Anna Salleh
ABC, Thursday, 15 September 2011
When it comes to protecting tropical species that are at risk from extinction there's no substitute for old growth forests, a new study has found.
The findings fly in the face of suggestions that loss of biodiversity can be tamed by the regrowth of forests in tropical areas.
Ecologist Professor William Laurance, of James Cook University in Cairns, and colleagues report their findings today in the journal Nature.
"This notion that we don't have to worry about the future of biodiversity in the tropics because there is forest regenerating in some areas is a very misleading argument," says Laurance.
"Most forms of forest degradation have an overwhelmingly detrimental effect on tropical biodiversity."
Old growth forests are those that have been undisturbed for centuries and contain enormous trees that in some cases are 1000 years old.
Laurance says about half of the world's old growth forest has been completely cleared, and much of the remaining forest is damaged.
"What we're witnessing in our lifetimes is a really massive transformation of the tropics," he says.
"An important question has been what the impacts are going to be on biodiversity."
Comparing habitats
Laurance and colleagues analysed 138 published studies that compared the abundance of species in old growth forests with that in other habitats, including areas given over to agriculture, regenerating forests, and selectively logged forests.
"When you compare these different types of habitat you find that the old growth forests are definitely the most important for sustaining biodiversity," says Laurance.
He says the findings feed into a recent debate about the importance of disturbed tropical forests in sustaining biodiversity.
Some biologists argue regenerating forests can sustain biodiversity. While this is true, says Laurance, they are no substitute for old growth forests.
He says old growth forests have 10 to 30 times more species of trees than other forests and have specialised habitats such as hollow trees.
Laurance and colleagues found that when it came to the species that are at greatest risk of extinction, old growth forests were the most important habitats.
"You tend to find the extinction-prone species almost exclusively in old growth rainforest," he says.
Laurance says a forest would need to be left undisturbed for around 300 years to qualify as old growth.
He says in the Brazilian Amazon, where an area the size of France has been cleared, a third of that is regrowth forest that is on average just six to seven years old.
Second best option
Laurance emphasises that while old growth forest is the most important habitat for at risk species, selectively logged forest may provide a second-best option.
In fact, Laurance and team found the least dramatic difference in species abundance between old growth and selectively logged forests, where only certain trees are cut out but much of the original habitat remains.
"That's important because it shows selectively logged forests should also be preserved as important habitat," says Laurance.
He says that Indonesia has 30,000 million hectares of selectively logged forests and most has been designated to be cleared on the basis it doesn't have any conservation value.
But, says Laurance, most of these forests are in areas where old growth has been completely removed and they represent the best hope for preserving biodiversity.
"To just write that stuff as being valueless is very bad policy."
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