No substitute for undisturbed tropical forests

Posted: 14 September 2011

Primary forests with little or no human disturbance are irreplaceable when it comes to maintaining tropical biodiversity, according to new findings by an international team of scientists.

Disturbed and degraded forests have received much attention recently for their potential conservation value. Secondary forests, in particular, had been thought by some to provide an essential complement to primary forests in supporting biodiversity. However, this new study found otherwise – with all major forms of disturbance reducing tropical biodiversity.

Publishing today in the journal Nature, the researchers from the University of East Anglia (UEA), University of Cambridge, Lancaster University, National University of Singapore (NUS), University of California, San Diego (UCSD), ETH Zurich, University of Adelaide, James Cook University and the Heinz Center for Science, Economics and the Environment conducted a comprehensive global assessment to estimate the impact of disturbance and land conversion on biodiversity in tropical forests.

Deforestation in the Brazilian Amazon
Deforestation in the Brazilian Amazon. Photo © Google Earth

Drawing on information from 138 scientific studies spanning 28 tropical countries, they compared biodiversity values between primary forests and a suite of degraded and converted forest types including secondary forests, selectively logged forests and forests converted to agriculture. Overall, biodiversity values were substantially lower in degraded forest types, highlighting the strong impact human land-use changes are exacting on tropical biodiversity.

“The next few decades will see a growing number of reserves being degraded, downsized, if not entirely degazetted, so holding onto the last remaining large tracts of primary forests within existing reserves might become a crucial part of the conservation mission this century,” said co-author Prof Carlos of the School of Environmental Sciences at the University of East Anglia.

Lead author Luke Gibson of the Department of Biological Sciences at the National University of Singapore said: “There is no substitute for primary forests. Our comprehensive assessment shows that all major forms of disturbance – with one possible exception – invariably reduce biodiversity in tropical forests.”

The exception is selective logging, which had a relatively small – but still negative – impact on biodiversity.

“Ecological restoration of selectively logged forests might represent an effective strategy to alleviate threats to tropical biodiversity, particularly when they are also rapidly growing in extent,” said Prof Lian Pin Koh of NUS and ETH Zurich.

As humans continue to degrade and convert tropical forests for timber extraction and agricultural production, tropical biodiversity is predicted to suffer negative consequences and continue to decline. For effective conservation in the tropics, where most species on Earth are found and where environmental threats are most severe, understanding how different kinds of human land-use activities affect tropical species becomes crucial.

Comparing human impacts across the key tropical forested regions, the authors found that Asia suffered the greatest loss in biodiversity but called for urgent research in understudied regions - particularly Africa, which sustains the second largest contiguous tropical forest in the world.

To protect the world’s remaining primary tropical forests, the study suggests a number of strategies, including the expansion and enhanced enforcement of protected areas. Curbing international demand for commodities obtained at the expense of primary forests is another strategy to protect tropical biodiversity.