Few people alive today know what is a stegodon. But it’s highly likely that the first generations of humans who arrived in East and Southeast Asia were quite familiar with these elephant-like creatures with bizarre tusks that ran parallel closely together towards the ground before curving upwards like a pair of long horns. On the continent, these pachyderms ranged from India to China. But when northern glaciers locked up enough seawater to turn Sundaland into a shallow shelf of immense river valleys and lowland rainforests, stegodons and other large land animals were able to walk or wade all the way to Luzon in the Philippines and Sulawesi in Indonesia. The returning seas trapped these footloose wanderers on islands such as Flores and Timor, where giants shrank over time to become jumbos the size of cows.
Between 40,000 and 50,000 years ago, modern hominids made their appearance in this region. The arrival of Homo in tropical East Asia coincided with a massive decline of large terrestrial mammals and the eventual disappearance of several species and even entire groups with no living remnants. Gigantopithecus was a massive ape that roamed East Asian forests in the same period as Homo erectus and now survives primarily in the imaginations of cryptozoologists and dreams of hopeful abominable snowmonsters. The stegodons survived for much longer, with the last known specimens dating from just 4,100 years before present from Yunnan in China, while dwarf species are known from Flores as recently as 12,000 years ago.
Elephants proper (Elephas sp.) coped better with the ascent of man, but with the rise of the Middle Kingdom, their habitats were cleared and hides so hunted (roasted trunk was a popular treat in ancient China) that a creature that once roamed as far north as Beijing now clings to a tenuous existence in the forests that border Myanmar. Elsewhere, Asian elephants rule over a fraction of their former range as expanding agri-industry and settlements force herds to find new homes in the hills or go head-on with humans to deadly effect.
Rhinos fared far worse, as East Asia’s two forest-dependent species teeter at the brink of extinction. Southern China had such high densities of Javan rhinos a few thousand years ago that their armour shielded the infantry of Chinese armies. Even in colonial times, casual big game hunters in the East Indies were able to boast of shooting six in a day. Today, no rhinos survive in China, whose medicinemen now cast their nets in Africa and other parts of Asia to satisfy their hunger for horny therapies. Just 300 or so Sumatran rhinos remain while Javan rhinos number in the mere dozens.
People go bananas over the old man of the forest today, but in past times, orang utans found favour with sympatric humans by providing easy targets for spears. Their bones litter the caves of Niah and older remains are recorded from Java, Southern China and northern Thailand. The great apes now swing only from shrinking old growth jungles in Sumatra and Borneo, where our insatiable appetite for energy and disregard of the planet’s carrying capacity fuel the furious fall of their forests.
Other megafauna that have long vanished from Asian rainforests include giant tapirs and giant pangolins. Surviving species, from rhinos and elephants to bears, tigers and apes, occupy slivers of rainforest across much of their historic range and struggle against poachers, plantations and logger barons that encroach with impunity even on protected parks and reserves.
It’s hard to imagine today that barely a century ago, animals like tigers and rhinos were so common in the region that they were regarded as agricultural pests and vermin. Many were trapped and hunted to near oblivion for sport or supper, but it’s telling that the rapid turn to near-extinction of oriental megafauna accompanied a period when the population of tropical East Asia enjoyed an exponential boost from a mere 100 million or so (for some 2,000 years) to nearly a billion in less than 200 years.
That said, it’s not all gloom and doom, yet. The world has been made far safer for children who no longer have to worry about big cats and horny beasts lurking behind every bush. Instead, they just have to deal with a future of environmental degradation, wilder weather patterns and intensified competition and conflict for shrinking land and marine resources. After all, it’s simply a matter of paying the price to replace ecosystem services lost by the extinction of species and destruction of habitats for progress and profit.
Seedy relationships and the future of Asian rainforests
But as Richard Corlett stressed in his welcome-back seminar on The Science of Seed Dispersal at the National University of Singapore, substantial chunks of tropical rainforest still persist in the region, albeit covering barely half the area they occupied a century ago. And apart from aberrant sites like Belum-Temenggor and Taman Negara, the modern-day visitor to these woods will appreciate a near-total freedom of fear from big and potentially dangerous animals save the odd boar or barking deer. In fact, the largest and deadliest creatures under the canopy are likely to be intrepid invaders from the urban jungle armed with bug repellent and an indefatigable spirit of reckless adventure.
