Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences Tropical Asia: an overview (Part1) Richard T Corlett Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences

XTBG

He explored S.E. Asia between 1854 and 1862. Alfred Russel Wallace was the “father of biogeography” (and the co-discoverer of the theory of evolution with Charles Darwin). He explored S.E. Asia between 1854 and 1862. In Singapore in 1862

His division of the world into “zoogeographic regions” (published in 1876 in his book The Geograpical Distribution of Animals) is still followed, with minor changes, today.

Wallace called Tropical Asia the “Oriental Region” Wallace called Tropical Asia the “Oriental Region”. His definition of this region also included most of the subtropics (from the Tropic of Cancer to c.30oN), because of the similar fauna.

North of the Oriental Region is the Palearctic Region, distinguished by cold winters and the resulting absence of most tropical animals.

Mean January minimum temperature

To the South-east is the Australian Region, distinguished by the absence of typical Oriental vertebrates (deer, monkeys, cats...) and presence of marsupials, unique bird groups etc.

‘Wallace’s Line’ separates the Oriental and Australian Regions

Botanists have generally ignored Wallace’s line altogether and include the whole region through to New Guinea in ‘Malesia’.

‘Wallace’s Line’ is not caused by climatic differences but by history.

Global sea-levels have fluctuated over the last 2 million years.

During periods of low sea-level, the land masses west of the Line were connected (or almost so, in the case of the Philippines), as were New Guinea and Australia to the east. The islands of Wallacea were still islands. Oriental “Wallacea” Australian

Even at the lowest sea-levels, most vertebrates could not cross the seawater gaps, but many plants could. Hence Wallace’s Line is a barrier to most vertebrates but not for most plants. [Invertebrate groups vary]. Oriental “Wallacea” Australian

The differences have been reinforced by the geological history, with the Oriental and Australian sides originally far apart. Continents in the Late Jurassic, 150 million years ago

Continents in the Late Cretaceous, 90 million years ago

Moving closer…. Continents in the Middle Eocene, 50 million years ago

Finally colliding in the Miocene…. Continents in the Early Miocene, 20 million years ago

The modern world….

The collision is still going on today… “Wallacea” c. 6 cm/yr

Volcanoes in Indonesia…. …which explains why Indonesia has so many volcanoes and earthquakes.

Wallace and subsequent authors have also recognized subdivisions within the Oriental Region

Wallace put the border between his Indo-Chinese and Indo-Malayan (or Sundaland) subregions near the top of the Thai-Malay Peninsula (14oN). Most recent zoologists have chosen the Isthmus of Kra (11oN). While botanists prefer a line near the Thai-Malaysia border (7oN). Indo-Chinese subregion Indo-Malayan (or Sundaland) subregion

Overall, there is a major change-over in species between 5-14oN on the Thai-Malay Peninsula. For plants (and some groups of animals) this is a more important boundary than Wallace’s Line. The simplest explanation is that it reflects the transition from an ever-wet Sundaland region, most covered in tropical rainforests until recently, and a more seasonal Indochina region, with a variety of generally more open forest types.

This is again determined by rainfall, with the Indian side much drier. The other major border within the Oriental Region is between the Indo-Chinese and Indian subregions. This is again determined by rainfall, with the Indian side much drier. Wallace included NE India and the Himalayas in the Indochinese subregion. Indian subregion Indo-Chinese subregion Indo-Malayan (Sundaland) subregion