An Irreplacable Habitat

Figure 3: Loss of orangutan habitat resulting from logging, plantations, rice-fields and mining operations in southern Kalimantan. Note that this map does not show the Tanjung Puting National Park or Lamandau Nature Reserve. The illustration mainly serves to demonstrate how the range of pressures work together.
Orangutans share their forests with a wide range of other threatened and ecologically important species. The tropical rainforests of Borneo and Sumatra have a biological richness and diversity (Table 1) that reflects their unique history, climate and ecology. The most species-rich are the lowland dipterocarp forests, so named because of the predominance of trees from the Dipterocarpaceae family. These dipterocarp trees tend to fruit simultaneously, producing very large amounts of fruit at the same time every two to five years. In these “mast years”, there is an abundance of food for seed-eaters, meaning that most of the seeds escape uneaten. Conversely, there is less fruit in other years, meaning that fruit-dependent animals such as orangutans need to occupy large ranges.

The peat swamp forests of Borneo and Sumatra have fewer endemic species than the dipterocarp forests, but they have a high density of fruiting trees, and do not have mast years which results in a more stable fruit supply, making them extremely important for orangutans. 

Orangutans play a crucial role in the forests they inhabit: their diet of fruit and their mobility means that they are excellent seed dispersers. Orangutans are thus responsible in part for maintaining forested ecosystems that provide important environmental services to humanity, from water resources to climate regulation.

Table 1: Species richness and endemism in Sumatra (475 000 km2) and Borneo (740 000 km2).

Island Birds Mammals Reptiles Freshwater fish Selected plant taxa
Number of native species
Sumatra 465 192 217 272 820
Borneo 420 210 254 368 900
Percentage of endemic species
Sumatra 2 10 11 11 11
Borneo 6 48 24 38 33

Source: Kapos & Caldecott 2005.

Flagship species of the lowland rainforests of Sumatra and Borneo

There are no more than 400 to 500 Sumatran tigers left in the wild (Macdonald 2006). It is thought that orangutans travel in the treetops to avoid tigers. Like the Sumatran orangutan, the Sumatran tiger is Critically Endangered according to the IUCN Red List (Cat Specialist Group 1996). The Bali, Caspian and Javan subspecies of tiger have already been lost.

The Sumatran rhinoceros is the smallest, hairiest and probably most endangered of the five rhino species. This is a mountain rain forest rhino, which browses on woody vegetation and occasionally fruit. At most 300 individuals remain in the wild and their numbers are declining as a result of illegal hunting and habitat fragmentation.

The Asian elephant has a widespread distribution, but the two small, forest-dwelling subspecies found in Borneo and Sumatra are unique. Elephants come into conflict with humans when their forests are destroyed and they seek food in croplands. Sumatran elephants made the news in 2006, when at least seven elephant deaths were associated with new oil palm plantations. The Indonesian government responded in June 2006 with a commitment to increase the size of the Tesso Nilo National Park.

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