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Decline of predatory species throws food chains out of whack, report says

July 14, 2011

Darryl Fears
Washington Post

The decline of large predators such as big cats, wolves, sharks and giant whales may be “humankind’s most pervasive influence on the natural world,” causing prey animals to swell in population and throw food chains out of balance, a new report says.

Humans have touched off the world’s latest mass extinction, according to the report, published Thursday in the journal Science, and the consequences are being felt on land and in water systems as large predators vanish.

“Recent research suggests that the disappearance of these animals reverberates further than previously anticipated,” says the report, “Trophic Downgrading of Planet Earth.” In addition to creating an overabundance of prey, the dwindling number of predators contributes to the spread of disease, wildfires and invasive species.

The decline of wolves in Yellowstone Park is cited as an example of what can happen. Elk and deer in the park once flourished on willow trees and saplings, threatening a crucial part of the forest on which other creatures rely.

The report also mentions the slaughter of lions and leopards by hunters and herders in parts of sub-Saharan Africa. As a result of the killings, disease-carrying olive baboons have thrived without their top predators and inched closer to food crops and people.

The decimation of sharks along the U.S. Atlantic Coast has allowed their main prey, the cow-nosed ray, to proliferate and dine heavily on the threatened Chesapeake Bay oyster.

A reduction of big herbivores such as buffalo and wildebeest in East Africa through hunting is also a problem, the report says. Their demise has led to increases in plants that fuel giant wildfires in the dry season.

Americans don’t have to visit federal parks or sub-Saharan Africa or plunge into seas to see the consequences, said Ellen K. Pikitch, a co-author of the report and a professor at Stony Brook University in New York. Many experience the problem every day in their own back yards.

“People who live in North America know it’s hard to grow a garden because deer will eat it,” said Pikitch, a marine biologist. “The lack of wolf populations throughout North America has led to an expansion of the deer population.

“You may hate wolves. You might think they’re dangerous. But without them, the land changes,” Pikitch said. “Deer carry ticks. We humans become more susceptible to diseases such as Lyme disease.”

Wildlife advocates say efforts to protect one species of predator in the United States were set back when the Obama administration signed a bill in April that removed 1,300 wolves from the endangered species list in northern Rocky Mountain states. It was the first time Congress had taken a species off the endangered list. The law allows limited hunting of the animals to begin this summer.

Other studies have examined the collateral damage caused by the near-extinction of large predators and herbivores. But the report in Science is the first to tie together the impact on land animals as well as salt and freshwater marine life, Pikitch said. It was conducted by an international team of 24 scientists and funded primarily by the Institute for Ocean Conservation Science at Stony Brook.

Much of the science in this area of study has focused on the threat to life at the bottom of the food chain, theorizing that small animals and plants are important because so many creatures rely on their survival.

Although “bottom-up” research is fundamental and important, the report says, “top-down” research deserves wider consideration “if there is to be any real hope for understanding and managing the workings of nature.”

The report acknowledges that top-down research of the food chain is difficult to conduct, noting that it can take decades to measure the effects of the disappearance of large predators.

“The irony . . . is that we often cannot unequivocally see the effect of large apex consumers until after they have been lost” and the ability to restore the species has also been lost, the report says.

Large predators, or apex species, include animals that people adore, such as otters, and others not so popular, such as vultures.

On the Pacific Coast, from Alaska to the southern tip of California, sea otters were hunted in the 1900s to near-extinction for their pelts. Their absence started a chain of events that nearly eliminated the kelp forests that nurture all manner of marine life on the coast.

Sea otters feed on sea urchins, which dine on kelp. Without otters, the sea urchin population exploded. The kelp forest started to disappear. When sea otter populations elsewhere were re-introduced to a few areas along the coast, the kelp started to rebound.

A telling consequences of the absence of large predators can be found on the Scottish island of Rum, where wolves have been gone for more than 250 years and red deer thrive, the report says. The once forested island is now treeless.

