The shift in the food chain carries steep consequences for many ecosystems, including humans
Published: (Friday, Jul 15, 2011)
Killing off the top predators in the
ocean and on land has sent huge ripples through nearly every type of
ecosystem in the world and disrupted the environment in ways researchers
are just beginning to understand, according to a report released
Thursday.
From sharks to wolves to lions, sharp
declines in the animals that normally sit at the top of the food chain
have triggered large-scale changes that ultimately affect humans by
altering the spread of disease, eliminating habitat, causing wildfires
and erosion, and spreading invasive species, said forestry professor
William Ripple of Oregon State University. Ripple is a co-author of the
study, which appears in the current issue of Science magazine.
“Regardless of where we look, we see very
strong effects of predators,” Ripple said. “From the tropics up to the
arctic, from the oceans to the higher mountains, regardless of the
ecosystem we’re seeing that the presence or the absence of these apex
predators can be extremely important.”
The loss of top predators has come largely
as the result of human activity through hunting or habitat loss, the
study says. The research suggests that the loss may be the most
pervasive influence that people have had on the landscape and
constitutes what is considered the sixth mass extinction in Earth
history.
The wide-ranging study, which involved 23
scientists from six nations, suggests that predator assessments should
play a larger role in evaluating ecosystem health and in decisions on
how to manage, or not manage, those ecosystems, Ripple said. It
concludes that in some cases the best way to restore biodiversity most
likely involves reintroducing top predators, a finding that’s sure to be
controversial in the ongoing debate over the return of wolves to Oregon
and other Western states.
The study looks at what are known as
“trophic cascades,” the domino-like changes that follow when a key part
of an ecosystem is altered. OSU has a research program in trophic
cascades and has documented their effects in specific Western
landscapes.
One such OSU study looked at two canyon
ecosystems in Utah that differed mainly in that cougar were present in
one and not the other. Where cougar were not present, deer and elk
populations rose and their browsing stripped away brush and streamside
vegetation, severely reducing habitat for native birds, eroding stream
banks, reducing streamside shade and reducing water depth, which made
the water too warm for native fish.
But where cougar were present, deer and elk
populations were held in check. That allowed streamside willows and
cottonwoods to survive, protecting stream banks from erosion, providing
shade and creating habitat for an array of birds, insects, amphibians
and fish.
Lead author James Estes, a professor of
ecology and evolutionary biology at the University of California at
Santa Cruz, said the effects of predators across large expanses reach
deep into plant and animal communities.
“The top-down effect of apex consumers in
an ecosystem are fundamentally important, but it is a complicated
phenomenon,” Estes said. “They have diverse and powerful effects on the
ways ecosystems work, and the loss of these large animals has widespread
implications.”
Ripple said the researchers acknowledge
that other things also occur in ecosystems that affect their overall
health, such as climate change, disease and habitat loss. But he said
the effect of reducing predator species hasn’t been given much
consideration in evaluations of ecosystem health.
“A lot of times predators are overlooked,”
he said. “There’s overwhelming evidence that in many systems in the
world these are very important species. We think that these top
predators provide a function, and when we lose the top predators we lose
ecosystem function.”
Predator loss also can affect people,
Ripple said. In Africa, the killing of lions and leopards has caused an
increase in olive baboons, which carry diseases that are passed to
people.
In the ocean, the loss of sharks to
overfishing in the North Atlantic resulted in a jump in the population
of cow-nosed rays, which feed on shellfish and caused a collapse of the
bay scallop fishery in the Northeast.
And the hunting of sea otters decimated
coastal ecosystems when kelp-grazing sea urchins, which the otters eat,
multiplied and destroyed kelp forests. Those forests act as a kind of
nursery for many ocean fish and other species harvested by people.
“We were very surprised to see the extent
of the effects of predators,” Ripple said. “It is just astounding to see
how these predators in very diverse types of ecosystems are so
important to the functioning of nature.”
The research is likely to be controversial
when applied to predators such as wolves. Wolves have only recently
returned to parts of northeast Oregon after an absence of many decades,
which has sparked a sharp debate between ranchers, who see wolves as a
threat to their herds, and environmentalists, who see them as a natural
part of the ecosystem.
Ripple said he understands the threat
wolves pose for ranchers and hunters and acknowledges there are areas
where reintroduction of top predators is not practical or feasible.
“Change is difficult, and some of these
predators are difficult to live with,” Ripple said. “What I’m suggesting
is we at least consider that predators have an important role in
ecology. It’s more than their charismatic return to the landscape. Once
that becomes part of the discussion, then it becomes more of a balanced
discussion for the management of top predators.”
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