A new study co-authored by a Montana State University ecologist concludes that less than a quarter of the world's tropical rainforest remains intact and undisturbed -- a situation that ecologists say could trigger extinctions for many forest-dwelling species. The study was published this week in the journal Proceedings of the National Academy of Sciences.

Andrew Hansen, emeritus professor in the Department of Ecology in MSU's College of Letters and Science, said the work builds on prior research into the relationship between the health of tropical forests and the well-being of the animals that live there. That work showed that intact, native tropical rainforests with minimal human disturbance -- referred to as high-integrity habitats -- are associated with low risk of extinctions.

However, the amount of high-integrity forest that remained undisturbed was unclear, so a team including Hansen and led by Rajeev Pillay of the University of Northern British Columbia decided to find out. The ecologists used remote sensors and detailed maps of rainforest animals' habitat ranges to quantify the amount of high-integrity tropical rainforest within the ranges of more than 16,000 vertebrate species.

The authors found that, while rainforest covered up to 90% of the vertebrates' ranges, only 25% of those rainforests were intact and free of major human disturbances. They also found that threatened species had smaller proportions of high-integrity habitat left in their ranges, compared to non-threatened species. Additionally, species with smaller habitat ranges had smaller proportions of high-integrity forest habitat remaining.

The scientists say the data suggest that further deforestation and forest degradation could trigger a cascade of extinctions and that conserving high-integrity tropical rainforests is critically important to protecting biodiversity.

Hansen said that high-integrity forests are particularly important for maintaining populations of birds, amphibians, reptiles and mammals.

"These findings are important for formulating international biodiversity policy," Hansen said. "They support recent calls for the United Nations-sponsored Global Biodiversity Framework to not only protect forest but to especially map and protect the high-integrity forest that is so important to native wildlife."

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