A lot of attention has been paid to how climate change can reduce biodiversity. Now MIT researchers have shown that the reverse is also true: Loss of biodiversity can jeopardize regrowth of tropical forests, one of Earth’s most powerful tools for mitigating climate change.

Combining data from thousands of previous studies and using new tools for quantifying interconnected ecological processes, the researchers analyzed numerous tropical sites where deforestation was being followed by natural regrowth, focusing on the role of animals such as birds and monkeys that spread plant seeds by eating them in one place and then defecating someplace else. Evan Fricke, a research scientist in the MIT Department of Civil and Environmental Engineering and the lead author of a paper on the work, has studied such animals for 15 years, showing that without their role, trees have lower survival rates and a harder time keeping up with environmental changes. 

Since tropical forests are Earth’s largest land-based carbon sink, such challenges make it harder to fight climate change. But the influence of biodiversity on forests’ ability to absorb carbon has not been fully quantified.

To do that, the researchers looked at data on where seed-dispersing animals live, how many seeds each animal disperses, and how they affect germination. Then they incorporated data revealing the impact of human activity such as hunting and forest degradation. They found, for example, that the animals move less, and thus spread seeds less widely, in areas with a bigger human footprint.

With the data, the researchers created an index that revealed a link between human activities and declines in seed dispersal. They analyzed the relationship between that index and records of carbon accumulation in naturally regrowing tropical forests over time, controlling for factors like droughts, fires, and livestock grazing.

“What’s particularly new about this study is we’re actually getting the numbers around these effects,” Fricke says. In particular, they found that naturally regrowing forests with healthy populations of seed-dispersing animals absorbed up to four times more carbon than those without as many. Meanwhile, in sites identified as suitable for reforestation, current levels of disruption to seed dispersal reduce the potential for regrowth by 57%.

These findings could help direct reforestation strategies. “In the discussion around planting trees versus allowing trees to regrow naturally, regrowth is basically free, whereas planting trees costs money, and it also leads to less diverse forests,” says César Terrer, a professor of civil and environmental engineering and a coauthor of the paper. “Now we can understand where natural regrowth can happen effectively because there are animals planting the seeds for free, and we also can identify areas where, because animals are affected, natural regrowth is not going to happen, and therefore planting trees actively is necessary.”

The researchers encourage action to protect or improve animal habitats, reduce pressures on seed-dispersing species, and potentially reintroduce them where they’ve been lost. Overall, they hope the study helps improve our understanding of the planet’s complex ecological processes.

“When we lose our animals, we’re losing the ecological infrastructure that keeps our tropical forests healthy and resilient,” Fricke says.