"Long before it's in the papers"
July 17, 2011

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Wipeout of top predators called No. 1 human effect on nature

July 17, 2011
Courtesy of the National Science Foundation
and World Science staff

Hu­man­ity’s “most per­va­sive” ef­fect on na­ture may be its de­struc­tion of large preda­tors and oth­er an­i­mals at the top of the food chain, which has dis­rupted ecosys­tems glob­al­ly, a study con­cludes.

The find­ing is re­ported in the July 15 is­sue of the re­search jour­nal Sci­ence.

While con­serva­t­ion meas­ures such as put­ting sharks in the seas may prove un­pop­u­lar to say the least, the re­search­ers say re-introducing some dec­i­mat­ed top preda­tors to na­ture may be the only way to un­do a host of un­wanted con­se­quenc­es that have al­ready come back to bite us.

A lake with large­mouth bass (right), and ex­per­i­men­tal­ly re­moved (left); the bass were found to in­crease wa­ter clar­i­ty. (Cred­it: Steve Car­pen­ter)

• 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.”

The stu­dy’s find­ings have pro­found im­plica­t­ions for con­serva­t­ion, he added. “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… it’s go­ing to re­quire large-scale ap­proaches.”

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Humanity’s “most pervasive” effect on nature may be its destruction of large predators and other animals at the top of the food chain, which has disrupted ecosystems globally, a study concludes. The finding is reported in this week’s issue of the journal Science. While conservation measures such as putting sharks in the seas may prove unpopular to say the least, the researchers say re-introducing some decimated top predators to nature may be the only way to undo a host of unwanted consequences that have already come back to bite us. Such measures, they add, require the restoration of large tracts to nature rather than piecemeal approaches. “These animals roam over large areas,” said James Estes, a marine ecologist and evolutionary biologist at the University of California, Santa Cruz, and lead author of the study. “You can’t restore large ‘apex consumers’ on an acre of land,” he added, using the scientific term for animals at the top of the food web. The study looked at research results from a wide range of land, freshwater and marine ecosystems and concluded “the loss of apex consumers is arguably humankind’s most pervasive influence on the natural world.” Large animals were once ubiquitous across the globe, Estes said. 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, the study found. These include 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, the researchers said. These animals include as wolves on land, sharks in the oceans, and large fish in freshwater ecosystems. There also are dramatic declines in populations of many large plant eaters, 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 “highlights the unanticipated effects of trophic cascades on Earth systems, including far-reaching processes such as biogeochemical cycles,” said David Garrison, director of the Biological Oceanography Program at the U.S. National Science Foundation, which helped fund the project. “The removal of predators like sharks and sea otters, bass and wolves has consequences,” he added, “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.” Among the examples Estes and co-authors cite: 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.” The study’s findings have profound implications for conservation, he added. “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… it’s going to require large-scale approaches.”