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Symposium tackles big question: how many species will survive our generation

An overview of the Smithsonian’s Symposium: “Will the rainforests survive? New Threats and Realities in the Tropical Extinction Crisis”

Nine scientists dusted off their crystal balls Monday at the National Museum of Natural History in Washington DC, weighing in on the future of the world’s tropical forest. Despite the most up-to-date statistics, prognosis for the future of tropical forests varied widely.

Debate’s Background

In the last few years a schism has occurred among biologists regarding the future of the tropics. No tropical scientist denies that rainforests and the species which inhabit them face unprecedented threats; neither do they argue that some of these forested regions and species will likely not survive the next fifty years. What has sparked debate, sometimes heated, is how bad will is it really? When the dust settles, what percentage of species will survive and how much forest will remain?

For years scientists have been warning of a mass extinction that could rival the previous five mass extinctions each occurring millions of years ago, including the extinction event that killed off the dinosaurs. Scientists have warned that within our lifetimes 50-75 percent of the world’s species could go extinct. This prediction is largely supported by the continuing destruction of the world’s tropical forest, where the majority of Earth’s biodiversity lives.

Deforestation for rubber plantations in Laos. Photo by Rhett A. Butler

Despite decades of conservation awareness, reserve creation, and pressure on policy makers, tropical deforestation has not abated but in fact has steadily risen globally. Some predictions have shown that by 2050 only 5-10 percent of old growth forests will remain. With the loss of these forests, scientists inevitably believe that the world will lose tropical species in startling numbers. This view has been promulgated by a wide variety of prominent and widely-revered biologists, including E.O. Wilson, Norman Myers, Peter Raven, and William Laurance.

Recently however a few notable biologists have begun to push back against this prognosis. Joseph Wright and Helene C. Muller-Landau began the controversy with a series of papers and presentations that argued against the common belief of an impending mass extinction. Wright’s initial study posited that current trends in deforestation would not continue in the future.

Using data from the UN, Wright argued that rural populations in tropical areas, such as those living off subsistence farming, will in the future abandon the rural life and move to cities. This exodus from the tropics by subsistence farmers, who often employ slash-and-burn techniques in rainforest areas for short-term agriculture, would alleviate the pressure of habitat loss on tropical species. Areas that were once agricultural will become secondary forest and capable of supporting high levels of biodiversity, argues Wright, a trend already seen in much of the world as forests take the place of agricultural land that was abandoned in the 1980s and 90s.

In a 2006 paper Wright and Muller-Landau asserted that “large areas of tropical forest cover will remain in 2030 and beyond…. We believe that the area covered by tropical forest will never fall to the exceedingly low levels that are often predicted and that extinction will threaten a smaller proportion of tropical forest species than previously predicted.”

Without widespread loss of habitat there would be no mass extinction, at least not to extent warned about. Wright’s modeling predicted that instead of looking at extinction levels from 50-75 percent, we would see levels of extinction that would be closer to 20-30 percent.

Before the symposium on Monday, this is where the lines had been drawn in the sand. However, the symposium served to expand upon many of the issues impacting the debate—including changing drivers in deforestation, regrowth of tropical forests, new ideas about extinction, and climate change—and even added a few new surprising conclusions about the fate of the global rainforests.

Changes in rainforest destruction trends

At present, deforestation is not slowing down. According to Gregory Asner of the Carnegie Institution, deforestation is still the dominant pattern in tropical forests worldwide. In fact, he told the audience in Baird Auditorium, that if one looks at statistics of global forest loss decade-by-decade than not only is deforestation continuing, but it is on the rise.

At the same time some of the demographic trends predicted by Wright and Muller-Landau are occurring in certain areas. Small agricultural plots in forest are being abandoned in some countries, as people move to the cities. So, why isn’t deforestation slowing down?

“The forces driving deforestation have changed significantly,” Thomas Rudel a Human Ecology and Sociology professor at Rutgers University said. He noted that between 1965 and 1985 tropical deforestation was largely due to “assisted small cultivators”. These are usually poor local people who moved into the forest to practice agriculture or ranching. These small cultivators were “assisted” by their governments, which at the time were concerned with securing sovereignty over remote forest settlements. Incentives, such as road-building, were often promised to the poor agricultural entrepreneurs.

Even though this trend has slowed, and in some places reversed, rainforest deforestation continues due to “enterprise driven” destruction. With the globalization of trade, Rudel notes that from 1985 to today deforestation increasingly occurs for industrialized agriculture, such as soy and palm oil, and for logging to produce wood products largely exported to the West. Rudel said that the trend of deforestation had gone from slash-and-burning for subsistence farming to “blocks of deforestation augmented by consumer demand”. Therefore it has become international consumption by wealthy nations, and not local needs, which is largely driving contemporary deforestation.

