Top predators are the "apex consumers" of the world's ecosystems and their decline in numbers has powerful repercussions on animals and plants lower down the food chain. The decline, from such activities as hunting and habitat loss, has had diverse effects, from changes in vegetation and wildfire frequency to water quality and nutrient cycles, the scientists said.
"Apex consumers... have powerful effects on the ways ecosystems work, and the loss of these large animals has widespread implications," said Professor James Estes of the University of California, Santa Cruz.
Professor William Ripple, of Oregon State University, said hunting larger animals to the edge of extinction takes away habitat and food from other animals. For example beavers are needed for tree growth, insects feed on large mammals and kills and whales provide a place for small fish to live. The death of larger animals also takes away the balance of the system, for example sharks being replaced by rays.
"We now have overwhelming evidence that large predators are hugely important in the function of nature, from the deepest oceans to the highest mountains, the tropics to the Arctic.
"In a broad view, the collapse of these ecosystems has reached a point where this doesn't just affect wolves or aspen trees, deforestation or soil or water.
"These predators and processes ultimately protect humans. This isn't just about them, it's about us."
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