Fear of dangerous predators is itself a key factor that helps keep ecologies in balance, a controversial study suggests.
The findings lend support to campaigners who want to see British forests "rewilded" with reintroduced wolves, lynx and bears.
Scientists in Canada carried out an experiment involving raccoons on British Columbia's Gulf Islands which have been decimating populations of songbirds, crabs and sea fish.
They suspected that the raccoon problem originated from the elimination of wolves and cougars from the islands a century ago.
To test the theory, the researchers played recorded domestic dog sounds for months at a time from speakers placed along two sections of shoreline where the raccoons habitually fed.
The effect was dramatic - hearing a "large predator" nearby frightened off the raccoons and reduced the time they spent feeding in their favourite coastal areas by 66 per cent.
This in turn had an enormous impact on the raccoons' prey. Shore crab numbers increased by 97 per cent, intertidal fish by 81 per cent and red rock crabs by 61 per cent.
The effect trickled down the food chain, reducing populations of other invertebrate species that were out-competed by the burgeoning population of crabs or eaten.
Study leader Professor Liana Zanette, from the University of Western Ontario, said: "These results have critically important implications for conservation, wildlife management and public policy.
"We have now experimentally verified that, by instilling fear, the very existence of large carnivores on the landscape - in and of itself - provides an essential 'ecosystem service,' and failing to consider fear risks dramatically underestimates the role large carnivores play in structuring ecosystems."
One of the most famous examples was the reintroduction of grey wolves to Yellowstone National Park in the US in 1995.
The wolves kept elks from over-consuming young willow plants which in turn benefited beavers, whose population had been reduced to just one colony in the park.
