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Press Release 11-141
Loss of Large Predators Caused Widespread Disruption of Ecosystems

Decline of "top consumers" may be humans' most pervasive influence on natural world

Image of sharks and smaller fish in a coral reef.

Fishing alters predator patterns; when large fish are removed, corals don't fare as well.
Credit and Larger Version

July 14, 2011

The decline of large predators and other "apex consumers" at the top of the food chain has disrupted ecosystems across the planet.

The finding is reported by an international team of scientists in a paper in this week's issue of the journal Science.

The study looked at research results from a wide range of terrestrial, freshwater and marine ecosystems and concluded "the loss of apex consumers is arguably humankind's most pervasive influence on the natural world."

According to lead author James Estes, a marine ecologist and evolutionary biologist at the University of California, Santa Cruz, large animals were once ubiquitous across the globe. They shaped the structure and dynamics of ecosystems.

Their decline, largely caused by humans through hunting and habitat fragmentation, has far-reaching and often surprising consequences, including changes in vegetation, wildfire frequency, infectious diseases, invasive species, water quality and nutrient cycles.

Plummeting numbers of apex consumers are most pronounced among the big predators, such as wolves on land, sharks in the oceans, and large fish in freshwater ecosystems. There also are dramatic declines in populations of many large herbivores, such as elephants and bison.

The loss of apex consumers from an ecosystem triggers an ecological phenomenon known as a "trophic cascade," a chain of effects moving down through lower levels of the food chain.

The research, funded in part by the National Science Foundation (NSF), "highlights the unanticipated effects of trophic cascades on Earth systems, including far-reaching processes such as biogeochemical cycles," said David Garrison, director of NSF's Biological Oceanography Program.

"The removal of predators like sharks and sea otters, bass and wolves has consequences," said Garrison, "not only for these species, but for all of us."

"The top-down effects 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."

Estes and co-authors cite a wide range of examples in their review, including:

  • The extirpation of wolves in Yellowstone National Park led to over-browsing of aspen and willows by elk; restoration of wolves allowed the vegetation to recover.
  • Dramatic changes in coastal ecosystems followed the collapse and recovery of sea otter populations. Sea otters maintain coastal kelp forests by controlling populations of kelp-grazing sea urchins.
  • The decimation of sharks in an estuarine ecosystem caused an outbreak of cow-nosed rays and the collapse of shellfish populations.

Despite these and other well-known examples, the extent to which such interactions shape ecosystems was not widely appreciated, scientists say.

"There's been a tendency to see it as idiosyncratic and specific to particular species and ecosystems," Estes said.

One reason for this is the top-down effects of apex predators are difficult to observe and study.

"These interactions are invisible unless there is some perturbation that reveals them," Estes said. "With these large animals, it's impossible to do the kinds of experiments that would be needed to show their effects, so the evidence has been acquired as a result of natural changes and long-term records."

Estes has studied coastal ecosystems in the North Pacific for several decades, conducting research on the ecological roles of sea otters and killer whales. In 2008, he and co-author John Terborgh of Duke University organized a conference on trophic cascades, which brought together scientists studying a wide range of ecosystems.

The recognition that similar top-down effects occur in many different systems was a catalyst for the current paper.

The study's findings have profound implications for conservation.

"To the extent that conservation aims to restore functional ecosystems, the reestablishment of large animals and their ecological effects is fundamental," Estes said.

"This has huge implications for the scale at which conservation can be done. 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."

The paper's coauthors include 24 scientists from various institutions in six countries.

The Institute for Ocean Conservation Science, Defenders of Wildlife, White Oak Plantation, NSERC Canada and NordForsk provided other support for the research.

-NSF-

Media Contacts
Cheryl Dybas, NSF, (703) 292-7734, cdybas@nsf.gov
Tim Stephens, UCSC, (831) 459-2495, stephens@ucsc.edu

The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering. In fiscal year (FY) 2014, its budget is $7.2 billion. NSF funds reach all 50 states through grants to nearly 2,000 colleges, universities and other institutions. Each year, NSF receives about 50,000 competitive requests for funding, and makes about 11,500 new funding awards. NSF also awards about $593 million in professional and service contracts yearly.

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Image of a wolf.
Wolves are among the many "apex predators" that have been affected by human activities.
Credit and Larger Version

Image of a diver taking notes while examining a study area on the ocean floor.
Kelp increases when sea otters are abundant; otters limit the number of kelp-eating sea urchins.
Credit and Larger Version

Image showing a lake with largemouth bass on right and without largemouth bass on left.
Lake with largemouth bass (right), and experimentally removed (left); bass increase water clarity.
Credit and Larger Version

Image of an intertidal zone with many seastars.
Intertidal zone with many seastars; seastars eat blue mussels and so prevent their dominance.
Credit and Larger Version

Image of Brier Creek, Oklahoma, with largemouth and spotted bass.
Brier Creek, Oklahoma, with largemouth and spotted bass; the bass promote a healthy stream.
Credit and Larger Version

Cover of the July 15, 2011 issue of the journal Science.
The researchers' work is described in the July 15, 2011 issue of the journal Science.
Credit and Larger Version



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