Around the world, about species have been identified as life-threatening. Of these, 4,328 species are classified as endangered and 2,853 are critically endangered. As many as 60 species have been identified as extinct in the wild. The main causes of the extinction of species are, destruction of habitat by human activities and pollution by toxic chemicals, such as chlorinated-hydro carbons concentrated in food webs and the interconnected food-chains that circulate through an ecosystem.