How a forest with more mammals stores more carbon

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To the many moral and practical reasons for protecting the creatures with whom we share this blue marble, add one more. They might help Earth safely store more carbon.

Writing in the journal Nature Ecology and Evolution, ecologists led by Mar Sobral and José Fragoso of Stanford University describe a three-year-long study of mammals and carbon cycles in southeast Guyana. Previous research suggested links between biodiversity and carbon storage; the more species of plants and animals there are in a given ecosystem, the more CO2 it seems to absorb. Exactly how this works, though, is still being revealed.

Sobral and Fragoso’s team wanted to learn more about the roles played by mammals. And so, across three years and 18,500 square miles, they conducted more than 10,000 surveys, counting some 218,000 individual mammals belonging to 28 species. They noted more than 43,000 feeding events and identified more than one million ground-deposited food remains. While in the field, they also measured soil carbon concentrations and tree size at hundreds of sites. Out of the number-crunching emerged a pattern: more mammal species meant more carbon ending up in soil or the bodies of trees.

This pattern reflects a vast, unceasing, little-appreciated animal labor — “moving plant matter across the landscape and processing it in ways that make it available to a larger diversity of invertebrates, fungi and microbes,” write the researchers. Eating, digesting, defecating. Spreading nutrients and, crucially, the seeds of trees that eventually breathe atmospheric CO2 and lock it in cellulose. Eventually dying and decomposing, nourishing yet more growth. Each action of minute significance but, like raindrops, adding up.

The researchers didn’t calculate exactly how much extra carbon a mammal-rich forest might store, but Sobral offered this rough estimation: a forest with 30 mammal species should sequester an extra 10,000 kilograms per hectare in above-ground tree biomass alone. Extrapolated to the entire Amazon, that translates to some 5.5 billion tons of carbon, roughly equivalent to U.S. emissions in 2015 — and that doesn’t even account for soil-bound carbon, which would drive the total much higher. It’s a back-of-the-envelope calculation, but it hints at the potential.

Far more remains to be learned, write the researchers. The relationships are not simple. They involve many scales and processes, and “cannot be easily deconstructed into individual function–service relationships.” Yet some implications are already clear: human-induced defaunation is potentially a climate problem, and conservation a climate solution.

Near roads and communities, where habitat was fragmented and disturbance higher and hunting more intense, trees were smaller and soils stored less carbon. “Mammal diversity should indeed be considered in management policies seeking to maximize carbon storage,” says Sobral. And while the study focused specifically on mammals and Guyana, she believes the findings hold for other creatures — reptiles, amphibians, birds, invertebrates — in other types of forest, though this this needs to be tested. “I believe the richness of any animal group would affect the biogeochemical cycles,” she says.

Source: Sobral et al., “Mammal diversity influences the carbon cycle through trophic interactions in the Amazon.” Nature Ecology & Evolution, 2017.

Image: Marissa Strniste / Flickr


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