"If you ask people who work on biodiversity how to conserve tropical forests, they will say protected areas," Hedley Grantham of Conservation International, US, told environmentalresearchweb. "But if you ask people who work on climate the same question, they will say carbon payments. We found this divergence of approaches curious, since it's the same forests providing both services."

Efforts to preserve biodiversity tend to protect areas of forest in nature reserves that will maximize the number of species at viable levels. Carbon payment schemes, on the other hand, such as the REDD+ (reducing emissions from deforestation and forest degradation) programme that emerged from the UNFCCC, use markets to incentivize marginal changes in carbon storage provision by land managers.

"There are very good reasons to conserve tropical forest biodiversity using parks and tropical forest carbon using payments," said Jonah Busch, also of Conservation International. "Species have characteristics such that they are best managed using intensive actions within compact, contiguous areas. Parks are well suited for this. On the other hand, carbon has attributes that make it best managed with less intensive actions dispersed over much larger areas."

According to the researchers, from a protection point of view, the four key differences between biodiversity and carbon storage are that rare species are more heterogeneously distributed than carbon; harder to measure and monitor; more sensitive to human disturbance and ecological processes such as habitat connectivity and climate stability; and that people value keeping a species’ population or habitat size large enough to ensure survival but are less bothered beyond that threshold, whereas the millionth tonne of carbon storage is valued as much as the first tonne.

In the case of Indonesia, the team found that because of this threshold effect along with habitat overlap, just 5% of the country’s forest area could provide a target level of 100,000 hectares of habitat for 80% of forest-dependent mammal species. In contrast, this amount of forest stores no more than 6.2–7.3% of the country’s carbon in above- and below-ground biomass.

"Although biodiversity conservation and climate change mitigation have chosen the approaches that they have for solid reasons, each can be improved by adopting a concept most associated with the other," said Grantham. "Parks can more effectively conserve biodiversity by considering additionality, that is, how much outcomes are improved by a park relative to if no park were put in place in that location. Payments can more effectively conserve carbon by prioritizing geographically."

So, for example, the location of parks could be improved by assessing the degree to which probable threat can be reduced, rather than the current skewing towards lands that are remote, unsuitable for farming, or otherwise facing little threat. It’s important to bear in mind that habitat would not be completely maintained under protection or, equally, completely lost without protection, the team says. Using this approach for Indonesia came up with a much higher (159%) reduction in habitat loss as a percentage of species’ initial range size, but slightly lower species representation – 20% fewer species for which a 1,000,000 hectare target is met with 10,000,000 hectares of forest.

REDD+ policies, meanwhile, are "often designed with little consideration for where they will be put in place", wrote Busch and Grantham in Environmental Research Letters (ERL). By looking at the spatial variation of forest cover, deforestation, carbon density, and emissions from peat soils, the team found that the Indonesian provinces of Riau and Papua could be the best targets for the nation’s pilot REDD+ programmes, as carbon payments there would have the greatest potential to reduce emissions relative to expected emissions without a REDD+ programme.

"Actions to conserve tropical forests for their multiple benefits should target the most effective actions to the most effective places," said Busch.

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