"By 2050, current trends in agricultural expansion will eliminate a total of 40 percent of Amazon forests, including six major watersheds and ecoregions," Britaldo Soares-Filho, of the Universidade Federal de Minas Gerais in Brazil, said in a report in the journal Nature.

A watershed is an area of land where all of the water that is under it or drains from it goes into the same place. It supplies water and habitats for plants and animals.

Soares-Filho and his colleagues used computer models to simulate what would happen to the Brazilian rainforests in the future under different scenarios.

"For the first time, we can examine how individual policies ranging from the paving of highways to the requirement for forest reserves on private properties will influence the future of the world's largest tropical forest," Soares-Filho said in a statement.

Without further checks, the scientists predict nearly 100 native species will be deprived of more than half of their habitats and nearly 2 million square kilometres (772,300 sq mile) of forest will be lost.

But if more is done to control expansion and increase protected areas, 73 percent of the original forest would remain in 2050 and carbon emissions would be reduced.

The scientists said better conservation of the rainforest would have worldwide benefits so developed countries should be willing to pay to make it possible.