Habitats ranging from coral reefs to tropical rainforests face mounting threats from pollution, expanding cities, deforestation, introduction of invasive species and global warming, says the report, issued Monday at the start of a U.N. meeting on biological diversity in Brazil.

"In effect, we are currently responsible for the sixth major extinction event in the history of Earth, and the greatest since the dinosaurs disappeared 65 million years ago," says the "Global Biodiversity Outlook 2" report.

Apart from the disappearance of the dinosaurs, the other "Big Five" extinctions occurred between 205 million and 440 million years ago, says the report.

Scientists suspect that asteroid strikes, volcanic eruptions or sudden climate shifts may explain them, it says.