March 2003

Vietnam’s Secret Life

Naturalists exploring the country’s mountains and
forests are finding that the keys to its extraordinary
biodiversity may lie deep in the past.

By Eleanor J. Sterling, Martha M. Hurley, and Raoul H. Bain

The Sichuan whipping frog has been assigned to the species Polypedates dugritei—but herpetologists are now realizing that these frogs actually form a group of species, not just one. The several species probably arose when climate change stranded ancestral frog populations on separate mountaintops; the confusion for zoology arose when the climate changed again, first warming and then cooling, enabling the new (but similar looking) frog species to disperse before they were isolated once more. Photo by Raoul H. Bain

Along Vietnam’s border with Laos runs the Truong Son range, known to the Laotians as Saiphou Louang and to much of the rest of the world as the Annamites. But the mountains are becoming known—to conservation biologists as well as to everyone else concerned with preserving the world’s species—as a region of exceptional biodiversity. In the early 1990s investigators began visiting Vietnam’s natural areas in greater numbers than at any time since the beginning of what is known to the people of the region as the Second Indochina War. And the investigators—ecologists, evolutionary biologists, and specialists in a broad spectrum of life-forms—soon confirmed what the local peoples had long known: an astounding array of organisms dwell in the country. For many biologists to this day, entering Vietnam is like entering uncharted territory, an area of vast biological abundance, where new species, it seems, can turn up virtually anywhere you look.

Biologists exploring the Truong Son have discovered—or, importantly, rediscovered—three previously unrecognized species of muntjac, or barking deer; one species of pig; and one species of rabbit. Those findings alone are remarkable; after hundreds of years of systematic biology, who would have thought that large or medium-size mammals would remain to be described? And that list doesn’t even include the saola, the sole member of Pseudoryx, a genus entirely new to the cattle family. Weighing in at about 220 pounds,

After hundreds of years of systematic biology, who would have thought that large or medium-size mammals would remain to be described?

the saola is the largest land-dwelling mammal introduced to science since the kouprey, or gray ox, was described in 1937. (That animal ranged through northern Cambodia and adjacent areas of Thailand, Laos, and Vietnam, but may now be extinct.)

But Vietnam promises more to biologists than just the windfall that is the Truong Son range. Ever since the mid-nineteenth century, up until the beginning of the Second World War, forays by Vietnamese and visiting naturalists had sketched a spotty but telling portrait of the country’s biodiversity. More recently, since peace came to Vietnam, further hints of biological abundance have come from collaborations between Vietnamese and foreign investigators.

But only in the past ten years have biologists understood that the newly recognized charismatic megafauna are only the tip of an iceberg of heretofore unknown species that live in the Truong Son as well as in other, primarily montane, areas of Vietnam. Among the organisms new to science (though, again, not to natives of the area) are three species of birds, nineteen species of amphibians, sixteen species of reptiles, and, just since the year 2000, at least twenty-nine species of fish and 516 species of invertebrates. And—perhaps just as intriguing—many of the species native to Vietnam do not occur anywhere else: a phenomenon known as endemism.

The ferment of scientific activity in Vietnam in the past decade is a result of several historical developments: the restoration of political stability after decades of war; the recent opening of strategic border areas to scientists; and the reopening of the country to foreign scientific collaborators, such as our group from New York’s American Museum of Natural History. Of course, Vietnam’s turbulent political history can only explain why so many discoveries are emerging just now. History and politics (aside from the destruction they wreak) have little to say about the country’s biodiversity—particularly about why so much of that biodiversity is endemic to it.

The heaving of mountains, the shifting courses of rivers, and the expansion and contraction of seas and forests have successively isolated and reunited populations of plants and animals.

The real roots of the region’s biodiversity lie in the dynamic interplay over time of geographic, geological, and climatic forces. The heaving of mountains, the shifting courses of rivers, and the expansion and contraction of seas and forests have successively isolated and reunited populations of plants and animals. As new habitats arise and old ones shift, existing organisms can disperse, adapt, or die. Those three options have largely created the unusually complex mosaic of life that exists in the region today.

Mountains and hills wrinkle the vast majority of Vietnam’s 127,000 square miles. Major mountain blocks include the highlands in the northeast, the Hoang Lien Son in the northwest (the southeasternmost extension of the Himalaya), and the Truong Son along the border with Laos. A range of forest habitats, each adapted to a different amount of yearly rainfall, blanket the slopes of these mountains.