Corlett had raised the question in a presentation last year, but this time he returned to the issue at length, wondering aloud what might be the long term fate and composition of Asian rainforests following the widespread extirpation of their megafauna in decades within living memory. “I have walked in forests in Southern China, where fruit just falls out of the trees and rots on the ground, because there’s no animals left to disperse them,” he recounted of his own observation.
Dipterocarps, the giant trees with coveted timber that form the predominant family in Asian forests, disperse via winged fruit that take flight with the wind. Despite their ubiquity, dipterocarps enjoy a high degree of ecological immunity from megafauna as their leaves are largely inedible to vertebrates and they seed at intervals too long to serve as primary food sources for frugivores. Other tree families have lives that are more closely linked with the beasts that prowl the forest floor. Fagaceae, Moraceae, Lauraceae, Myristicaceae, Annonaceae, Anacardiaceae, Clusiaceae, Rubiaceae, Bombacaceae and Sapotaceae are unfamiliar names whose members yield favourite fruits such as acorns, chestnuts, figs, jackfruit, rambutan, mango, durian, rambutan, mangosteen, chiku, guava, jambu, nutmeg and coffee as well as other species that depend on the appetites of forest creatures to spread their seeds to new locations.
Corlett likened the complexity of wind-dispersed seeds to rocket science, with physics that are “much more complex than a jet fighter.” Dipterocarp fruit, for instance, have high wing loading that causes them to “spin to the ground within a few metres of the parent tree”. But Corlett suspects this is only half the tale, as the family has managed to cross considerable water barriers to reach Papua and the distant Philippine islands.
To further complicate things, the study of seed dispersal gets much more complicated “as soon as you add animals into the equation.” Take a dispersal agent such as the frugivorous coppersmith barbet. “The plant has to attract the bird, reward it so that it comes back again, and the seeds have to survive passage through the gut, which in a small bird like that takes about 20 minutes,” noted Corlett. Not all birds are created equal though. Ducks, for instance, are utterly useless as seed dispersers, unlike imperial pigeons, virtually extinct in Singapore, which can swallow several large fruit whole and pass out the seeds through the gut. Similarly, hornbills consume large fruits although the seeds are regurgitated rather than excreted as droppings. For trees like nutmegs and palms that succeed in drawing such birds, the payoff comes in the dispersal of their seeds over vast distances.
With mammals, Corlett pointed out an additional complication, in teeth. “A bird couldn’t eat longans because it can’t remove the outer peel, but fruit bats can,” he noted. “They hang upside down in the tree, hold the fruit on one foot and cut the peel off with their teeth and drop it under the tree – no bird can do that.” The largest flying foxes (Pteropus sp.) can cover 50 km in a single night, while smaller fruit bats occupy more modest territories. Even handier are the primates, such as gibbons, who would peel a citrus the way humans do and gulp down the rind. After 24 hours or so, the seeds emerge with the droppings in hopefully another part of the forest.
Macaques, however, spit rather than swallow. “Feed an orange to a macaque and it will peel the orange but it won’t swallow the seeds,” remarked Corlett. “And this doesn’t make for very good seed dispersal because the seeds tend to fall under the parent tree.” Qualifying that though, he adds that macaques have cheek pouches in which they stuff food, so “there is some seed dispersal” as the animals move through the forest and spit out seeds after a snack attack. “And since these are the only primates in Singapore [save the virtually extinct leaf monkey], they are important here,” noted Corlett.
Except for cats (and the probably rural legend of tigers and durians), most carnivores are not averse to fruit. Masked palm civets, a relative of the local toddycat, “eat a lot of fruits particularly in winter in Central and Southern China,” said Corlett, showing a slide of civet scat on a rock in Hong Kong. Sun bears too are highly frugivorous, with some populations heavily reliant on the fruiting cycles of forest figs. “Carnivores are good seed dispersal agents,” stressed Corlett, “because they don’t damage seeds – basically their jaws just go up and down and the seeds pass through undamaged.”
In comparison, terrestrial herbivores like deer “tend to damage or destroy a lot of seeds” through their vigorous chewing. Some trees, however, have evolved protection against this by having fruit with very hard seeds that deer swallow whole. While chewing their cud, the animals bring up the fruit into the mouth, “chew off the flesh and drop the seeds in little piles on the ground.”
With giants like elephants and rhinos, there’s no subtlety. “They just swallow the whole lot down and the seeds pass through in one to four days,” stated Corlett. “This means that seeds can be dispersed for 10 km or more inside an elephant.” Some seeds are adapted to survive this rigorous passage and Corlett shows a slide of elephant dung at Khao Yai National Park in Thailand with mango seedlings germinating from them. “Elephants and rhinos are probably the major dispersal agents for mangos,” he said.
