Java-Bali rainforests

• Category: Forest Biomes.
• Geographic Location: Southeast Asia.
• Summary: A highly fragmented southeast Asian rainforest, home to several endemic species, this biome faces significant threat from rapid encroachment of high-density human populations.

Set between the eastern Indian Ocean and the Java Sea is the Indonesian island of Java, an elongated and rugged strip of land with one of the highest-density human populations in the world. Off its eastern coast, across the 1.5-mile-wide (2.4-kilometer-wide) Bali Strait, is another Indonesia island, the much smaller Bali. Eastern Java and Bali are home to the biome known as the Java-Bali rainforest, which takes the form of scattered patches of deciduous and semi-evergreen forests. These support many endemic (found nowhere else on Earth) and near-endemic species; the lush tropical ecosystem is under pressure from agriculture, poaching, logging, and related human land-use activity.

The climate of the Java-Bali region, classified as tropical wet and dry, and affected by seasonal monsoons, receives less rainfall than adjacent rainforest biomes in western Java. The region is densely mountainous and actively volcanic, with the highest elevations being 12,060 feet (3,840 meters) at Mt. Semeru in Eastern Java, and 10,300 feet (3,140 meters) at Mt. Agung on Bali. Both volcanoes have histories of recorded activity over the last 100 years.

Vegetation

Variations in elevation and its subsequent effects on rainfall result in microclimates across the region. Mountainsides that face toward the south receive higher rainfall and are able to better support forest growth. Such localized variations result in tiny, isolated pockets of dry deciduous, moist deciduous, and semi-evergreen forests scattered in parts of the lowlands and on the mountains.

Most of the original plant growth in these rainforests has been cleared through logging or burning for agricultural purposes. Although volcanic soil is highly fertile, repeated burning and shifting cultivation and grazing under the control of the increasing human population have contributed to prevalent loss of habitat and soil erosion.

The diversity of the flora and fauna in the Java-Bali rainforests is considered low to moderate, especially compared to other rainforest biomes in southeast Asia. In Bali, the small size of the island, dwindling natural growth, and human disturbance of what remains of the forests put extreme pressure on the survival of the few endemic species.

The benchmark strata and characteristics of a healthy rainforest ecosystem, such as a distinct emergent layer, canopy, understory, and forest floor, do not exist or are difficult to identify in the Java-Bali Rainforest biome. Although a few semi-evergreen forests survive at elevations higher than 1,600 feet (490 meters), the remaining deciduous forests in the lower elevations are composed mostly of light stands of trees, only a few of which are taller than 80 feet (24 meters). Such stands have open canopies, allowing sunlight to reach the forest floor and support herbaceous undergrowth. There is no dominant tree type in this biome, unlike in many equatorial or tropical rainforests. Some of the common genera here include Homalium, Acacia, Ailanthus, and Dipterocarpus.

Characteristic Animals

There are at least 100 mammal species here, with two that are strict endemics: the critically endangered bawean or Kuhl’s deer (Axis kuhlii), and the endangered Javan warty pig (Sus verrucosus). The wild banteng (Bos javanicus), though not endemic, is also endangered. The now-extinct Balinese tiger (Panthera tigris balica) was the smallest and rarest tiger subspecies in the world, recorded now only through a few bones and skins kept in museums. Once strictly endemic to the island, it is believed to have succumbed to hunting and human encroachment on its already-limited habitat.

The Javan tiger (Panthera tigris sondaicus), which was larger in size and had a more extensive habitat than its Balinese cousin, is also extinct, while a Javan leopard subspecies is considered endangered. Today, no large predators exist on Bali. The largest undomesticated mammals that remain on the island are the Javan rusa deer (Rusa timorensis) and the wild boar (Sus scrofa), both of which thrive in stable populations here and elsewhere.

Fifteen bat species have been identified in the Java-Bali region. These include fruit bats and flying foxes, which residents hunt both for their meat and for medicinal use. The cave fruit bat is economically important as a pollinator of durian trees, which contribute to the region’s agriculture and economy. In spite of this, the fruit bats are exterminated by orchard owners, who believe that the animals are harmful to their crops.

Approximately 350 bird species are present in the biome, 10 of which are endemic or near-endemic. Although some of the near-endemic species are also found in the montane rainforests of western Java, they continue to face significant conservation pressure there as well, with threat levels ranging from threatened to critically endangered. Among the bird species here that are under threat are the Bali starling, the Bali myna, Javanese lapwing, and Javan hawk eagle. The critically endangered Bali starling, a well-known songbird which is endemic to the island, has been successfully bred in captivity, and subsequently released into the forest. However, illegal poaching continues to take its toll, and it is unknown how well the released birds have been able to breed in the wild.

Conservation and Threats

Eighteen protected zones, the largest of which is Meru Betiri National Park, cover a total of 900 square miles (2,330 square kilometers), or 4 percent of the land area of the biome. Several of these zones are rather small, 12 square miles (31 square kilometers) or less, and are scattered throughout the region. Despite the designation of these protected sections, poaching and settlement continue apace.

The Java-Bali Rainforest biome faces significant ecological threats. The biome’s conservation outlook is negatively affected by the absence of much of its original forest cover, and the extreme and continuing fragmentation of remaining stands. Ironically, lost rainforest means lost carbon sink, thus less ability to “apply the brakes” on the increasing global greenhouse gas concentrations in the Earth’s atmosphere.

Climate change presents uncertain challenges that complicate strategies for how to mitigate the largely human-caused stresses this biome already endures. It is projected that East Java and Bali will experience rising average air temperatures, later seasonal onset of rainfall and monsoons, decreased rainfall in East Java but increased rainfall in Bali, a longer dry season in both areas, increased fires, and more extreme weather events overall. Each of these phenomena has been recorded as already underway.

Impacts will include expanding hillside erosion, greater insect and disease stress upon some forest flora, shift in habitat elevations, and opportunities for invasive species at the expense of some native ones. For example, the Javan hawk-eagle is known to breed slowly, and to prefer living in a single, familiar area throughout its adult life. These tendencies make the species vulnerable to each of the habitat impacts listed above.

Bibliography

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Hendricks, Lauren B., Gusti Z. Anshari, and Daniel G. Gavin. “Fire in the Rainforest: A 3200-Year History of Fire in a West Kalimantan, Indonesia, Tropical Rainforest.” Ecosphere, vol. 15 no. 6, 17 June 2024, doi.org/10.1002/ecs2.4918. Accessed 26 Dec. 2024.

“Indonesia Disaster Agency Warns of Drought in Java, Bali Amid El Nino.” Reuters, 14 Aug. 2023, www.reuters.com/world/asia-pacific/indonesia-disaster-agency-warns-drought-java-bali-amid-el-nio-2023-08-14/. Accessed 26 Dec. 2024.

Newman, Arnold. Tropical Rainforest. New York: Checkmark Books, 2002.

Van Balen, S., Vincent Nijman, and Resit Sozer. “Conservation of the Endemic Javan Hawk-Eagle Spizaetus Bartelsi Stresemann, 1924 (Aves: Falconiformes): Density, Age-Structure and Population Numbers.” Contributions to Zoology 70, no. 3 (2001).

Wikramanayake, Eric, Eric Dinerstein, and Colby J. Loucks, et al. Terrestrial Ecoregions of the Indo-Pacific: A Conservation Assessment. Washington, DC: Island Press, 2002.