Forests at risk as seed-spreading animals disappear

Forests don’t plant themselves. In fact, the survival of many forests depends on a hidden network of animal workers that most people never think about. Birds, mammals, fish, and even a few amphibians are nature’s original gardeners, moving seeds from place to place and giving forests a chance to grow.

But this quiet system is breaking down. Populations of seed-dispersing animals are shrinking. Without them, many trees won’t survive, forests will lose their diversity, and the fight against climate change will become even harder.

Forests rely on animals to grow

In the Amazon and Atlantic Forest, about 90 percent of trees depend on animals to spread their seeds. In the Cerrado, the number is 60 percent.

These creatures aren’t just helping forests regenerate – they’re also indirectly helping the planet store carbon. When seed-dispersing animals disappear, forests change. Their ability to absorb carbon dioxide weakens, reducing their role in controlling global warming.

Yet, despite their importance, seed-spreading animals rarely get the attention they deserve in large-scale forest restoration plans. That’s a big oversight, noted an international team of researchers.

Mauro Galetti is co-director of the Center for Research on Biodiversity Dynamics and Climate Change (CBioClima), based at the Institute of Biosciences of São Paulo State University (IB-UNESP) in Rio Claro, Brazil.

“There’s a lot of talk today about carbon credits and forest restoration, but who ‘plants’ the carbon? It’s the toucan, the agouti, the tapir, the jacutinga,” said Galetti. “To have a copaiba tree, for example, the forest needs toucans and monkeys to disperse its seeds.”

“Therefore, we need to include frugivorous animals in the restoration equation, as there’s already enough science to quantify how much forest carbon is planted by animals.”

The alarming decline of seed movers

A growing collection of research warns that the loss of frugivorous animals – animals that eat fruit – could sabotage efforts to protect and restore ecosystems.

A study published in the journal Science found that the worldwide loss of birds and mammals has already reduced seed dispersal by 60 percent.

“We’ve made great strides in addressing these problems of seed disperser loss, and although Brazil is the country with the most scientific studies on seed dispersal, we need to delve deeper into the problem,” the researchers said. “In addition, we need to identify which strategies best restore seed dispersal.”

Animal digestion boosts seed success

When an animal eats a fruit, the seed goes on a journey. It might get softened by stomach acids, scratched by a bird’s gizzard, or carried to a safer location. This treatment increases the seed’s chances of sprouting – and thriving away from its parent tree.

“Therefore, seeds consumed by animals will germinate more, faster, and will establish themselves in safer places to grow,” said Galetti. “And if there’s no animal to ‘bruise’ the seed and take it away from the mother plant, it won’t germinate.”

“The Brazil nut, for example, has only one disperser: the agouti. If the agouti becomes extinct locally, the Brazil nut’s seed-dispersal service will succumb. We therefore depend on a fundamental ecological service provided by the agouti.”

The overlooked partners in restoration

Like bees, seed-dispersing animals are vital for plant reproduction. Both groups face threats from habitat loss and human activity.

But their challenges differ: pollinators are more impacted by pesticides, while seed dispersers are hit harder by habitat destruction and hunting.

The loss of pollinators often gets more attention because the consequences for food production are immediate and measurable. The decline of seed dispersers is slower and less visible, but the long-term effects on biodiversity and carbon storage are severe.

“Both are important and should be taken into account in restoration and conservation projects, Galetti said.

“However, the decline of pollinators is more easily measured in the short term, generating immediate economic impacts such as loss of crop productivity, while the effects of seed-disperser loss occur slowly and broadly, compromising the functionality and resilience of ecosystems.”

A forest needs more than just trees

Right now, the economic costs of losing seed-dispersing animals remain largely unknown. What is clear is that restoration won’t work without them.

“Restoration isn’t just about planting trees; you have to consider who will maintain the future of that forest, which are the dispersing animals,” Galetti said. “A few years ago, it was believed that by planting the forest, these animals would come to it.”

“But that’s not how it works. It’s much more complex to have a restored forest functioning.”

The researchers stress that ignoring seed dispersers could lead to forests that fail to recover from fires, degrade more quickly, and lose biodiversity.

“Addressing the decline of seed dispersers is critical to preserving animal biodiversity, ensuring forest connectivity, and balancing plant communities,” concluded Galetti.

The full study was published in the journal Nature Reviews Biodiversity.

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