment, species richness
declined on burned and fertilized areas, whereas grazing
maintained diversity under these conditions. In a
second experiment, loss of species diversity due to
frequent burning was reversed by bison, animals that
Collins calls keystone herbivores in North American
grasslands.
In North American tallgrass prairies, diversity and
productivity are controlled to a large extent by nitrogen
availability, Collins explains. Historically, nitrogen
availability in prairies was driven by interactions
between frequency of fires and grazing by large herbivores.
In general, spring fires enhance growth of certain
grasses, and herbivores such as bison preferentially
graze these grasses, keeping a system of checks and
balances working properly, and allowing many plant
species to flourish.
"Extirpation of native grazers, habitat fragmentation,
increased nitrogen deposition from the atmosphere,
and altered fire frequency have disrupted grassland
ecosystems worldwide," adds Collins. "Although burning
is essential to maintaining tallgrass prairies, fire
alone is not a sufficient management solution for
restoring prairie diversity, as some have proposed."
Whereas fire is used as a conservation tool throughout
much of the tallgrass region, the use of grazing by
bison or cattle as a management tool for maintaining
species diversity is less common. Yet herbivores such
as bison historically served as keystone species in
tallgrass ecosystems.
"This research indicates that by adding or maintaining
grazing," states Collins, "at least in ecosystems
like grasslands that were impacted historically by
these herbivores, diversity in native vegetation can
be retained under conditions that would otherwise
lead to a decline in species richness."
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