Thursday, January 15th, 2009 | Author:

Biologists debate the scale of extinction in the world’s tropical forests

A RARE piece of good news from the world of conservation: the global extinction crisis may have been overstated. The world is unlikely to lose 100 species a day, or half of all species in the lifetime of people now alive, as some have claimed. The bad news, though, is that the lucky survivors are tiny tropical insects that few people care about. The species that are being lost rapidly are the large vertebrates that conservationists were worried about in the first place.

This new view of the prospects for biodiversity emerged from a symposium held this week at the Smithsonian Institution in Washington, DC, but the controversy over how bad things really are has been brewing since 2006. That was when Joseph Wright of the Smithsonian Tropical Research Institute in Panama and Helene Muller-Landau of the University of Minnesota first suggested that the damage might not be as grim as some feared. They reasoned that because population growth is slowing in many tropical countries, and people are moving to cities, the pressure to cut down primary rainforest is falling and agriculturally marginal land is being abandoned, allowing trees to grow.

This regrown “secondary” forest is crucial to the pair’s analysis. Within a few decades of land being abandoned, half of the original biomass has returned. Depending on what else is nearby, these new forests may then be colonised by animals and additional plants, and thus support many of the species found in the original forest.

Dr Wright and Dr Muller-Landau therefore reckon that in 2030 reasonably unbroken tropical forest will still cover more than a third of its natural range, and after that date its area—at least in Latin America and Asia—could increase. Much of this woodland will be secondary forest, but even so they suggest that in Africa only 16-35% of tropical-forest species will become extinct by 2030, in Asia, 21-24% and, in Latin America, fewer still. Once forest cover does start increasing, the rate of extinction should dwindle.

There are, however, two criticisms of this analysis. The first questions whether the raw data about forest cover are a good indicator of biodiversity, at least for big animals. William Laurance, a colleague of Dr Wright’s, pointed out to the symposium that birds and mammals are more vulnerable to alterations in their habitat than are insects and other small animals. His data suggest that even in some of the world’s best-protected primary forests, these species face severe pressures.

Elizabeth Bennett, of the Wildlife Conservation Society, an American conservation group, agreed and mentioned that for large birds and mammals, uncontrolled hunting for food and for trade is causing a phenomenon known as “empty-forest syndrome”. She said that although many forests look healthy when viewed from a satellite, they are actually falling silent because many of their large animals have been removed for subsistence or profit.

Nonetheless, the symposium’s participants agreed that the number of species of large animals may no longer be reliable indicators of the status of the millions of other species that live in a forest, and about which far less is known. Most species, as Nigel Stork of the University of Melbourne pointed out, are insects—and these are more resilient and much less threatened.

A diversity of opinion

The second criticism of Dr Wright’s and Dr Muller-Landau’s view questions their interpretation of the link between a country’s population and its rate of deforestation. Dr Laurance suggested this link may not be as strong in the future as it once was. In the past two decades, he said, many parts of the tropics have seen a rise in industrial forms of land use such soya-bean farming, oil-palm plantations, and oil and gas development, together with the road-building and other construction projects that these bring in their wake. Such activities vary with the demands of international markets, not the size of the local population. In particular, Dr Laurance is worried that the liberalisation of agricultural trade and the rise of biofuels could lead to a huge increase in demand for land in the tropics.

What everyone agreed, though, was that climate change is a threat. Even the optimistic Dr Wright is worried. He warned the meeting that because many tropical species evolved in an environment that has very little temperature variation, they are not equipped to cope with an increase of as little as 3ºC, which is the sort of change that many climate scientists predict. Such species may thus have to migrate long distances if they are to survive. He said that by the end of the century, 75% of tropical forests will be warmer than today, and what will remain in these hot, wet places is unknown.

Source: The Economist

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