Sumatra and Associated Islands -
Biodiversity and Tropical Forests in Indonesia

Sumatra, which has an area of 473,607 km² and is the sixth largest island in the world, is extremely rich in biotic habitats. It has extensive lowland dipterocarp forests throughout the island, which form the vegetation matrix of the island. The east coast is dominated by extensive Mangrove Forests, but also with patches of Peat Swamp Forest. The sandy west coast supports various types of Coastal Forests. Large peaty freshwater swamps are found in the south. It also has a number of mountains, some formed by uplift of sedimentary deposits (Barisan Range) and some by volcanic action (Mts Kerinci, Sinabung, Merapi, Singgalang). Further, it has had different associations to many of the small islands that surround it, ranging from recent land connections with them to no connections at all. It also has a number of major rivers, which in the lowlands have caused ecological barriers to the distribution of animals and plants, as also have some of the mountain ranges. Barriers to movement of animals and plants have afforded opportunities for their speciation in Sumatra. This, coupled with the wide range of habitats in Sumatra, has been in large part responsible for the island’s rich fauna and flora.

Terrestrial
Sumatra has the most mammal species in Indonesia (201 spp.), of which nine are endemic to the Sumatran mainland and a further 14 are endemic to the associated Mentawi islands. Sumatra also has 22 other species of Asian mammals found nowhere else in Indonesia, attesting to the close links with Peninsular Malaysia. It is also the repository of the Sumatran Rhinoceros, Elephant, Tiger (Panthera tigris) and the Forest Dog (Cuon alpinus), all of which are virtually extinct in other parts of Indonesia (FAO/ van der Zon 1979).

Small islands with animals and plants are closely associated with those on Sumatra, include Bangka, Biltung and Anambas/Natuna. These have had recent land bridges with both Sumatra and Borneo (including Kalimantan). They have an essentially Sumatran/Bornean fauna and flora, albeit with much fewer species.

The small islands that have been more isolated from Sumatra, such as the Mentawi Islands and the smaller Simeulue and Enggano Islands, have developed more distinct faunas. For example, the Mentawi islands, which have been isolated for long periods without any land connection to Sumatra, have a unique assemblage of endemic mammals, with 85% of their non-flying species endemic. These endemic mammals are: Pagai Islands Horshoe Bat (Hipposideros breviceps), Mentawi Macaque (Macaca pagensis), Mentawi Snub-nosed Monkey (Simias concolor), Mentawi Leaf Monkey (Presbytis potenziani), Mentawi Gibbon (Hylobates klossii), Loga Squirrel (Callosciurus melanogaster), Soksak Squirrel (Lariscus obscurus), Mentawi Black-cheeked Flying Squirrel (Iomys sipora), Mentawi Orange-cheeked Flying Squirrel (Hylopetes sipora), Mentawi Civet (Paradoxurus lignicolor), Giant Mentawi Rat (Leopoldamys siporanus), Mentawi Forest Rat (Maxomys pagensis), Mentawi Rat (Rattus lugens), and Mentawi Pencil-tailed Tree Mouse (Chiropodomys karlkoopmani).

The two small islands of Simeulue and Enggano Islands, which probably have never been connected by land to Sumatra, have an impoverished fauna. For example, there are no squirrels on either island. However, Simeulue has three endemic species of snakes, an endemic bird and a morphologically distinct macaque monkey and a pig, which may also be taxonomically distinct (Mitchell 1981). Enggano has three endemic mammal species (Sody 1940), two endemic bird and one endemic snake species (Lieftinck 1984).

Sumatra also has an extremely rich bird fauna. Its bird list of 580 species is second only to New Guinea. A total of 465 of these bird species are resident and 21 are endemic. At least 31 species of birds of Asian origin are found on Sumatra, including the Great Hornbill (Buceros bicornis), which occurs nowhere else in Indonesia (FAO/ MacKinnon and Wind 1979). Sumatra is also extremely important for migratory species, mostly from the Palaearctic region, but also from tropical South and Southeast Asia. Nisbet (1974 ) suggests that 11 of these migrants principally winter in Sumatra.