The seasonality of the rainfall is, in part, a consequence of the monsoon circulation pattern, the dominant climatic feature of southern and eastern Asia for at least the past seven million years. In the winter, strong northeast monsoon winds blow, as air flows from cold, high-pressure areas in Asia along the eastern edge of the Tibetan Plateau toward a hot, low-pressure zone over Australia—a process that brings cold, dry winds to Vietnam. In the summer, air masses move in from the opposite direction, from Australia and the Indian Ocean; passing over Vietnam the air releases moisture picked up along the way, hence the country’s summer rains. Those dynamic circulation patterns interact with the terrain and the surrounding ocean to expose Vietnam to widely varying amounts of rainfall.

Temperature and humidity vary with topography; in general, the higher the elevation, the cooler and wetter the climate. The interaction of upland areas with moisture-laden moving air masses creates a “rain shadow”: windward slopes wring most of the moisture from the clouds and remain substantially wetter than the leeward slopes. In the Truong Son, for example, the coastal-facing eastern slopes are wetter than those facing west.

Meanwhile, a variety of climates dot Vietnam’s interior. Regions without prolonged dry periods support moist evergreen forests; seasonally dry forests—mixtures of evergreen and deciduous trees—grow in areas of more mixed dry and wet periods; and dry deciduous forests range across southwestern regions with expanded dry seasons. In addition, Vietnam also hosts ecosystems as diverse as temperate coniferous forests, mangrove forests, grasslands, and coral reefs.

Geologically speaking, Southeast Asia lies at the interface of three converging continental plates: the Eurasian, Indo-Australian, and Philippine Sea. Continental plates, which are formed from Earth’s rigid, brittle lithosphere, or crust, move gradually across the planet, buoyed by movements from below, in the Earth’s mantle. Sometimes the plates break into pieces; an aggregation of continental fragments broke off from the prehistoric supercontinent called Gondwanaland about 400 million years ago. Over the course of the ensuing 200 million years, some pieces of the broken continent migrated north to collide and fuse with Asia at higher latitudes, in the process creating much of what is now Vietnam. The Truong Son range arose during collisions that took place between the late Paleozoic and early Mesozoic Eras, between 340 million and 255 million years ago.

Later, between 55 million and 40 million years ago, what is now the subcontinent of India broke off from Gondwanaland and migrated north, colliding with Eurasia. The fusion of the two landmasses led to a major influx of new species, which dispersed through India and into Southeast Asia. As the collision of India with Asia continued—and it continues to this day—the rising Himalaya and the Tibetan Plateau essentially isolated Southeast Asia from invasions by species from the north and west.

While the Himalaya were rising, Earth’s climate began to fluctuate between cool and warm phases. Continental glaciers formed and retreated and, in response, sea levels fell and rose. When sea levels fell, the shallow Sunda continental shelf became exposed (today it lies beneath the seas south of Vietnam). The Sunda shelf linked landmasses that are now separated, forming bridges that joined mainland Southeast Asia to the Sunda Islands—including Borneo, Java, and Sumatra. A mixture of rainforest and grassland, woodland and sedge blanketed the emergent land.

During such glacier-forming cold periods, when much of the Gulf of Thailand and parts of the South China Sea disappeared, the monsoons picked up substantially less moisture than they do today. Seasons became more distinct. Forests previously limited to mountain elevations descended to lower levels, and grasslands proliferated, pushing out lowland evergreen rainforests. The most recent cold period took place about 18,000 years ago, the climax of the last ice age.

In contrast, during the warmer periods between glacial advances the climate became wetter and less seasonal, and the evergreen rainforests expanded, reaching into the higher elevations and latitudes. Sea levels rose, covering the continental shelves and land bridges, splitting up and isolating populations and individual species. Those processes have continued throughout the Cenozoic Era up until what is, geologically speaking, the present day. In Southeast Asia, long-term cycles of isolation and recolonization have been the evolutionary norm.

Those cycles fueled the rise of new species, led to the extinction of others, and, in general, determined the distribution of the present-day flora and fauna of Southeast Asia. Thus the geological and climatic history of the region can serve biologists as a kind of Rosetta stone, helping them to decode and disentangle the patterns of Vietnam’s biodiversity as well as to pinpoint where new species might be found.

