They say habitation loss, over-hunting, over-fishing, climate change caused by fossil-fuel gases, and the spread of germs and viruses and introduced species.
Until mankind's big expansion 500 years ago, mammal extinctions were very rare; on average, two species died out every million years. But, researchers say in the last five centuries at least 80 out of more than 5,000 species have died out.
Scientists say that unless action is taken now, a full-blown mass extinction could occur within a few centuries.
