, the 'Evil Quartet' is a term used to describe the four major causes of biodiversity loss . Among these, habitat loss and fragmentation is explicitly identified as the most important cause driving animals and plants to extinction . The sources provide the example of tropical rain forests, which once covered 14 per cent of the earth's land surface but have been reduced to 6 per cent due to destruction . The other factors in the quartet include over-exploitation , alien species invasions , and co-extinctions .