Rudel argued that Wright’s theory was built on “period specific model” when rural populations did most of the deforestation and local population were more directly linked. The new drivers of deforestation, however, have changed the situation.

The importance of secondary forests

One of the arguments of the Wright and Muller-Landau theory that fared better during the symposium was that the extinction crisis may not be as bad as predicted due to the significance of secondary forests and other degraded landscapes, which may allow the preservation of certain species.

In an illuminating talk, Robin Chazdon, a professor at the University of Conneticut who has studied secondary forests for twenty five years, stated that secondary forests and other non-primary growth landscapes will prove essential to biodiversity.

“These are the areas that we need in order to conserve most of our biodiversity,” Chazdon said.
She pointed to several important studies in order to prove her point that secondary forests and other degraded landscapes were not lost causes in terms of biodiversity. A study in Veracruz, Mexico found that bird biodiversity was actually greater in shade grown coffee farms than in the forest. This larger diversity was due to the shade grown coffee farms retaining a good number of bird forest species while attracting non-forest species as well. Chazdon noted that agroforestry, like shade grown coffee, can be a “mixed story but still can protect a lot of species in ecosystems and use them for agricultural products”.

In the Western Ghats of India, where cultivation has occurred for 2,000 years, arecanut agriculture retains 90 percent of the bird biodiversity of the forest. In the largely degraded and devastated Atlantic Forest of Brazil chocolate grown under the canopy provides homes for 70 percent of many species, including birds, bats, butterflies, mammals, ferns, lizards and frogs.

Chazdon added that plant species also fared well, especially in secondary forests. In a forest less than twenty years old in Costa Rica, scientists discovered 90 percent of forest tree species either already growing or as seedlings.

On the other hand she told the audience that some landscapes were simply devastating for wildlife: for example soybean fields are “devoid of biodiversity” and “astonishingly poor” biodiversity exists in palm oil plantations. Palm oil plantations have been shown to retain a paltry 15 percent of species from the lost forest.

While Chazdon illustrated the potential of secondary forest and agroforestry to provide a haven for biodiversity in a fluxuating system, Gregory Asner emphasized that regrowth in forests was still in the minority compared to the larger trend of deforestation. Currently two percent of original forest cover is in process of regrowth, according to Asner’s satellite studies. Most of the regrowth is occurring in hills or mountainous areas, leaving tropical lowland species in a less advantageous position.

More skeptical of secondary forests, Bill Laurance told the media that: “Rainforest regrowth is indeed occurring in regions but most old growth is destroyed. In biodiversity terms, this is akin to a barn door closing after the horses have escaped”.

Patterns of extinction: who will survive?

Entomologist Nigel Stork from the University of Melbourne agrees with Joseph Wright that the mass extinction crisis has been exaggerated. Stork argues that the scientists who predicted extinction rates of 50-75 percent did not take into account that certain groups of species, such as birds and mammals, are more prone to extinction than other groups like insects.

Stork pointed out that half of all species described on Earth, including plants and fungi, are insects. Half of these, or a quarter of all species, are beetles—Stork’s particular passion. “Most organisms are very small,” Stork said, arguing that this fact alone offers a buffer against the loss of half the world’s species.

Studies have shown that certain traits make a species more vulnerable to extinction, including large body size, small restricted range, low number of young, top of the food chain, high specificity to another organism, and low physiological adaptation. Many of the world’s birds and mammals fall under these categories, especially the more ‘charismatic’ animals which have become poster-children for the extinction crisis: elephants, tigers, polar bears, whales, eagles and condors.

Scientists have estimated that on average a species of mammal survives around 1-2 million years, whereas invertebrates average around 11 million years before facing extinction. Another study compared various taxons in Great Britain and found that it was seven times more likely for mammal or bird to go extinct than an insect, spider, or mollusk.

In his talk Stork highlighted another point of Wright’s theory: many of the world’s most vulnerable species have already succumbed to extinction, so any species left are perhaps more resilient than scientist’s have given them credit for. The Quaternary extinction event, which occurred between 50,000 and 10,000 years ago already killed off half of the world’s megafauna, including famous species like the woolly mammoth and the saber-tooth tiger.

Stork concluded that birds and mammals are not good indicators of extinction for other species, but that the high numbers of 50-75 percent species loss assumed that an equal threat of extinction for mammals, birds, insects, fungi, etc. So far none of these taxons have proven to be a good indicator for overall species extinction.

However, Stork also acknowledged that beetles survived the last three major extinction events largely unharmed, yet such events are still referred to as mass extinctions. Often, such extinctions greatly affect particular groups of species, rather than all groups similarly. For example, while the Crectaceous-Tertiary extinction event wiped every species of dinosaur off the earth, mammals, amphibians, reptiles, and insect survived. There is also simply a lack of data on the extinction of insects.

