A plea to restore populations of some of the world's most dangerous animals has been made by scientists who claim the loss of large carnivores is damaging ecosystems.
More than three-quarters of the 31 species of large land predators, such as lions and wolves, are in decline, according to a new study. Of these, 17 species are now restricted to less than half the territory they once occupied.
Large carnivores have already been exterminated in many developed regions, including western Europe and eastern United States - and the same pattern of "carnivore cleansing" is being repeated throughout the world, said scientists.
Yet evidence suggests carnivores play a vital role in maintaining the delicate balance of ecosystems which cannot be replaced by humans hunting the animals they normally prey on.
"Globally, we are losing our large carnivores," said lead researcher Prof William Ripple, from the department of forest ecosystems and society at Oregon State University in the US.
"Many of them are endangered. 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."
Humans have waged a long-standing war with large carnivores that kill livestock and threaten rural communities.
But the international team from the US, Australia, Italy and Sweden called for a global initiative to conserve large predators.
The scientists suggested it could be modelled on the Large Carnivore Initiative for Europe, an expert group affiliated with the International Union for the Conservation of Nature (IUCN). It is committed to helping European predators including the wolf, lynx and brown bear reoccupy many of their former habitats.
Prof Ripple and his colleagues focused on seven key species whose ecological impact has been extensively studied - the African lion, leopard, Eurasian lynx, cougar, grey wolf, sea otter and dingo.
A review of the evidence showed how the decline of cougars and wolves from Yellowstone and other North American national parks led to an increase in browsing animals such as deer and elk.
This in turn had a cascading effect, disrupting vegetation growth and upsetting populations of birds and small mammals.
Studies of the European lynx, Australian dingo, lions and sea otters have shown similar effects, said the researchers whose findings were reported in the journal Science.
Lynx were strongly linked to the abundance of roe deer, red fox and hare, while in Africa the loss of lions and leopards had coincided with a dramatic increase in olive baboons, which threaten farm crops and livestock.
In the waters of south-east Alaska, a decline in sea otters hunted by killer whales had led to a rise in sea urchins and the loss of kelp beds.
Ecosystems had responded quickly where large carnivores had been returned to their former habitats, said Prof Ripple. Two examples were the reintroduction of wolves in Yellowstone and the Eurasian lynx in Finland.
"I am impressed with how resilient the Yellowstone ecosystem is," said the professor. "It isn't happening quickly everywhere, but in some places, ecosystem restoration has started there."
The classic concept of predators being harmful is outdated, the scientists claimed.
Prof Ripple added: "Human tolerance of these species is a major issue for conservation. We say these animals have an intrinsic right to exist, but they are also providing economic and ecological services that people value.
"Nature is highly interconnected. 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."
Disruption of large carnivore populations had led to crop damage, altered stream structures, and changes to the abundance and diversity of birds, mammals, reptiles and invertebrates, said the scientists.
By keeping herbivores in check and allowing woody plants to flourish and store more carbon, carnivores also acted as a buffer against climate change.
The researchers accepted that getting human communities to accept the reintroduction of large carnivores was a "major sociopolitical challenge".
They wrote: "It will probably take a change in both human attitudes and actions to avoid imminent large carnivore extinctions. A future for these carnivore species and their continued effects on planet Earth's ecosystems may depend on it."