Indonesia's Tesso Nilo tropical forest, one of the world's most biologically diverse, could disappear within four years if logging persists at the current rate, the Worldwide Fund for Nature (WWF) said. The 1,800 sq km forest on the island of Sumatra contains the highest level of lowland forest plants known to science, with up to 218 vascular species in just one 200 sq metre plot, a recent survey by the conservation organisation showed. But it said heavy logging for timber and pulp was having 'devastating effects on both plant and animal life.'