On the other end of the scale, some seeds are dispersed by insects. Globba, a genus of small gingers, “have a lipid-rich aril which acts as an attractor for ants, which disperse the seeds a few metres away.” Rats and mice, of which there are dozens of forest-dwelling species besides the familiar urban pests, also consume great numbers of seeds, destroying many in the process but burying their surplus in caches that stand a chance of germination.
Some animal that eat fruit do the tree no favours. ‘Bad guys’ include squirrels, green pigeons, parrots and pigs, which “destroy all or most seeds in fruits they eat,” although Corlett adds that he is keen to do experiments with captive green pigeons “to see the extent to which this is true.”
A science lost and found
The study of seed dispersal was a popular discipline amongst late 19th and early 20th century biologists. These included a certain Charles Darwin, who undertook experiments such as feeding seeds to birds, killing the poor fowl and floating their carcasses on seawater for weeks to determine if the seeds would still be viable. He also fed seeds to fish which were then fed to herons to learn if the seeds could survive passage through two vectors, according to Corlett, who did not say whether the herons were sacrificed in the process.
Singapore’s very own Henry N. Ridley wrote a classic study on The dispersal of plants throughout the world in 1930, which Corlett highly recommends. Besides animal agents, Ridley also discussed at length the role of the orang asli, observing their practice of spitting out seeds as well as uncovering seeds in human droppings. “Apparently, on forest fruits where the flesh is strongly attached to the seed, the orang asli swallow them whole the same way as gibbons and orang utans, stated Corlett dryly, “It’s an adaptation on the part of the plant to make sure that the seed is swallowed.”
After Ridley, however, ecologists came to regard seed dispersal as being “too idiosyncratic and unpredictable to be a science”, leading to about 40 years of neglect. It got so bad that when Corlett was an undergraduate in botany, seed dispersal was never mentioned at all. “It was a general belief that given time, seeds would be dispersed to anywhere, so seed dispersal didn’t really matter,” he recounted, “What determined whether a plant grew in a particular spot is not whether the seed got there but whether the environment was suitable.”
What’s changed since then? asked Corlett. He believes this occurred as ecologists unravelled the biodiversity of tropical forests and sought to explain what drives this wealth of species. In Lambir Hills National Park in Sarawak, the Smithsonian Tropical Research Institute maintains a half-square-km plot in which every tree is accounted for. “This half-square kilometre has 1,175 tree species,” noted Corlett. “That’s more tree species than in North America, more tree species than in Europe, more than Central and Northern China – it’s more tree species than the entire temperate forest of the northern hemisphere!”
Not only are tropical rainforests incredibly diverse, they contain many very similar species. According to classical ecological theory, complete competitors cannot coexist in a shared habitat – one would drive the other to extinction, unless one or both species adapt to occupy slightly different ecological niches. Thus, each of the over thousand species at Lambir must have a unique set of adaptations to grow, survive and disperse their seeds. In a paper published in Science in 2000, Condit et al found that many trees species in Lambir as well as six other rainforest plots in Asia and Central America had distributions that indicated some form of dispersal limitation. “These species occur in circular clumps that do not correspond with topography,” wrote Condit, who pointed out a negative correlation between dispersal effectiveness and tree aggregation of a single species. Dipterocarps (which are wind-dispersed) were noted to be more aggregated than non-dipterocarps.
One of the first serious attempts to explain this diversity was a paper in 1970 by D.H. Janzen, with whom Corlett shares a dubious parental connection. Janzen saw that for any given species, regardless of whether it was wind- or animal-dispersed, the number of seeds that land at any point in the forest declines with distance from the parent tree. At the same time, seeds that fall right under the parent tree or nearby have a very low chance of survival due to intraspecific competition and predation by herbivores. “Also, pests and diseases can spread from the parent tree to seedlings of the same species,” added Corlett.
These two processes allow the plotting of a chart that indicates seed dispersal and seed survival in relation to distance from the parent tree. Thus, each tree species would have a surrounding zone in which none of its seeds survive, as well as an outer limit beyond which no seeds get dispersed at all. This resulting spatial structure “leaves space for the other trees,” remarked Corlett. In short, the hypothesis known as the Janzen-Connell Effect (Joseph Connell made a similar suggestion independently) explains the species diversity of rainforests by allowing for the coexistence of numerous similar species of trees “as long as they have different natural enemies and as long as the natural enemies are efficient enough to keep each species at a low density, so no one species can take over.”
