Top-down Regulation of Ecosystems by Large Carnivores
(Adapted from Dave Foreman’s 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 Soulè sees trophic cascades, when top-down regulation of ecosystems is truncated by the loss of large carnivores, playing out in four ways:
- 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.
- 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.
- 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. Soulè calls this phenomenon, midsized predators multiplying in the absence of large predators, mesopredator release.
- 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.)
We are pleased that Martha Ruszkowski offers her Ukrainian translation of this text.
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. Soulè 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.
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.
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. Click here for pdf
PDFs not yet available:
Michael Soulè 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.)