The region has long been a focus for case studies of evolution and the impact of economic growth on the integrity of tropical forests. This really began when Alfred Russel Wallace (1823-1913) travelled widely in the region. He was the co-publisher with Charles Darwin (1809-82) of the theory of natural selection, coming to the same conclusions as Darwin by recognising a clear-cut division of species distribution in the area.
During the last ice age the countries of mainland tropical Asia and the Greater Sunda Islands - Sumatra, Borneo and lava - were connected by dry land; they were separated as melting ice caused sea levels to rise again. Before this wildlife was able to move along the land bridges, and the islands still have plant and animal species in common with those of the mainland, characterized by tall dipterocarp trees and by monkeys, native deer and hornbills. These habitats, together with their plants and animals, fall within the Indomalayan biogeographical region.
By contrast the islands of eastern Indonesia - Irian Jaya (western New Guinea), Kai and Am - lie on the Australian continental shelf and belong within the Oceanian biogeographical region. Here there are mound-building birds, bowerbirds, parrots and birds of paradise, while wallabies take over the ecological niche of deer.
Wallace's dividing line, separating these two biogeographical regions, runs through the islands between Bali and Lombok, Borneo and Sulawesi, and Palawan and the rest of the Philippines. Still recognized as the biogeographical boundary for many families of plants, and of birds, mammals, insects and other animals, it is a reminder of Southeast Asia's dynamic geological past. On the edges of these two biogeographical regions the islands of Sulawesi, the Philippines, the Moluccas and the Lesser Sundas form a mixing ground for plants and animals from both east and west. They include many species that are found only here.
Southeast Asia was created between 15 and 3 million years ago when outlying fragments of ancient drifting supercontinents collided in the vicinity of the island of Sulawesi. Moulded by its geological past, at a crossroads for animal and plant migrations, the region has some of the most spectacular and diverse tropical habitats in the world. These range from muddy coastal mangroves and peat swamp forests to moss-draped cloud forests and shrubby alpine plant communities. There are tall, lowland dipterocarp (two-winged fruited) forests The tops of these giant dipterocarp trees do not overlap. One explanation for this "crown shyness" is that it inhibits the spread of leaf-eating caterpillars. It also allows light to penetrate through the trees, and has led to the evolution of jumping and gliding mammals and reptiles. The eastern islands have palm-thick jungles. There are tidal wetlands and multicoloured crater lakes of still-active volcanoes; craggy limestone hills with spearlike pinnacles and vast underground cave systems to harsh, nutrient-poor heathlands. The cloudy moss forest at 3,500 m in Northern Borneo is one of the wettest places on Earth, and in the highlands it is also very cold. The trees are stunted, being no more than 10 to 15 m (30 to 45 ft) high; as there is only a single canopy layer enough light penetrates to promote the growth of hanging lichens, mosses and other epiphytes, as well as ground plants. Lava and ash on a volcanic peak weather to a fertile soil, the radiating ridges and channels formed in the lava flows are colonized by wind-blown seeds, and vegetation begins to cover the mountainside.
- 1017 reads
