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, "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.