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Top-down Regulation of Ecosystems by
Large Carnivores
(Adapted from Rewilding North
America)
Recent field research shows that ecosystem integrity is often
dependent on the functional presence of large carnivores. John
Terborgh of Duke University has studied the ecological effects of
eliminating large carnivores (jaguars, pumas, and harpy eagles) from
tropical forests. He tells us that big cats and eagles are major
regulators of prey species numbers—the opposite of the
once-upon-a-time ecological orthodoxy that saw them as unimportant.
He has also found that the removal or population decline of large
carnivores can alter plant species composition, particularly the
balance between large- and small-seeded plants, due to increased
seed and seedling predation by superabundant herbivores that are
normally regulated by large carnivores. This is called top-down
regulation. Terborgh and his co-authors in Continental
Conservation write, “‘Top-down’ means that species occupying the
highest trophic level (top carnivores) exert a controlling influence
on species at the next lower level (their prey) and so forth down
the trophic ladder.”
In contrast, bottom-up regulation is driven by energy moving
up the food web (tropic levels) from plants to herbivores to
carnivores. Although bottom-up regulation is a part of the big
picture, those wildlife managers who stress it may be influenced by
political considerations to keep “carnivore numbers artificially low
or eliminate them altogether,” according to Brian Miller. Top-down
or bottom-up, however, is not an either-or situation, but a
both-and, as David Brower liked to say. Brian Miller, a conservation
biologist and a fellow of The Rewilding Institute, and his
colleagues point out that “forces flow in both directions
simultaneously and interact while doing so.” Because top-down
regulation has been so slighted by wildlife managers in the past, we
now need to appreciate and celebrate its proper role in maintaining
and restoring ecosystem health.
There are “two fundamentally different ways” in which the
consequences of the loss of top-down regulation occur: competitive
exclusion and trophic cascades.
Competitive exclusion: Through predation, carnivores can
moderate competition among similar species so that more species are
able to use a certain habitat. On the other hand, when freed from
control by its predators, one species in a guild of species may be
able to outcompete and thus eliminate the others. As Brian Miller
and company write, “Removing the predator will dissolve the
ecological boundaries that check competition.”
Trophic cascades: In top-down regulation, different trophic
levels are limited by the next level up. For example, in a simple
system of three trophic levels—plants, herbivores, and
carnivores—plants are limited by herbivores, which are limited by
predators. A classic example of a trophic cascade has been described
from years of research on the relationship between wolves, moose,
and balsam fir on Isle Royale National Park in Lake Superior. When
the wolf population is low, growth in the firs is depressed because
of heavy browsing by moose.
Michael Soulč sees trophic cascades, when top-down regulation of
ecosystems is truncated by the loss of large carnivores, playing out
in four ways:
1. Numerical release of prey species. Predators can control
population growth of prey species. When big hunters disappear, their
prey species may boom in numbers and degrade their habitat.
2. Behavioral release of prey species. In the presence of their
predators, prey species wisely behave timidly. When large carnivores
disappear, prey species may act more boldly and hence cause harm to
their habitat.
3. Numerical release of mesocarnivores. Populations of smaller
carnivores are often held in check by larger carnivores. When large
carnivores disappear, smaller predators may have population
explosions, thereby increasing their predation on other species.
Soulč calls this phenomenon, midsized predators multiplying in the
absence of large predators, mesopredator release.
4. Behavioral release of mesocarnivores. Smaller carnivores are very
careful when larger carnivores are out and about. In the absence of
large carnivores, smaller carnivores may act more boldly and prey
more heavily on vulnerable species.
The elimination of large carnivores often leads to more than one of
the effects. Following are three case studies [more examples are
given in Rewilding North America and the other references
below]:
Coyotes were removed on a Texas grassland where twelve species of
burrowing rodents coexisted. Twelve months later, only Ord's
kangaroo rat was found. Without predation by coyotes, it was able to
outcompete and exclude the other burrowing rodents.
