"Rain forest trees can live for centuries, even millennia, so none of us expected things to change too fast," Laurance said in a statement. "But in just two decades -- a wink of time for a thousand year-old tree -- the ecosystem has been seriously degraded," he said.

Writing in the Proceedings of the National Academy of Sciences, Laurance and his team said fragmenting the forest creates more edges, exposing trees that would normally have been protected by other trees.

"When you fragment the rain forest, hot winds from the surrounding pastures blow into the forest and kill many trees, which just can't handle the stress," said Henrique Nascimento, a team member from Brazil's National Institute for Amazonian Research in Manaus.

"Also, winds build up around the fragment and knock down a lot of trees," Nascimento said.

The international team of researchers has been studying Brazil's rain forest for 22 years, covering nearly 32,000 individual trees.

"The rain forests of central Amazonia contain some of the most biologically diverse tree communities ever encountered, averaging 250 species that attain a diameter of at least 10 cm (4 inches) per 1 hectare (2.5 acres)," they wrote in their report.

"These communities are also being cleared and fragmented at alarming rates as a result of large-scale cattle ranching, slash-and-burn farming, rapid soya expansion, industrial logging, and wildfires," they wrote.

They found that nearly a fifth of some of the most common tree genera -- the larger groups comprised of several related species -- declined in abundance over the 22 years. Only one-tenth of the genera became more abundant.

Those within 300 feet (100 metres) of the forest edges were the most vulnerable.

The researchers did not report on saplings or seedlings but said these would be even more vulnerable.

"When you fragment a forest, the winners are common pioneer and generalist species that like forest disturbance," said Laurance. "The losers are rare, slow-growing tree species that provide fruit, nectar, and homes for a diversity of rain forest animals."