The mass extinction underway could be similar, while insects could survive largely intact, other groups may not be so lucky. In fact, Joseph Wright stated during his talk that a mass extinction of topical montane frogs was already underway.

Climate change: the unpredictable elephant in the room

One place where the scientists at the Symposium largely agreed was the threat posed by climate change to the tropics and the inability to know how it would affect biodiversity in the region.
Though Stork believed that mass extinction warnings had been exaggerated, at the end of his talk he admitted, “I think climate change will change all of that this century, tipping the scale toward mass extinction”.

Wright, the other skeptic when it came to mass extinction in the tropics, agreed: “climate change, I believe, is a much greater threat to biodiversity in the tropics than habitat destruction”.

Wright noted that currently temperatues in the tropical forests had risen 0.6 degrees Celsius, and “conservative” estimates showed that by the end of the century 75 percent of tropical forests would experience a rise of 3 degrees Celsius.

“Tropical species are much more sensitive to small increases in temperature than temperate species”, Wright said, explaining why this expected shift was so alarming. In addition, he presented data showing that tropical species would have to travel much greater distances than temperate species to find habitat within their normal range of temperatures. Wright called these two factors—greater sensitivity to temperature change and larger migrations for suitable habitat—a “double whammy” for tropical species.

By the end of the century, Wright predicted, climate change would cause rainforests to become a “novel climate”: with temperatures similar to those in deserts but still receiving the requisite rainfall for tropical forests.

“How species will respond to novel climate we don’t know,” Wright admitted.


Although it was never stated outright, it appeared by the end of the symposium that all of the speakers foresaw mass extinction in the future of the tropics, unless drastic actions are taken. While the extinction may not reach 50-75 percent—since insects dominate the world—it would have a devastating effect on the world’s vertebrates.

While none of the scientist disagreed that the major drivers of extinction would be climate change and habitat loss, the role each would play remained contentious. While it may seem a minor disagreement, such discussions directly affect how to deal with the crisis, i.e. should the world invest in more tropical reserves or drastic measures to ensure climate change mitigation?

Fortunately, the symposium was not without concrete ideas for going forward, in fact there were many.

Robin Chazdon argued that in order to ensure enough habitat, secondary forests and agroforestry should be supported and deserved conservation attention. Though she believed primary forest should still remain primary. Furthermore, she saw great potential in reforestation projects undertaken by humans in order to speed and aid the process along; she described such projects as buffers for biodiversity and general mitigation against climate change. She hoped reforestation projects in degraded rainforest would increase rapidly.

Thomas Rudel argued that the shift in responsibilty for deforestation from small cultivators to industrial companies allowed an opportunity for governments and conservation groups to really pursue those responsible for the damage. Such actions he stated was “not available during early period of deforestation”. He emphasized the importance of supporting international agreements, such as REDD, payment for ecosystem services, and organic and certified products.

Bill Laurence viewed reserves as the key, calling them “islands of survival”. He argued for a general enlargement of tropical reserves and more support for tropical reserves, which he sees declining in health due to human pressures such as roads, pollution, and population growth along reserve edges. Furthermore, he argued that the preservation of primary forest would greatly aid in global warming mitigation.

Joseph Wright disagreed, stating that existing reserves were largely effective, so the focus most turn solely to climate change. While he stated that wide-scale ecosystem changes due to global warming were inevitable, one way to mitigate them was through forest regeneration and regrowth, a process which sequesters carbon. For the preservation of biodiversity he argued the emphasis must be put on linking “warm hot lowland areas with cooler montane areas”, so that species would have places to take refuge as their ecosystem heats up.

By the symposium’s end, Laurance stated that he was pleased with the “surprising amount of agreement”. Despite disputes about details, it did appear that the scientists were not in opposition about the most important matters: rainforests, providing many ecosystem services, are essential for global environmental health and the preservation of biodiversity; they must be protected as much as possible against the major threats which appear to be overwhelming them. In addition, while the extinction crisis underway may not affect every group of animals, it is already affecting particularly groups far and above the normal extinction rate and the cause can be squarelhy laid at the feet of a single species, ourselves.

Christian Samper, head of the National Museum of Natural History, told the media that “by bringing together the world’s foremost authorities on different aspects of rainforest science, we hope to achieve new insights into a situation with potentially profound implications for all species, ours include.”

Although questions (and debates) remain, the symposium’s goal was achieved.

If species and forests are to survive, pollinators will be essential. The 5th Annual Frugivore and Seed Dispersal Symposium in 2010 will cover this issue in great detail:

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