Siberut island has been geographically isolated from mainland Sumatra since the Pleistocene age, approximately 500 000- 2 000 000 years ago. As a result of this geographic isolation the island has experienced a unique evolutionary transformation that has resulted in the distinctiveness of the island as we know it today. The process of isolation and lack of colonization of flora and fauna from mainland Sumatra has encouraged the formation of a high level of endemic species with exceptional ecological uniqueness. This can be seen, for example in Siberut’s unique fauna, which is more primitive and ancient than that found on mainland Sumatra.

The most famous endemic animals on Siberut are four endemic primate; Kloss's Gibbon or Pygmy Siamang (Hylobates klossii), Bokkoi or Siberut macaques (Macaca siberu), Simakobu or Pig Tail Macaque (Simias concolor), and Joja or monkey Mentawai (Presbytys potenziani). The Pig Tail Macaque is among the 25 most endangered primates in the world.

At least 29 land mammals and four known species of marine mammals, 116 species of birds, a crocodile, two turtles, 3 turtles, 34 snakes, 22 lizards, 16 frogs, and two Caeciians have been recorded. The total number of flora on the island is not known but around 846 species, 390 genera and 131 families of trees, shrubs and herbs, liana and epiphytes have been recorded.

Siberut contains four main types of forest namely;

1. Primary Dipterocarp Forest in the hilly areas dominated by the Dipterocarpaceae tree family, especially from the type Dipterocarpus;
2. Primary Mixed Forest found on the slopes and low hills below the Primary Dipterocarp Forest. In this forest there is no dominant species, but the most common are from the Myristicaceae, Euphorbiaceae, Dillenciae and Dipterocarpaceae families;
3. Brackish Freshwater Forest with limited and specific types of flora, dominated by Terminalia phellocarp which is a feather palm, rattan, pandanus and araoids and;
4. Mangrove forests along the eastern coastline which includes shallow sea and coral reefs.