Sea otters were a top predator in the
kelp forests of the North Pacific. When they were hunted out, sea
urchin populations exploded. Because urchins graze on kelp, this
critical habitat was largely lost and species using it—raptors,
shorebirds, fish, and invertebrates—declined. With the return of sea
otters under strict protection, sea urchin populations dropped and
kelp forests began to recover—along with the diversity of species
associated with kelp.
With the extermination of wolves and the near extermination of
mountain lions sixty years ago in Yellowstone National Park, elk
populations built up. Lacking their predators, elk grew lazy and
lackadaisical, loafing in large herds in river meadows. Their
behavior changed so much, it was hard to call them elk. Not only did
they overgraze the grasslands, their browsing of willow shoots
hampered beavers from reestablishing themselves in Yellowstone.
However, with the recent reintroduction of wolves to Yellowstone,
elk have become elk again. They're awake! They're moving. They're
looking over their shoulders. They aren't loafing in big herds in
open river valleys. Wolves have changed elk behavior for the
better—to a more natural set of behaviors—and thereby are bringing
integrity back to the ecosystem. For example, willows are again
growing along streams, and researchers expect beavers to return. In
addition, wolf-killed elk are a smorgasbord for many species,
ranging from grizzly bears to insect-eating songbirds. Between 1921
and 1999, there was “no significant recruitment of new stems into
the aspen overstory” in Yellowstone. Oregon State researchers
William Ripple and Eric Larsen write, “We hypothesize that
disturbance to predator/prey relationships, especially between
wolves and elk, has been a major factor in [Yellowstone National
Park] aspen decline.”
As Terborgh and his co-authors point out, “Both top-down and
bottom-up regulation can operate concurrently in the same system.”
Nonetheless, solid research by top biologists has shown and
continues to show that when large cats, canids, weasels, raptors,
and other predators are removed from an ecosystem—marine, tropical,
or temperate—the ecosystem is deeply wounded. Continental
Conservation puts it bluntly:
The absence of top predators appears to lead inexorably to
ecosystem simplification accompanied by a rush of extinctions.
Brian Miller and his colleagues write, “In short, management
policies based on reducing carnivore numbers have caused, and will
continue to cause, severe harm to many other organisms.”
Restoring large carnivores begins to heal ecosystems. It's that
simple.
(Adapted and condensed
from Rewilding North America by Dave Foreman [Chapter 7].
Copyright © 2004 by the author. Reproduced by permission of Island
Press, Washington, D.C. Quotes are from the books and papers below.)
Books
Rewilding North
America: A Vision for Conservation in the 21st Century
by Dave Foreman (Island Press 2004). Order from The Rewilding
Institute.
Continental
Conservation: Scientific Foundations of Regional Reserve Networks
edited by Michael E. Soulč and John Terborgh (Island Press
1999). Particularly Chapter 3 “The Role of Top Carnivores in
Regulating Terrestrial Ecosystems.” Order from The Rewilding
Institute.
A Sand County
Almanac by Aldo Leopold (Oxford 1949). Early insights from the
20th century’s greatest conservationist on the importance
of large carnivores. Order from Amazon.
Articles
Available as PDF:
Available in a book
listed above:
John Terborgh et al.,
“The Role of Top Carnivores in Regulating Terrestrial Ecosystems,”
Chapter 3 in Continental Conservation. Another excellent
overview.
PDFs
Brian Miller et al.,
“The Importance of Large Carnivores to Healthy Ecosystems,”
Endangered Species Update Vol. 18 (2001). An excellent overview
of research showing top-down regulation by large carnivores.
Download
PDFs not yet
available:
Michael Soulč and Reed
Noss, “Rewilding and Biodiversity as Complementary Goals for
Continental Conservation,” Wild Earth, Fall 1998, 22. Good
examples of top-down regulation; the classic paper on rewilding.
Will soon be available as PDF.
John Terborgh et al.,
“Ecological Meltdown in Predator-Free Forest Fragments,” Science
294 (November 30, 2001): 1923-1926. Results of ecosystem
collapse on newly formed islands in a Venezuelan reservoir due to
the loss of jaguars, pumas, and harpy eagles.
(Many more papers
soon to be listed.)
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