Washington Post article

Also see:
News release



More Press

Time Econcentric blog

How Human Beings Are Downgrading Life on Planet Earth
Posted by Bryan Walsh

We live in the Anthropocene, as some scientists have come to call our new geologic era. The term acknowledges the fact that human beings—nearly 7 billion strong and growing—have so much influence over the life, geography and even chemistry of planet Earth that we're now essentially responsible for the whole show. For good and for ill, the planet from now on will be what we make of it.

But just because our actions are shaping the future of the planet—and everyone and everything else that rides on it—doesn't mean that we always know the effects of what we're doing. Case in point: in a new review in the July 14 Science, a group of researchers persuasively argue that one of the biggest impacts that human beings have had on the planet isn't necessarily deforestation or climate change or pollution. It's the extirpation—through hunting, habitat loss or disease—of apex predators, species like tigers or wolves or sharks that skit high on the food chain. When those species go, there can be drastic knock-on effects for animals and plants below them, as the paper argues:

The loss of apex consumers is arguably humankind's most pervasive influence on the natural world. This is true in part because it has occurred globally and in part because extinctions are by their very nature perpetual, whereas most other environmental impacts are potentially reversible on decadal to millennial time scales. Recent research suggests that the disappearance of these animals reverberates further than previously anticipated (6–8), with far-reaching effects on processes as diverse as the dynamics of disease; fire; carbon sequestration; invasive species; and biogeochemical exchanges among Earth's soil, water, and atmosphere.

More from TIME: Farewell to Sharks

Removing those apex consumers produces what's called "trophic downgrading," which refers to the cascade of damage that can occur when an intact ecosystem is disrupted in such a significant way. It's a relatively new concept for ecologists and conservationists, who long studied nature one plant or animal at a time. But as we now know, everything is connected. Actually, revise that—everything is connected, but some species are more connected than others, and when they're taken out of the picture, ecosystems can change utterly or even collapse altogether. A few examples from the Science paper:

  • The reduction of cougars in Utah has led to a sharp increase in the number of deer, a loss of vegetation and an overall decline in biodiversity.
  • Whaling in the 20th century—when it reached industrial levels—actually changed the diet of killer whales, which then led to a decline in sea lions, seals and sea otters.
  • The hunting of lions and other big cats in Africa led to a population explosion in olive baboons, which then came into closer contact with human beings—and spreading disease along the way.
  • A rinderpest epidemic decimated the populations of wildebeest and other ungulates in the Serengeti, resulting in more woody vegetation—which then led to stronger and more frequent wildfires, until the disease was eliminated.
  • The collapse of sharks—remember them?—has been followed by a collapse in shellfish populations.

Photos from TIME: In the Time of Trees

The lesson here is that in nature, almost everything comes with a price. We can't harvest an entire species—or "manage" them, as we long did with predators like wolves—without blowback, to use the defense parlance. The trouble, of course, is that these changes are very hard to model, because the web of species interactions is invisible—until we come along in our bumbling fashion and perturb it, like a hiker walking through a spiderweb. What the Science authors are suggesting is that the burden of proof has shifted—we should assume that top-line predators are vital to their ecosystems, and think twice before disturbing them.

Ideally, we'd disturb nothing, and leave nature to its perfect balance. Except—as Emma Maris writes in this lovely post—the idea of untouched nature is a myth, and has been ever since the first human being started using fires and tools. We're not going back to Eden, however much we may long for it, because we never lived there. In his new book, the writer Mark Lynas describes us—accurately, I think—as the "god species." We're nearly all-powerful—but we're not all-knowing, unfortunately. We've seized the keys to the kingdom, and it's our responsibility to safeguard the planet—for ourselves, as much as for everything and everyone else.

More from TIME: The New Age of Extinction
Bryan Walsh is a reporter at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME's Facebook page and on Twitter at @TIME.

PBS Newshour

Loss of Top Predators Has Far-Reaching Effects
By: Jenny Marder

Sea otters eat sea urchins and sea urchins eat kelp. When sea otters are present, the coastal kelp forests maintain a healthy balance. But when the fur trade wiped out the otters in the Aleutian Islands in the 1990s, sea urchins grew wildly, devouring kelp, and the kelp forest collapsed, along with everything that depended on it. Fish populations declined. Bald eagles, which feed on fish, altered their food habits. Dwindled kelp supplies sucked up less carbon dioxide, and atmospheric carbon dioxide increased.

