It was the first and most extreme ecological disaster. Easter Island, in the south Pacific, once lush with subtropical broadleaf forest, was left barren and vast seabird colonies were destroyed after the arrival of man.

But now there is new evidence that human beings may not have been responsible for the destruction after all. Although Easter Island has long been held to be the most important example of a traditional society destroying itself, it appears that the real culprits were rats - up to three million of them.

This contradicts the belief that the native population's obsession with carving, constructing, and transporting its famous statues around the island led it to deplete its own natural resources, going into what has been called "a downward spiral of cultural regression".

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"A theme of self-inflicted, pre-European contact ecocide is common in published accounts," says the anthropologist Dr Terry Hunt, who led the research at the University of Hawaii. "Easter Island has become a paragon for prehistoric human-induced ecological catastrophe and cultural collapse. Scholars offer this story as a parable of today's global environmental problems."

He has examined new data from the Hawaiian and other Pacific islands that shows that by early historic times the deforestation of Easter Island was already complete, or nearly so. A dense forest of palm trees and more than 20 other types of trees and shrubs had mostly disappeared. As many as six land birds and several seabirds had also become extinct.

The island had a relatively simple ecosystem with vegetation once dominated by millions of palms. The original ecosystem of the island, with a limited range of plants, and few if any predators, would, says the report, have been particularly vulnerable to alien invasions.

Almost all of the palm seed shells discovered on the island were found to have been gnawed by rats. Thousands of rat bones have been found, and crucially, much of the damage to forestry appears to have been done before evidence of fires on the island. Evidence from other Pacific islands also confirms how devastating rats can be.

Exactly how rats got on to the island is not known, although one theory is that they arrived as stowaways in the first canoes of Polynesian colonists. Once they arrived, the rats found palm nuts offered an almost unlimited high-quality food supply.

Under ideal conditions, rats reproduce so rapidly that their numbers double every 47 days; unchecked, a single mating pair can produce a population of nearly 17 million in just over three years. Research in the Northwest Hawaiian Islands shows that when available food is taken into account, populations can reach 75 to the acre.

"At 75 rats per acre, the rat population of Easter Island could have exceeded 3.1 million," says the report. The Hawaiian research demonstrates that rats were capable, on their own, of deforesting large lowland coastal areas in about 200 years or less. "In the absence of effective predators, rats alone could eventually result in deforestation."

Dr Hunt says the environmental catastrophe of Easter Island has been masked by speculation about the intentions of people cutting down the last tree: "Indeed, the last tree may simply have died. Rats may have simply eaten the last seeds.

"The evidence points to a complex historical ecology for the island, one best explained by a synergy of impacts, particularly the devastating effects of introduced rats. This perspective questions the simplistic notion of reckless over-exploitation by prehistoric Polynesians and points to the need for additional research.

"I believe that there is substantial evidence that it was rats, more so than humans, that led to deforestation."