Seeding the forest for the trees
From a non-subject after Ridley, seed dispersal enjoyed a resurgence in ecological studies in the 38 years since Janzen’s seminal paper. Numerous researchers have formulated plausible explanations for the diversity of tree species in tropical rainforests, “and almost all of them include something to do with seed dispersal,” said Corlett, adding that several experimental studies using seed traps disproved the old idea that “seeds eventually get everywhere.” Corlett noted, “Seed dispersal is extremely limited; you can run hundreds of seed traps for tens of years and most seeds don’t reach most of the forest.” Dispersal limitation has therefore become a core factor to account for the co-existence of a huge number of tree species in rainforests.
Dispersal limitation offers an explanation for why a particular species fails to grow at a certain site where it should in theory do well, while another species for which the site is not-optimum occurs in the same place. Stephen Hubbell, however, has made dispersion limitation not just a factor but the core determinant in his Unified Neutral Theory of Biodiversity and Biogeography. According to Corlett, Hubbell “took this idea of similar species being able to coexist to its extreme and said, ‘What if all species were ecologically equivalent but dispersally limited?’ Then he modelled what the consequences of this were and showed that with dispersal limitation, large numbers of species can coexist for very long periods, and as long as you have got some process to replace species which are eliminated by competition, numerous species can coexist.”
Besides helping to explain tree diversity in tropical forests, seed dispersal serves a vital role in maintaining gene flow in isolated populations. “Molecular studies in the tropics have shown that in general, gene flow through seed dispersal is of a similar magnitude to gene flow in pollen,” stated Corlett. Thus, tree species that rely heavily on animal dispersal agents may suffer from fragmented and shrinking gene pools when the animals that transmit seeds between populations are extirpated or unable to traverse between forest patches due to land clearing or other man-made barriers.
In Singapore, seed dispersal and its limitations may be crucial to the survival of existing nature reserves, where numerous mammalian and avian species have been lost in the last century. Certain trees in Bukit Timah, for instance, while extant, might be members of the living dead as there are no suitable animals left to disperse their seeds. Should a species become extinct, the only hope of re-establishing it naturally is “if an imperial pigeon brings a seed from Johor,” adds Corlett wryly.
Another side effect of seed dispersal systems that Corlett revealed is the way invasive plants have co-opt native animals to serve as their dispersal agents. “There are three American plants now established in Singapore, which are dispersed by bats in the Neotropics,” he said. “But they are dispersed by birds and bats here, despite the fact that our bats are totally unrelated to the fruit bats in the Americas.”
So whether you are a native tree bereft of bosom seed buddies or an alien species needing a partner in crime, it matters what animals are around to disperse your seeds. In the degraded woods of Hong Kong, for instance, a student of Corlett’s, Jackie Weir, tracked the movements of birds using radio telemetry and measured seed residence time in their guts. The hwamei (a species of laughing thrush) and two species of bulbuls were compared and Weir found that whilst the former remained on average no more than 50 m from their feeding territories during a 10 minute gut passage time, bulbuls wandered about twice as far, thus functioning as effective seed dispersers for trees in Hong Kong’s fragmented landscape.
Trouble in the jungle?
A considerable amount of data has been gathered for smaller birds and fruit bats. But in the light of the field’s late resurgence, there is still much ground to cover, notably for seed dispersers that are now so scarce that one wonders if their forests will outlive the memory of their extinction. For the elephants, rhinos, tapirs, hornbills and flying foxes may be gone, but the trees that depend on their far-flung foraging could well survive for centuries like Ents before they finally fall to signal the loss of both individuals and their entire lineage.
So while Corlett stressed that “there’s still quite a lot of forest left in the region,” what has happened is that the “vast majority of that forest has lost most or all of its large vertebrates.” What’s more, many of these forests are in a state of fragmentation, which hinders the movement of potential dispersal agents into them from forests that do harbour the animals.
“I am trying to work what proportion of trees in the forest depend on these large vulnerable vertebrates,” said Corlett of his research agenda. “The forest is still there, but these species are hunted out” and the question is whether these trees will survive the passing of their partners. In Thailand’s Khao Yai National Park, a three-year study noted a negative relationship between seed size and the number of frugivores that feed on a fruit. Larger, single-seeded fruit were not consumed by smaller frugivores that still thrive in degraded forests, echoing Corlett’s own observation of barren South China forests that fruit to no avail.