The animal that sits at the top of the food chain matters, and its loss has large, complex effects on the structure and function of its ecosystem, according to an article published on Thursday in the online issue of the journal, Science.

That the presence or loss of an ecosystem's top predator is linked to surges and crashes in the food chain is nothing new. The term for the phenomenon is "trophic cascade," and it's been applied to coastal sea otters, as well as the gray wolves in Yellowstone and the mountain lions in Zion National Park, to name just a few.

But what is new, authors of the paper say, is that this is ubiquitous across all ecosystems. "We see it on land, we see it on water, we see it in high latitudes, we see it in low latitudes," said James Estes, a research scientist at the Institute for Marine Sciences at the University of California, Santa Cruz and the paper's lead author. "We do not not see it anywhere."

The paper says that ecosystems are built around "interaction webs" within which every species can influence many other species. And the full impact of the loss of a top predator cannot be fully understood until the species has disappeared, but once gone, its absence can have far-reaching effects on water quality, air quality, disease patterns and fires.

Among the examples cited in the paper: A rinderpest epidemic devastated the population of wildebeest in the Serengeti, resulting in a growth of woody plants, which has led to more frequent wildfires. The decline of lions and leopards in Africa has corresponded with changes in the behaviors of olive baboons, leading them to interact more with human food and farms, and most likely causing a rise in intestinal parasites.

The article is a synthesis of the work of more than 20 scientists, and an outgrowth of a symposium held at the White Oak Plantation, near Jacksonville, Fla. in 2008 to study the impacts of large predators across global systems. "At the end of the symposium, we were all sitting around, and there was just this overwhelming sense that there really is a message here that needs to be integrated and put out there," Estes said. "There was frustration that some of our colleagues didn't realize the importance of large consumers. So we said, 'let's get a collection of credible people from around the world, mostly senior people who have worked in a diversity of global ecosystems, and see what consensus they may have.'"

The team included theoreticians and scientists who study forest, marine and freshwater ecosystem ecology in North America, South America, Africa and Europe.

"It's not reporting on any new findings, but I would say its value is that it is a synthesis," said Matthew Kauffman, a professor at the University of Wyoming, who is not part of the study. "It's showing us that there are top-down effects of large predators and large herbivores among many different ecosystems, functioning in many different ways. It allows us to see the full scope of the value of having top predators in ecosystems."

William Ripple, professor of forestry at Oregon State University, and a co-author of the study, has studied the disappearance and reintroduction of gray wolves in Yellowstone, and the influence these events have had on the surrounding animals and plants. "We cored the trees, counted the tree rings and found that the aspen trees stopped regenerating after the wolves were killed off," he said. By connecting the dots, his team developed a hypothesis: aspen tree growth and wolves are linked. Without wolves as predators, elk populations thrived, eating seedlings and wiping out many of the young aspen trees.

Since the wolves were reintroduced to Yellowstone, some aspens, cottonwoods and willows appear to be growing back. Ripple believes this is because elk, which are fewer in number and more skittish when wolves are present, are eating fewer seedlings, allowing for more tree growth.

And it doesn't stop at the plants, Ripple said. The resurgence of the plants has corresponded with more insects, birds and beavers. The beavers dam up the streams and make ponds, altering the stream ecology and fish habitat.

Scientists don't all agree on these mechanisms. Kauffman's research, for example, found that the behavior of the elk has not changed significantly since the wolves returned. More important to new tree growth, he said, is that wolves are directly reducing the elk population through predation.

But most scientists do agree that the influence of the presence or absence of top predators is far reaching. "It's intuitive, it's very obvious, yet nobody wants to talk about it," said Paul Dayton, a professor of marine ecology at the Scripps Institution of Oceanography, who was not a part of the study. "People like me will give talks about it and wave our arms around. "But I've never seen all these ecosystems and identical patterns merged into one paper."

