"Small-population languages remaining in economically developed regions are seriously threatened by continued speaker declines."

In Alaska, for example, there were only 24 active speakers by 2009 of the Athabaskan people's indigenous language, which children were no longer learning. And the Wichita language of the Plains Indians, now based in the US state of Oklahoma, had only one fluent speaker by 2008.

In Australia, aboriginal languages like the recently extinct Margu and almost extinct Rembarunga were "increasingly disappearing", the team wrote.

"Economically developed regions, such as North America and Australia, have already experienced many language extinctions," they said.

"Nevertheless, small-range and small-population languages still persist in hotspots within these regions. Those languages need immediate attention because of their high extinction risk."

Also at risk were developing parts of the world undergoing rapid economic growth, such as much of the tropics and the Himalayan region, said the team - citing Brazil and Nepal.

The researchers had gathered data on the number of speakers of a language, their geographical range, and rates of growth or decline. They then considered possible influences like globalisation or environmental and socio-economic changes.

The data comparison showed that "levels of GDP [gross domestic product] per capita correlated with the loss of language diversity: the more successful economically, the more rapidly language diversity was disappearing," said a press statement from the University of Cambridge.

Study co-author Tatsuya Amato explained that as economies developed, one language often came to dominate.

"People are forced to adapt the dominant language or risk being left out in the cold - economically and politically."

One saving grace was bilingualism, which the team said must be encouraged to preserve the world's linguistic diversity.