Reading the Zoogeography of Sumatra

Sumatra is one of the regions with the highest biodiversity in the world, as well as a natural laboratory for studying zoogeography and evolution. The wealth of wildlife inhabiting this island was not formed by chance, but is the result of long-term interactions between geological changes, climate dynamics, and evolutionary processes that have taken place over millions of years.

The distribution of each species is a reflection of the Earth’s history, from the movement of tectonic plates, the rise and fall of sea levels, the formation of the Bukit Barisan mountains, to changes in river networks that form natural boundaries for various animal groups (Whitten et al., 2000; Corlett & Primack, 2011). Therefore, understanding the distribution of animals in Sumatra is not enough just by knowing their locations, but also requires seeing how the history of the landscape shaped these distribution patterns.

During the Pleistocene, around 2.6 million to 11,700 years ago, Sumatra was still connected to mainland Asia via the Sunda Shelf. A drop in sea level of more than 100 metres created a land bridge that allowed various types of mammals, birds, and plants to migrate from Asia to Sumatra (Voris, 2000). Elephants, tigers, rhinoceroses, tapirs, deer, bears, primates, and many other animal groups entered the island through this route. When the ice age ended and sea levels rose again, the land connection was severed, so that the populations that had settled in Sumatra evolved in isolation. This geographical isolation then triggered genetic differentiation and local adaptations that gave rise to new species or subspecies. The Sumatran orangutan (Pongo abelii) evolved separately from the Bornean orangutan, while the Tapanuli orangutan (Pongo tapanuliensis) was recognised as a distinct species after genetic research showed that its population had been isolated for hundreds of thousands of years (Nater et al., 2017).

In addition to sea level changes, the formation of the Bukit Barisan mountains, which stretch for more than 1,700 kilometres from Aceh to Lampung, also shaped the zoogeographic patterns of Sumatra. These mountains create differences in rainfall, temperature, humidity, and vegetation types between the western and eastern sides of the island.