Based on available studies and his own data, Corlett hazards a preliminary estimate that 15-25% of tree species in the region are “either solely or largely dispersed by larger vertebrates.” In the latter group, he also ponders the effect on forest structure and composition when trees that used to be dispersed over long distances have to rely solely on small rodents or birds that forage not too far from the parent tree. “If you remember the Janzen-Connell effect, it made survival very much less is dispersal is over a short distance.”
Turning to Janzen again, Corlett highlights a 1982 Science paper in which Janzen postulates a link between plant species distribution trends and the extinction of gomphotheres (giant mastodon-like animals), horses, ground sloths and other megafauna in Central America. About 12,000-10,000 years ago, when modern humans reached the continent, these animals vanished, and according to Corlett, Janzen argued that “as a consequence of this, species that produce fruits dispersed by gomphotheres and other now extinct giant vertebrates have suffered.” Some trees have found new ecological equivalents in introduced horses and cattle, but many experience dwindling populations due to ineffective dispersal.
“What happened 11,000 years ago in Central America is still happening now as we speak in Southeast Asia,” reminded Corlet. The obvious question, therefore, is “Will protecting these forests without their large vertebrates be enough? Can the forests survive without their megafauna?”
At this point of time, “We really don’t know,” acknowledged Corlett, who plans to investigate this question with studies in the few remaining sites where terrestrial megafauna are still present in near natural densities: parts of Taman Negara, Huai Kha Kheng in Thailand’s western forest complex and Danum Valley in Sabah. “It’s too late to look at forest rhinos though,” he added. “Nowhere has a natural density of Sumatran rhinos.”
These ‘pristine’ sites could be compared with areas which have lost most of their megafauna such as Pasoh Forest Reserve in Negeri Sembilan or Lambir Hills in Sarawak. As detailed ecological data is available, “we could identify tree species which we believe depend on megafauna, look at these species in Pasoh where there’s 18 years of demographic records and see if the effects of the loss of megafauna show up in the distribution of tree species.”
With some parts of the region, it’s too late to tell as the effect has probably been erased by centuries of new equilibriums. “There’s areas like Sulawesi which had elephants and stegodons in the Pleistocene,” noted Corlett. “Luzon had elephants, stegodons and rhinos.” Singapore probably did not have any permanent elephant or rhino populations, but Corlett said that in Bukit Timah, which has lost nearly half of its mammal, bird and amphibians species, “it’s clear that there are things which are not being currently dispersed.” Formerly cultivated plots in the Central Catchment Reserve are also “dominated by small-fruited, small-seeded species dispersed by small passerine birds and fruit bats – on these sites none of the big fruits have come in.”
In the Smithsonian’s study plot in Bukit Timah, trees in the nutmeg family (Myristicaceae) “show evidence of lacking a dispersal agent,” as hornbills vanished from the site more than a century ago. “Now we have hornbills coming back, and it’d be interesting to see if there’s a signal from the loss of hornbills and a signal from their recovery,” said Corlett, although he questions whether newly re-established oriental pied hornbills could tackle the larger fruits that were handled by rhinoceros hornbills.
These impact studies are vital where it to decisions whether to reintroduce megafauna into a forest that has lost its species. “We know of 11,00 domestic elephants in the region, more than in the wild,” stated Corlett. “We could re-establish elephants, though I am not sure it would work in Bukit Timah…” Sambar deer are a more plausible option, he opined. For all our ingenuity, it seems the hardest task for humans is to make space for even a fraction of the great and grand creatures that share this planet. Their loss, which seems eminent with each day, would cost little to those few who can afford to survive without nature’s bounty and merely mar the memory of man as one who suffers no restraint in his pursuit of profit, pleasure and populations to the tipping point of calamity.
proof has been unearthed that man and dinasour did walk the earth together.
Posted by: Dr.Q | 18 September 2008 at 01:46 AM
Dr.Q: A likely tale indeed. Pull the other one. Where is this "proof" you speak of?
As an aside, nice summary of the talk. I'd wanted to do a post of my own, but figured that you had all the necessary materials and notes. =P
Posted by: Hai~Ren | 18 September 2008 at 04:30 PM
don't put to much info or people won't come to your website and make the site a little more decorative because it looks like a granny grandpa site BORING!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! YOU GUYS SUCK I HATE THIS SITE LUV YA ANONOMYS
Posted by: ANONOMYS | 28 March 2009 at 04:53 PM