In ecosystems around the world, the decline of large predators including lions, wolves, otters, and bears is changing the face of landscapes from the tropics to the Arctic. Experts on ecosystem health from around the world have warned that predator declines are threatening the environment and that scientific understanding is only just beginning to appreciate their true value.

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More than 75 percent of the 31 large-carnivore species studied by researchers at Oregon State University and their colleagues in Europe, Africa and Australasia are declining, and 17 species now occupy less than half of their former ranges. Although there are a few exceptions to this trend and in some cases carnivores are slowly returning to their former habitats, for the most part larger predators have already been exterminated from much of the Global North, including Western Europe and the eastern United States.

"We are losing our large carnivores," warned William Ripple, lead author of the paper and a professor in the Department of Forest Ecosystems and Society at Oregon State University. "Many of them are endangered," he said. "Their ranges are collapsing. Many of these animals are at risk of extinction, either locally or globally. And, ironically, they are vanishing just as we are learning about their important ecological effects."

Prof Ripple worked with other experts from across the United States, Australia, Italy and Sweden in his analysis. The group last week called for an international initiative to conserve large predators, and facilitate their coexistence with people.

'Trophic cascades' from predator loss affect other wildlife

The researchers singled out seven species that have been studied for their widespread ecological effects, including 'trophic cascades' which result from their decline or loss. These include African lions, leopards, Eurasian lynx, cougars, gray wolves, sea otters and dingoes.

Ripple has documented the impacts of cougars and wolves on the regeneration of forest stands and riparian (water-edge) vegetation in Yellowstone National Park and elsewhere in North America. Fewer predators, he discovered, lead to an increase in browsing animals such as deer and elk, which in turn disrupts vegetation, shifts birds and small mammals and changes other parts of the ecosystem in a widespread cascade of impacts.

Studies of Eurasian lynx, dingoes, lions and sea otters have found similar effects, Ripple said. In Europe, Lynx populations have been closely tied to the abundance of roe deer, red fox and hare. In Australia, the construction of a 3,400-mile dingo-proof fence has enabled scientists to study ecosystems with and without the animals, which are closely related to gray wolves. In some parts of Africa, the decrease of lions and leopards has coincided with a dramatic increase in olive baboons, which threaten farm crops and livestock and are also thought to carry disease.

The Science study's authors called for a deeper understanding of the impact of large carnivores on ecosystems, a view that they trace back to the work of landmark ecologist Aldo Leopold. The classic concept that predators are harmful and deplete fish and wildlife is outdated, they said. The team argued scientists and wildlife managers must recognise a growing body of evidence for the complex roles that carnivores play in ecosystems and for their social and economic benefits.

Leopold believed that the relationships between predators, their prey and the ecosystems which they share, are more complex than had been appreciated, but his observations were largely ignored for decades after his death in 1948.

"Human tolerance of these species is a major issue for conservation," Ripple said. He added that, although conventional wisdom suggests otherwise, "These animals have an intrinsic right to exist [and] they are also providing economic and ecological services that people value." Among the 'services' that have been documented in other studies are carbon sequestration, riparian restoration, biodiversity and disease control.

"Nature is highly interconnected," said Ripple. "The work at Yellowstone and other places shows how one species affects another and another through different pathways. It's humbling as a scientist to see the interconnectedness of nature."

Struggle over returning predators in Europe

In France, much attention has been paid in recent years to the recovering wolf population, with the government being forced to introduce and revisit its 'wolf plan' over farmers' claims that the animals are interfering with farming activities and killing livestock.

Although French farm organisations have been quick to call for wolf culls, and their calls have been backed by many politicians, the ecosystem value of predators which have typically been hunted out of many European States remains a relatively new area of study. A paper published in March 2012 by scientists in the United States revealed "The loss in the Northern Hemisphere of large predators, particularly wolves, [means] current populations of deer and other large herbivores far exceed their historic levels and are contributing to disrupted ecosystems."

Environment and wildlife NGOs have called for efforts in France to focus on fence building, hiring shepherds and training dogs, rather than allowing the killing of wolves; there are thought to be around 250 wolves in France and the animals are already being subjected to illegal persecution. Even so, farming unions have demanded more draconian measures, including proposed increases to the number of wolves legally allowed to be killed each year.

Researchers from Oregon State University, who were also involved in the 2012 study, looked at forest ecosystems in Northern Europe, North America and Asia and concluded that, in addition to the cultural values of such predators, their presence keeps numbers of large herbivores in check. When predators are persecuted, herbivore populations increase, which has implications for the growth of young trees and can reduce biodiversity and even affect the biome's ability to sequester carbon. They said, "New analysis makes clear that the potential beneficial ecosystem effects of large predators is far more pervasive, over much larger areas, than has often been appreciated."

Bill Ripple discusses the importance of large predators further in this video from Oregon State University.