Consider, for example, some of the distribution patterns of Vietnamese amphibians and reptiles. Northeastern Vietnam shares more than twenty of these species with China’s Hainan Island and Guangxi autonomous region. China’s Yunnan Province, however, which borders both Vietnam and Guangxi, shares none of them. Sea-level changes, in concert with climate, help explain the patterns. Low sea levels during the Pleistocene Epoch, beginning 1.8 million years ago, enabled flora and fauna to migrate back and forth from the mainland to Hainan Island. But the drier, cooler climate of Yunnan Province limited the westward dispersal of at least some of the species.

Sea levels far lower than they are today also enabled amphibians and reptiles to travel freely back and forth between the Greater Sunda Islands and mainland Southeast Asia. As a result, the two areas share roughly a fifth of their amphibian and reptile species. In fact, investigators comparing present-day species distributions with measurements of contemporary ocean depths can demonstrate how low sea levels must have fallen during some geological periods. Moving across the sea would have been hard for reptiles and practically impossible for amphibians, whose permeable skin does not tolerate saltwater.

Inland, away from the seas, climate and geography can also play a role in the rise of new species as well as in endemism. Here, however, Vietnam’s climatic cycles drove organisms up and down mountains rather than back and forth across land bridges that were later flooded.

For example, when the climate warmed, montane forest communities contracted and moved up the mountain slopes, where the climate was relatively cool. Those contractions could keep forest populations living on separate slopes isolated for substantial periods of time—in some cases, long enough to differentiate into distinct species. Later, when the climate cooled again and the forests expanded downslope, those now-distinct species could disperse throughout the lowlands. If further warming phases pushed the new species back up to high elevations, many might end up populating new, or more widespread, areas than the ones where they originated. The effect, of course, is virtually identical to that of rising and falling sea levels, which led to island endemism. But the mixing of evolutionary lineages and the resultant diversity means zoologists are faced with a tangle that they are only now sorting out.

Bird endemism also seems to follow such a tangled pattern. Montane forests above about 3,000 feet in the Truong Son are rich in songbird diversity, notably in the flycatchers and Old World warblers and related species. Two of the three new bird species described in the central Truong Son (the black-crowned barwing, the golden-winged laughingthrush, and the chestnut-eared laughingthrush) are endemic to the area. Biologists have recognized eleven endemic subspecies of babblers there as well. These discoveries have led the conservation organization Bird-Life International to designate this region of the Truong Son a high priority for bird conservation.

The patterns of endemism and speciation observed in Vietnam are set in motion when the separated populations become trapped by some kind of barrier that develops between them. But that barrier need not be elevation, or a land bridge that has been flooded. In the Truong Son another kind of barrier has arisen, in the form of a moist refuge surrounded by a drier environment.

As we noted earlier, the Truong Son act as a barrier to the monsoon winds, giving rise to high rainfall and a reduced dry season on the eastern slopes. As a result, moist evergreen forests have grown historically at all elevations to the east of the main ridge (though today much of the land at lower elevations is deforested). In contrast, on the western slopes drier semi-evergreen forests are more common.

The moist evergreen forest may have been able to persist on the eastern slopes of the Truong Son despite increasingly dramatic climate fluctuations during the past three million years. Those tropical rainforests may have provided a refuge for forest-dependent species during colder, drier, more distinctly seasonal periods. Such an extra dimension, along which habitat may have expanded or shrunk with time, could help explain why, even in such an Eden of biodiversity as Vietnam, the Truong Son range outshines the rest of the country. (Indeed, in addition to the species we mentioned earlier, the rainforests of the Truong Son harbor about a dozen endemic species of frogs.)

Unfortunately, some patterns of endemism are not so easily explained. Vietnam is host to several principal kinds of primates: gibbons (lesser apes), langurs and macaques (which are both monkeys), and lorises (which are prosimians). The ranges of many of those animals are clearly restricted, yet strangely, for many of them, no discernible geographic barrier has been identified.

Several but not all of Vietnam’s primate species live only east of the Mekong River, even though there is no apparent reason that they could not raft across.

Several but not all of Vietnam’s primate species live only east of the Mekong River, even though there is no apparent reason that they could not raft across. Even more strangely, the ranges of all but two of Vietnam’s primates—the lesser slow loris and the bear macaque—are restricted to either the north or the south of the country. A 195-mile-wide transition zone, between 14 and 17 degrees north latitude, serves as a barrier between north and south, but no climatic or geographic reasons have been identified for the existence of this barrier.

