A team of researchers from Singapore, Australia, Switzerland, the UK and the USA has carried out a comprehensive assessment to estimate the impact of disturbance（干扰，骚乱） and land conversion on biodiversity in tropical forests. In a recent study published in Nature, they found that primary forests -- those least disturbed old-growth forests -- sustain the highest levels of biodiversity and are vital to many tropical species. Rampant rates of logging and agricultural expansion have transformed the world's tropical forests, leaving little remaining primary forests unaltered by humans. The value of these rapidly expanding degraded and converted forest landscapes is hotly debated, and was the subject of the study.

"Some scientists have recently argued that degraded tropical forests support high levels of biodiversity," says Luke Gibson, the lead author from the National University of Singapore (NUS). "Our study demonstrates that this is rarely the case," he adds.

Drawing on information from 138 scientific studies spanning 28 tropical countries, Gibson and his colleagues compared biodiversity in primary forests to that in regenerating forests and forests degraded by logging and converted to agriculture. Overall, biodiversity values were substantially lower in disturbed forests.

"There's no substitute for primary forests," says Gibson. "All major forms of disturbance invariably reduce biodiversity in tropical forests," he adds.

Selective logging, in which machinery is used to extract a limited number of trees from the forest, appears to be the least harmful human disturbance. "As selective logging is rapidly expanding throughout the tropics, ecological restoration of such areas might represent an effective strategy to alleviate（减轻，缓和） threats to biodiversity," says Lian Pin Koh of ETH Zurich.

Parks, however, will remain a critical conservation strategy in protecting the world's remaining primary tropical forests. "We urgently need to expand our reserves and improve their enforcement," says Tien Ming Lee, co-lead author at the University of California, San Diego. "Effective reserves have the added benefit of reducing overall carbon emissions," adds Lee.