Estes says that there needs to be a "gross rethinking" of the way management decisions are made."

Dayton's hope is that the research will prompt land managers and conservationists to focus on species interactions, rather than extinctions. "Right now, we manage through the Endangered Species Act," he said. "And it's a horrible way to manage ecosystems. We're not managing them, we're trying to save little fragments in zoos. What we need to do is manage these interactions."

Mother Nature Network

Food Chain of Fools

Humans have reshaped the Earth in many ways — replacing forests with farms, for example, or carving canals and building dams — but according to a new study in the journal Science, one activity marks our single "most pervasive influence on the natural world": the systematic killing of large "apex" predators. Without big cats, wolves, sharks and whales to regulate ecosystems, the report says, populations of prey animals have exploded and run wild, throwing food chains out of whack.

"We now live in a world, really for the first time, where these big apex consumers are missing," says lead author James Estes, an evolutionary biologist at the University of California-Santa Cruz. "We see it on land, we see it on water, we see it in high latitudes, we see it in low latitudes." While humanity's big-game bloodlust isn't exactly breaking news, the report's brilliance lies in its scope, says University of Wyoming ecologist Matthew Kauffman, who wasn't involved in the research. "It's not reporting on any new findings, but I would say its value is that it is a synthesis," Kauffman tells PBS NewsHour. "It's showing us that there are top-down effects of large predators and large herbivores among many different ecosystems, functioning in many different ways. It allows us to see the full scope of the value of having top predators in ecosystems." As UC-San Diego oceanographer Paul Dayton tells USA Today, "I think this might be the most important paper Science has published in a long time."

The problem is rampant worldwide: The loss of wolves let deer take over Yellowstone, the loss of sharks let rays decimate Chesapeake Bay oysters, and even the loss of big herbivores like wildebeest in East Africa boosted plants that fuel bigger wildfires in the dry season. But it isn't confined to wildlife parks. "People who live in North America know it's hard to grow a garden because deer will eat it," study co-author Ellen Pikitch tells the Post. "You may hate wolves. ... But without them, the land changes. Deer carry ticks. We humans become more susceptible to diseases such as Lyme disease." Fixing the problem, Estes says, will require a new kind of wildlife conservation. "This has huge implications for the scale at which conservation can be done," Estes says. "You can't restore large apex consumers on an acre of land. These animals roam over large areas, so it's going to require large-scale approaches."

(Sources: Washington Post, USA Today, PBS NewsHour, National Science Foundation)

New York Times editorial

The Peak and Life Below It

A familiar metaphor for nature is the pyramid of life, with large predators living at the peak because they’re few in number and eat species lower on the pyramid. Like most simple metaphors, this one has a perceptual flaw. It creates the illusion that large predators have an effect only on the prey species immediately below them. The truth, as a growing body of scientific studies shows, is that the presence, and absence, of top predators cascades all through nature in surprisingly complex ways.

Our species has done a sadly efficient job of removing top predators: wolves, bears, lions, tigers, sharks and many more. According to the authors of a new article in Science magazine, “the loss of these animals may be humankind’s most pervasive influence on nature.”

The loss of cougars in what is now Zion National Park led to an “eruption” of mule deer, which reduced riverbank vegetation and, ultimately, changed the shape of stream channels. The loss of sea otters along the Pacific Coast led to the destruction of kelp forests and the many creatures they supported. The effect includes herbivores. When disease decimated wildebeest in East Africa in the late 19th century, grassland turned to shrubs and into fuel for wildfires, changing the ecosystem.

In the rare cases where top predators have been reintroduced, the benefit is profound. The success of gray wolves in Yellowstone changed many things. Grizzlies fed on their kills. Coyote numbers dropped and the numbers of small mammals climbed. Elk spent less time in creek bottoms, where they were more vulnerable, and streamside ecology changed as a result.

It is now clear that biological diversity increases when top predators are present. The pyramid is healthiest when its peak is still present and when humans aren’t the only top predators around.

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