What is the future of Asian rainforests? In this second instalment, MARCUS NG shares University of Hong Kong researcher, Richard Corlett's insights into species distribution and invasion, and the loss of megafauna.

Written by Marcus Ng on 5 Mar 2007 with 0 comments. Be the first!

Part II: Ecological Musings on Species Shared, Neglected and Extinct

The world's major rainforest zones share very few species between them, having diverged as biogeographical zones when the southern supercontinent Gondwana broke up 160 million years ago. Corlett's first question, "Why is what where," asks whether the resulting differences in the world's forest plants and animals translate into functional consequences that affect ecosystem dynamics such as nutrient recycling, seed dispersal and responses to forest fragmentation. In the Neotropics, leaf-cutter ants harvest leaves to grow the fungi on which they feed; in the Old World, subterranean termites are the fungi farmers. "There's no obvious reason why these two groups shouldn't be found together, since they live very different lifestyles, but they don't!" wonders Corlett aloud.

Other cases of ecological analogues include the hovering hummingbirds and perching sunbirds; fruit-eating tanagers and bulbuls (the latter swallow fruits whole, while the former remove the seeds before swallowing); and frugivorous bats, which are represented in the Old World by non-echolocating pteropodids but have emerged from an entirely different clade (Phyllostomidae) with wider-ranging diets in the New World. Corlett concedes that these are very difficult questions to which scientists have "quite spectacularly failed to answer". What little we know, however, points to the pressing fact that every rainforest region is simply unique and irreplaceable!

Neglected families: the oaks and babblers

Back in 1885, the explorer Henry O. Forbes wrote of the oaks "in full blossom that characterised the Rupit" region of Sumatra and plentiful clumps of oak along the Rawas river. The ecology of the Fagaceae is Corlett's 2nd example of a neglected sphere in rainforest research. Often associated with the temperate zones, oaks and chestnuts are in fact at their speciose in the tropics. "Singapore alone has got more species in this family than the whole of Europe," notes Corlett, adding that the Fagaceae in Borneo outnumber those in North America. He finds them an odd rainforest group - unlike the majority of other rainforest plant families (except for the Dipterocarps), the Fagaceae are ectomycorrhizal (in which fungal mycorrhizae form an external layer around the roots rather than penetrating the root wall). And like the Dipterocarps, the Fagaceae mast fruit after long intervals, although their fruit are markedly large, fleshy and mammal-dispersed. But in contrast to an unending deluge of new studies on the ecology of Dipterocarpaceae, the tropical Fagaceae are woefully overlooked, with but one paper every 3-4 years, "despite the fact that they are almost as diverse, have a similar distribution and in some ways similar ecology."

Babblers are another understudied group in Corlett's view. Bird that twitchers know as babblers, however, are being reclassified by systematic ornithologists, who now regard white-eyes, laughing-thrushes and Sylvia warblers as babblers, while rejecting the babbler status of old favourites such as the white-bellied yuhina (much of this work is being done by Alice Cibois of the American Museum of Natural History). Yet, babblers receive "very little attention" from ecologists, despite being a major component of Asian forest avifauna. "In the sub-tropical regions of Asia, babblers can make up 30-40% of the birds," declares Corlett. "You get many similar co-existing species - five different fulvettas in a forest in Vietnam, three different Minias…"

But little is known about babbler ecology, despite suggestions of peculiar breeding strategies and social structures. And despite their image as insectivores, Corlett has found that fruit is eaten by a significant number of species. "So they are probably very important seed dispersal agents in the rainforest understorey." Sadly, time is running out, as many babblers are completely dependent on forests. "If you clear your forest, most babblers stay in the little fragments and they don't occupy urban or agricultural habitats with a few exceptions," stated Corlett. A further complication is their popularity as cagebirds, which has resulted in vast numbers being released in places like Taiwan and Hong Kong (Corlett estimates about 50,000 a year for Hong Kong), with unknown ecological consequences.

Big, bad and (increasingly) extinct: forest megafauna in Asia

Long gone from Asia's rainforests are their unique megafauna. What remains are gaping questions about their role in forest ecology. About 200,000 years, Asia's rainforests harboured elephants, rhinoceroses, ungulates, deer, pigs, cats, bears and orangutans, as well as now extinct beasts such as stegodons (strange relatives of elephants), giant pangolins, giant tapirs and the largest known ape: Gigantopithecus blackii. "All these species could co-exist at one site," revealed Corlett. Stegodons survived into the Holocene, while the giant tapirs, pangolins and Gigantopithecus became extinct much earlier. The surviving megafauna, such as elephants, tiger and orangutans, also occupy but a small fraction of their original distribution in Asia ("there were elephants over all China just 5,000 years ago"), with many reference study sites such as Lambir and Pasoh in Malaysia having lost all their megafauna save deer and (a surfeit of) pigs thanks to human impact ("Gigantopithecus would have been extremely easy to kill…")

For Corlett, this loss entails vast ignorance about the ecological function of megafauna, especially in areas such as seed dispersal. Some mangos, for instance, are almost entirely dependent on elephants. Large browsers such as elephants and rhinos are also major natural disturbance agents, clearing fresh trails and patches in the understorey. As a result, their disappearance mean that "tropical forests today must function in a very different way from the tropical forests which had all these megafauna," and Corlett wonders how on earth could this broken ecological chain be studied ("maybe we can make models of stegodons and push them through the forest," he suggests in jest).

Forest invasions

The next question involves invasive species. Corlett's surmises that whereas continental rainforests are resistant to invasive plants ("Singapore is full of exotic species, and yet only one exotic plant [Clidemia hirta] has got into Bukit Timah") but this is not the case for animals. In both Singapore and Hong Kong, exotic fauna, in particular birds, have established themselves in the forests. It may be that for some birds, it doesn't matter what species of tree there are as long as they are structurally similar to those in its native region. And as Corlett stressed, the subject is "not something you really want to experiment on," other than imaginary scenarios, considering the ecological implications. Nonetheless, it implies for Corlett a frustrating inability to study and understand the dynamics of forest invasives.

Richard Corlett is a researcher at the University of Hong Kong who has spent a quarter century in tropical East Asia. In a talk at the National University of Singapore last January, Corlett outlined what he felt people ought to know about Asian forests. It is clear that we still know dreadfully little of these immensely rich ecosystems.