According to first author James Estes, a professor of ecology and evolutionary biology at the University of California, Santa Cruz, large animals were once ubiquitous across the globe, and they shaped the structure and dynamics of ecosystems. Their decline, largely caused by humans through hunting and habitat fragmentation, has had far-reaching and often surprising consequences, including changes in vegetation, wildfire frequency, infectious diseases, invasive species, water quality, and nutrient cycles.

The decline of apex consumers has been most pronounced among the big predators, such as wolves and lions on land, whales and sharks in the oceans, and large fish in freshwater ecosystems. But there have also been 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 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 his coauthors cite a wide range of examples in their review, including the following:

• The extirpation of wolves in Yellowstone National Park led to over-browsing of aspen and willows by elk, and restoration of wolves has allowed the vegetation to recover.
• The reduction of lions and leopards in parts of Africa has led to population outbreaks and changes in behavior of olive baboons, increasing their contact with people and causing higher rates of intestinal parasites in both people and baboons.
• A rinderpest epidemic decimated the populations of wildebeest and other ungulates in the Serengeti, resulting in more woody vegetation and increased extent and frequency of wildfires prior to rinderpest eradication in the 1960s.
• Dramatic changes in coastal ecosystems have 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 ecosystems are shaped by such interactions has not been widely appreciated. "There's been a tendency to see it as idiosyncratic and specific to particular species and ecosystems," Estes said.

One reason for this is that 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 been studying coastal ecosystems in the North Pacific for several decades, doing pioneering work on the ecological roles of sea otters and killer whales. In 2008, he and coauthor 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 have been observed in many different systems was a catalyst for the new paper.

The study's findings have profound implications for conservation. "To the extent that conservation aims toward restoring 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. Support for the study was provided by the Institute for Ocean Conservation Science, Defenders of Wildlife, White Oak Plantation, U.S. National Science Foundation, NSERC Canada, and NordForsk.