It has been the one environmental question that everyone could agree on: cutting down the rainforest is a disaster for biodiversity. Or is it? According to two respected tropical researchers, the future may not be so bleak after all. In fact, pretty soon the rainforests of the world will start to grow back, and many species on the brink of extinction will recover.
Not surprisingly, this declaration â first made in Joseph Wrightâs presidential address to the Association for Tropical Biology and Conservation in 2004 â ignited a blazing row among tropical biologists. Some were happy to see an end to more than 20 years of doom-mongering. Others have attacked the research as flawed, irresponsible and liable to make a bad situation worse, providing ammunition for companies that want to make a quick buck out of forests and an excuse for governments to let them.
One thing is certain: deforestation is flattening tropical forests at an alarming rate. The UN Food and Agriculture Organization (FAO) estimates that more than 12 million hectares of tropical forest were lost each year between 1990 and 2000. Wright doesnât dispute this. What he did dispute in his 2004 address, and later in a paper co-authored with Helene Muller-Landau from the University of Minnesota in Minneapolis-St Paul (Biotropica, vol 38, p 287), is the received wisdom that what happens next is always devastating for biodiversity.
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When areas of old-growth forest are cleared, specialist species become marooned in the isolated islands that remain. Without access to new territories, mates and food sources they die out. In time, secondary forest may grow up to fill in the gaps and link the islands back together, but most biologists believe that the damage to the ecosystem is irreversible. The new growth superficially looks like the forest it has replaced but in fact is a far inferior habitat that can support only a fraction of the original species. Looking to the future, most biologists see only increasing loss and fragmentation of the worldâs old-growth forests and a widespread loss of biodiversity.
Wright and Muller-Landau disagree. Todayâs tropics have some 6 million square kilometres of old growth and 5 million square kilometres of secondary and partially logged forests. âThis ratio will change,â says Wright. âMost future forest will be secondary. The good news is that this is not the disaster everyone predicted.â
No one doubts the duoâs credentials. Wright has been a research biologist at the Smithsonian Tropical Research Institute in Balboa, Panama, for more than 20 years and is well known for his studies of tropical plant biology. Muller-Landau has published widely on seed dispersal and forest regeneration. The idea for this study came from Wrightâs experience of forest in Panama. âThere has been widespread old-growth forest loss and consistent migration to Panama City as people have abandoned marginal agricultural lands,â he says. âWith people gone, secondary forest has regenerated. Crucially, if protected from hunters, nearly every bird and mammal species found in primary forest has also been found in secondary.â
Forest futures
Rural slash-and-burn agriculture, the pair concluded, was the main cause of tropical deforestation, in Panama at least. But as rural people move on and secondary forest reclaims the land, biodiversity recovers. Wright and Muller-Landau wanted to know whether the same could be said for the rest of the tropics. To find out, they compared UN projections of future population with FAO projections of tropical forest distribution. This revealed a strong relationship between rural population density and loss of forest cover, a link that held across 45 developing countries in Africa, the tropical Americas and Asia. Since the UN forecasts that populations in most of these countries will become increasingly urban over the next quarter century, the researchers projected a decline in deforestation and an increase in secondary forest.
The pair then estimated the number of species likely to be found in the regenerated forest areas, based on the relationship between species numbers and habitat area. This is a measure often used in large-scale estimates of species numbers, although some ecologists say itâs too blunt an instrument to be of any use in this kind of forecast. Considering the forecasted changes in demographics, and calculating the area of farmland that would thereby be abandoned and likely to be under secondary and regenerating forest, Wright and Muller-Landau predicted that the widely touted biodiversity cataclysm would be largely avoided. Instead of losing more than three-quarters of all tropical forest species, as some studies had predicted, the study predicted 21 to 24 per cent of Asiaâs forest species, and 16 to 35 per cent of Africaâs, would be threatened with extinction â big losses, but nothing like as bad as the 80-plus per cent predicted by some other models. The outlook for the tropical Americas was even more reassuring, with a predicted loss of less than 20 per cent.
Yet far from being welcomed as good news, the study has met fierce opposition from other researchers. Bill Laurance, a colleague of Wrightâs at the Smithsonian Tropical Research Institute has been one of the sharpest critics. With many yearsâ experience studying tropical forest loss in Amazonia, Africa and Australia, Laurance speaks of the âheroic assumptions and extrapolationsâ made by Wright and Muller-Landauâs study (Trends in Ecology and Evolution, vol 22, p 65).
Rainforest row
Lauranceâs main objection goes to the heart of Wright and Muller-Landauâs premise: that rural communities practising slash-and-burn are largely to blame for deforestation. He gives the example of logging in the species-rich old-growth forests of Cameroon and the Congo region. âThis is not happening because their populations have suddenly exploded,â he says. âItâs because countries like China are buying and importing massive quantities of central African timber. Much of this then becomes furniture thatâs sold to North America and Europe. In a globalised world, you donât need a lot of people living locally to really hammer a countryâs forests.â
âIn a globalised world you donât need a lot of people living locally to really hammer a countryâs forestsâ
Alexander Belokurov, WWF Internationalâs forest protection programme officer, agrees. âSuch major promoters of tropical forest destruction as the increasing demand for timber, charcoal and food for urban populations, and such large-scale industrial drivers as cattle ranching, soy production, oil-palm plantations, timber exploitation and infrastructure development have all played a much higher role in the disappearance of tropical forests in recent years,â he says.
The effects of government economic incentives should not be underestimated either, says Sean Sloan, a geographer at McGill University in Montreal, Canada, who specialises in the causes of deforestation and forest regeneration. âCows and soy in the Brazilian Amazon in the 1980s and 90s, Central American teak plantations in the 1990s, in the future maybe government-subsidised biofuels: these have very real effects but bear no relationship to Wright and Muller-Landauâs simple âdensity-destructionâ model,â he says.