Sumatra lies in the West Malesia plant region along with Peninsular Malaysia, Borneo, Philippines and part of southern Thailand. Sumatra probably has more than 10,000 species of higher plants, most of which are found in lowland forest. The number of tree species per unit area equals that of Borneo, and like other Sunda forests are probably the richest plant communities in the world (Whitmore 1984). The Sumatran forests have a species diversity that is comparable to Borneo (Meijer 1981), but it has far fewer endemic genera of plants (17 versus 59). It does have some spectacular plants, including Rafflesia arnoldii, the largest flower in the world, and Amorphophallus, the tallest flower in the world.

The Peat Swamp Forests of Sumatra, which are mainly ombrogenous (gaining nutrients from rainfall), are large areas mostly concentrated along the southern and central parts of the east coast of Sumatra. They are mostly drained by blackwater rivers. These rivers are low in nutrients, containing fewer inorganic irons, lower levels of dissolved oxygen and higher concentrations of humic acids than clear rivers (Janzen 1974). Vegetation in these swamps generally is both floristically and structurally concentric as plants respond to the poorer fertility towards the center of a given peat swamp (Anderson 1976). Trees may be large and as high as 45 m, or stunted and only about 12 m high. Common species are Tristania obovata and Ploiarium alternifolium. No vegetation appears to be confined to these peat swamps with perhaps the exception of several species of palm (Salacca conferta and Livistona hasseltii). Blackwater rivers have an impoverished fauna characterized by airbreathers, including the fish (Johnston 1967). They also have an impoverished fauna with low densities of mammals and birds (Merton 1962).

Freshwater Swamp Forests are physically similar to Peat Swamp Forests, but there is a lack of deep peat; they receive water from both rainfall and rivers. They are mainly on riverine alluvium and occasionally on alluvium deposits of larger lakes, such as ‘lake’ Bento, Kerinci. Their distribution is generally contiguous with peat swamps. Few plant species are restricted to these forests, but their species composition is more similar to lowland forests than to Peat Swamp Forests (Whitten 1982). Structurally they are also variable and range from grassy marshes, pandan dominated forests, to a lowland forest form. They are richer in animals than Peat Swamp Forest and appear to retain a slightly impoverished assemblage of those found in lowland forests. They used to have large populations of the Estuarine Crocodile (Crocodylus porosus) and the False Ghavial (Tomistomus schlegeli), but their populations are now low; the endangered White-winged Wood Duck (Cairina scutulata) also occurs there. The high agricultural value of their soils (Burnham 1975) has meant that they have suffered greatly from human activities, such that in 1982 as little as 22% of the original extent of this formation survived.

The lowland forests of Sumatra are, like those in Borneo, extremely diverse in both plants and animals. They form the matrix vegetation community for the island. The vegetation is characterized by thick buttressed trees dominated by tall, up to 70 m, dipterocarp species (Dipterocarpus, Parashorea, Shorea, Dryobalanops), of which there are 112 species, including 11 endemics (Ashton 1982). Other dominants are usually in the family Caesalpiniaceae (Koompasia, Sindora, Dialium). They are dynamic stratified plant communities constantly suffering perturbations that produce a range of different gap types. Huc and Rosalina (1981, 1981b) calculated that at various locations in Sumatra, the growth cycle in forests averaged 117 years, which is similar to the 108 years calculated by Noordwijk and Schaik (in Whitten et al. 1996) at Ketambe. Variations in the particular phases of the growth cycle can be considerable. Even minor variations in such forests in Siberut Island are apparent to the gibbon monkeys (Whitten 1984). Animal diversity is not as high as for some other tropical regions though. For example, the birds at seven Sumatran sites were less diverse than in Africa and South America (Pearson 1982), although Wells (in Whitten et al. 1996), contests that conclusion. Many animals, particularly those that feed on fruits and pollen, are nomadic and roam over the forests in search of food. Most large mammals in Sumatra live primarily in the lowland forests. And some areas have up to eight species of primate. These are the Orangutan (Pongo pygmaeus), tarsier (Tarsier bancanus), Siamang (Hylobates syndactylus), Whitehanded Gibbon (H. lar) or the Dark-Handed Gibbon (H. agilis), Silvered Leaf Monkey (Presbytis cristata), Thomas’ Leaf Monkey (P.thomasi ), Banded Leaf Monkey (P. melalophos), Eastern Leaf Monkey (P. femoralis), Long-tailed Macaque (Macaca fascicularis), Pig-tailed Macaque(M. nemestrina), and Slow Loris (Nycticebus coucang). Sumatra has 10 species of distinctive hornbill birds; eight can be found in a single area of forest.