Ecologists, confronted with such a checkerboard distribution, have speculated that competition for food or other resources might limit the expansion of one species into another one’s range. Although that explanation sounds plausible, it is undermined by the observation that species such as the Assamese macaque and the pigtailed macaque, whose ranges do not overlap in Vietnam, live side by side in Laos and China. Interspecific competition cannot be the only answer.

Other investigators have speculated that glaciation during the Pleistocene Epoch—accompanied by colder temperatures, depressed rainfall, and increased seasonality—might have forced primates into ecological refuges. Species that ended up in the northern areas were presumably better able to manage the cooler weather year-round than were the species that favored the south.

How well do the available data support that hypothesis? The answer is not yet clear. A combination of competition among species for available resources and climatic factors were probably at work, but more study needs to be done before biologists have a solid picture of primate evolution in Vietnam. Biodiversity, it seems, is not merely a result of geographical and climatic obstacles and effects. The irony is that, although geographic ranges are better known for primates than for any other group of organisms in the region, those ranges are perhaps the ones biologists are least able to explain.

In seeking to understand the origins of Vietnam’s biodiversity, biologists have to be wary of snap judgments. Particularly for species identified only in the past decade, it is virtually impossible to determine whether their ranges are restricted by long-term geological or climatic factors, or simply because of habitat loss or degradation. Some species may be “bastard endemics”—occupying only a subset of a formerly larger geographic range—simply because they have retreated, say, to mountaintop islands in a sea of cultivated or seriously degraded land.

Acknowledging the ability of environmental degradation to create endemism brings a practical urgency to the theoretical study of the factors that create endemic species. In short, the study of endemism is not just a pursuit for evolutionary biologists; it is an issue for conservationists and environmentalists to consider as well.

Vietnam has a relatively long history of seeking to redress environmental degradation. As far back as 1962, Ho Chi Minh, the revolutionary general and president of North Vietnam from 1945 until 1969, established the country’s first protected area, Cuc Phuong National Park [see “This Land,” by Nguyen Thi Dao, page 70]. By 1990 more than ninety reserves, covering some 4 percent of the country (or about 3.2 million acres), had been placed under government protection. The Vietnamese government still has plans to roughly double that protected area, but in the region’s second most populous nation, the demands of economic development must compete for land with efforts to conserve biodiversity.

In that context, the study of endemism can help governments and others set priorities about what to protect. Certainly it makes sense to determine those priorities, as conservationists have done in the past, on the basis of which species are endemic. After all, if a local population of a widespread species were to go extinct, at least there would still be other populations in the world, but endemic species have much more restricted ranges. The loss of a single population, or a couple of populations of such species, could easily lead to the species’ extinction.

The trick, then, is to identify areas of endemism preemptively, without waiting for some remarkable discovery to send everyone scrambling to save a rare and obviously threatened organism. By applying the study of endemism, rooted in the slow passage of deep time, to the problem of conservation—which strives to match the double-time pace of economic development—we might be able to protect areas before they’ve been thoroughly explored, perhaps leaving some of Vietnam’s diversity for the future to discover.

Conservation biologist Eleanor A. Sterling is the director of the Center for Biodiversity and Conservation (CBC) at the American Museum of Natural History in New York City. She has spent the past fifteen years engaged in field research, studying threats to biodiversity. Coauthor Martha M. Hurley is a postdoctoral research fellow at CBC. She is part of the team analyzing data from the CBC’s biotic survey in Vietnam, under way since the late 1990s. Together with Sterling and their colleague Minh Duc Le, Hurley is also coauthor of a forthcoming book that will highlight Vietnam’s remarkable biodiversity. Coauthor Raoul H. Bain is a herpetologist who earned an M.Sc. in zoology from the University of Toronto, with a focus on the diversity of Southeast Asian amphibians. He began doing scientific fieldwork in British Columbia as a technician in paleontology and has since worked in Alberta, Tennessee, and the Bolivian Andes. Bain has made four field trips to Vietnam since 1995. The publication of Sterling, Hurley, and Bain’s article coincides with the exhibition “Vietnam: Journeys of Body, Mind, and Spirit,” which will open at the American Museum of Natural History on March 15.

Copyright © Natural History Magazine, Inc., 2003

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