These omissions also concern Barry Brook from Charles Darwin University in Australiaâs Northern Territory. Brook led a team that wrote the first rebuttal of Wright and Muller-Landauâs paper (Biotropica, vol 38, p 302), in which they additionally criticised the study for leaving known biodiversity hotspots out of the analysis and for ignoring climate change. Wright counters that hotspots, he believes, comprise some 51 per cent of the pre-agricultural distribution of tropical forest, so by definition will have been included in the study. âConservation biologists love to think that hotspots are tiny. This just isnât the case in the tropics,â he says. However, critics claim that his study deliberately excluded the most species-rich and extinction-prone hotspots, such as Madagascar, the Philippines and the Brazilian Atlantic forests. Wright insists that they were included.
Sloan also points out that rural migration only helps if nearly everyone deserts a particular area and lets it regrow. In some places, he says, that is unlikely. âIn parts of south-east Asia, rural population densities are so high that you could have widespread migration to the cities and there would still be little appreciable forest regeneration.â Wright, however, is confident that his and Muller-Landauâs assumptions were reasonable. âWe included rural-only projections because we were stunned at the strength of the relationship between forest cover and rural population density for the three tropical continents. The relationship for the three continents was amazingly similar. It was something that we had just not expected.â
This confidence in the UN data underlying their predictions has also been attacked by some critics, who feel that Wright and Muller-Landau stepped out of their fields of expertise and so were insufficiently cautious with the data. âI know no demographer whoâd consider UN population data to be as watertight as Wright and Muller-Landau did,â says Robert Engelman of the Worldwatch Institute, based in Washington DC, which researches sustainable development and social justice. âUN population projections are conditional forecasts, not actual predictions. The studyâs supposition that UN projections will unfold as âpredictedâ are based on a grave misunderstanding of what projections actually are.â
Despite these concerns, not everyone is against the study. âIt is the best analysis available of the species extinction issue,â says Ariel Lugo, director of the US Forest Serviceâs International Institute of Tropical Forestry in Puerto Rico. He praises the authors for making their assumptions so explicit. âPrevious studies were extremely deficient, but many scientists ignored the poor analyses while embracing the gloom-and-doom conclusions,â he says. As to the aspects that the model did not cover, Lugo feels that they âare not fundamental: they represent a challenge to future analysesâ. He cites data from Puerto Rico: âWhen rural people moved to the cities, forest cover increased from 7 per cent to the current 45 per cent,â he says. More importantly, Puerto Rico has lost 99 per cent of its primary forests and 93 per cent of its total forest cover, has a population density 10 times the highest number on Wright and Muller-Landauâs graphs, âyet the island has experienced very low rates of species extinctions, losing no more than 10 per cent of its bird species and 5 per cent of its plantsâ, he says. Moreover, his recent studies reveal that the islandâs emerging secondary forests host more species than the original forests, though alien species help to make up the numbers.
Others have similar data, such as Torben Larsen, an internationally recognised authority on tropical butterflies. West Africa, he reports, has less than 3 per cent of its original primary forest cover but appears not to have lost a single species of forest butterfly. âThough this may be an example of enormous extinction debt and the worst may be yet to happen,â he cautions.
Trouble ahead
It may turn out that many species can survive well in secondary forest, but a lot of conservationists argue that plenty canât. In their study, Wright and Muller-Landau estimated species numbers in old-growth forest, regenerating forest and plantations to be the same. âEcologically, such systems are rarely comparable,â says Laurance, pointing out that a host of species are known to be unable to live in secondary forest. âThese range from individual specialist mammals like Australiaâs lemuroid ring-tailed possum to whole communities of understorey birds. You just never find them outside primary forest.â Laurance considers Wrightâs notion that this might not be the case to be âalarmingâ.
Wright counters that he and Muller-Landau were just criticising the idea that partially logged and regenerating forests support little biodiversity. âThis implicit assumption is rarely questioned. This is why we have stirred up such a hornetâs nest,â he says.
Yet perhaps the most serious charge against Wright and Muller-Landauâs study is that it violates one of conservation biologyâs most sacred tenets, the precautionary principle. âWeâre rapidly losing the most biologically important real estate on the planet, and itâs important that people understand the gravity of this,â says Laurance. He worries that the study could be seized on by political interests whose agenda includes marginalising conservation. âOver the past 30 years tropical conservation biology has been immensely successful at getting areas protected,â he says. âA study such as this could be widely cited as an excuse to do nothing, or worse. I just cringe at the thought of Wrightâs arguments being used by developers to argue, say, that a massive new Amazonian highway should proceed.â
Wright doesnât see it that way. âItâs the obligation of every scientist to report what theyâve discovered,â he says. âOur analysis indicates land-use change will not lead to vast decreases in tropical forest cover. Reporting this does not violate the precautionary principle.â
He also feels his study has been misinterpreted. âI believe there might well be a truly massive biodiversity crisis fairly soon. The issue is what the cause will be. Climate change is the big global worry, not deforestation rates.â The goal of the study, he explains, was âto change the focus away from old-growth forests to recovering logged and secondary forests, and away from deforestation to climate changeâ. A second study on the effect of climate change is in the works.
This doesnât convince Bill Laurance. He accuses Wright of changing his message in light of the massive critical response and feels that itâs far too early for this kind of optimism. âIâd much rather our great-great-grandchildren shook their heads at how needlessly we worried, rather than have them wonder why we didnât do enough when we still could,â he says.