Heath Forest and Padang forest are largely restricted to Bangka and Belitung islands, although small areas exist in eastern Sumatra. Heath Forests are on infertile white sand soils relictual of ancient eroded sandstone beaches. They usually have a vegetation that is an impoverished and stunted assemblage of trees found in the lowland forest matrix. No species of animals are known to be restricted to Heath Forests. Padang is a shrubby and low (less than 5 m tall) vegetation. It is commonly thought to be a degraded Heath Forest (Whitmore 1984).

The lowland Iron Wood forests of Sumatra are dominated by the species, the Iron Wood (Eusideroxylon zwageri) generally found on sandy tertiary soils in the central southernpart of Sumatra. Little is known of the fauna occupying this forest type, although, due to its dominance by a single tree species, it could be expected to be an impoverished assemblage of the typical lowland fauna. However, it appears to have bird fauna similar in diversity and number to lowland forests (Whitten et al. 1996).

The extensive mountains in Sumatra are covered from 1200-2100 m above sea level with lower Montane Forests, from 2100-3000 m with Upper Montane Forest, and above 3000 m with Sub-alpine Forest. Low Montane Forests are characterized by the Fagaceae and Lauraceae (laurels), diminution of dipterocarp trees and increase in tree ferns. The Upper Montane Forests are characterized by the order Coniferae (pines and related trees) and the families Ericaceae (e.g., bilberries Vaccinium, Rhododendron) and Myrtaceae. Trees are often quite low and gnarled, and mosses and liverworts are common. The Sub-alpine Forest is characterized by having dwarf forms of the Upper Montane Forest as well as grasses, rushes and sedges. A number of the mountain flora species are shared with mountainous flora in many countries, both tropical and subtropical (Rhododendron, Deschampsia, Gentiana and Primula). The invertebrate and lower vertebrate fauna on Sumatran mountains iare likely to be impoverished compared to lower altitudes (Whitten et al. 1996). Chasen and Hoogerwerf (1941) showed that this was decidedly the case with birds in Aceh, northern Sumatra. They showed that below 12000 m, 134 species were found, while above this altitude only nine were recorded, all but one also found in the lowlands. In the sub-alpine zone, 11 species were found but only two of these were shared with birds found in the lower altitudes, indicating that the Sub-alpine Forest has a distinct community of birds, including Sunda Whistling Thrush (Myiophoneus melanurus), Scaly Thrush (Zoothera dauma) and Island Thrush (Turdus poliocephalus) (van Strein 1977). The mammal community on Sumatra becomes impoverished above about 1,200 m above sea level (Robinson and Kloss 1918). At least 11 species of mammals are more or less restricted to mountains in Sumatra. These are the Grey Shrew (Crocidura attenuata), Grey Fruit Bat (Aethalopes alecto), Sumatran Rabbit (Nesolagus netscheri), Volcano Mouse (Mus crociduroides), Giant Sumatran Rat (Sundamys infraluteus), Edward’s Rat (Leopoldamys edwardsi), Hoogerwerf’s Rat (Rattus hoogerwerfi), Kerinci Rat (Maxomys hylomyoides), Kerinci Rat (Maxomys inflatus), Kinanbalu Rat (Rattus baluensis), Mountain Spiny Rat (Niviventer rapit), and Serow or Mountain Goat (Capricornis sumatraensis).

About 30% of the plant species from similar forests in Malaysia have some economic value (Burkill 1966); a similar proportion of economically useful trees could be expected to be found in Sumatra. Unfortunately, the previously extensive dipterocarp forests, which provide about 25% of the hardwood timber on international markets, have been so over-exploited that it is expected that the supply of timber from Lowland Rainforests will be exhausted by 2005 (World Bank 2001).