The Leuser Ecosystem in the northern part of Sumatra is one of the richest expanses of tropical rainforest in the world. Covering an area of approximately 2.6 million hectares (roughly the size of Belgium), it stretches from the shores of the Indian Ocean almost to the Straits of Malacca, and consists of majestic lowland forests, alpine meadows, freshwater swamps, spectacular rift valleys and several volcanoes.

It is thought that this unusual mix of habitats, coupled with the fact that until recently no people lived in the area, explains the extraordinarily rich biological diversity of the Leuser Ecosystem. The Leuser Ecosystem is for instance, the last place where sufficient numbers of rare species, such as the Sumatran species of Orangutan (4,000), Elephant (500), Tiger (500), and Rhino (60) can maintain or reestablish viable populations. In other words, the loss of the Leuser Ecosystem will mean the extinction of these species and probably many others along with them.

Apart from its rich biodiversity, the Leuser Ecosystem provides life support functions for the sustainable development of approximately four million people that live around it. Some examples of these functions are: the regular supply of fresh water, erosion and flood control, germ-plasm protection, local climate regulation, carbon sequestration, fresh water fisheries, and natural beauty (supporting tourism). These functions, sometimes called ecological services, have recently been valued at around 300 million Euro per year.