Most trees in the Amazon (90%), the Atlantic Forest (90%), or the Cerrado, the Brazilian savannah-like biome (60%), depend on animals to disperse their seeds, ensure their reproduction, and keep the forest standing. Birds, mammals, fish, and even a species of amphibian play a crucial role in forest diversity around the world. However, this process has been disintegrating as populations of seed-dispersing animals have declined dramatically.
The loss of frugivorous animals (whose diet consists mainly of fruit) has another effect: it alters the composition of forests, weakening their ability to absorb carbon dioxide and thus reducing their role in combating climate change.
Yet, major global efforts to protect and restore ecosystems continue to underestimate seed-dispersing animals in biodiversity conservation and forest restoration strategies.
"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. 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," says Mauro Galetti , one of the directors of the Center for Research on Biodiversity Dynamics and Climate Change ( CBioClima ), a FAPESP Research, Innovation, and Dissemination Center ( RIDC ) based at the Institute of Biosciences of São Paulo State University (IB-UNESP) in Rio Claro, Brazil.
Galetti and researchers from the United States, Switzerland, Panama, Germany, Spain, and Portugal published an article in Nature Reviews Biodiversity warning about the consequences of losing seed dispersers in a changing climate. According to the researchers, the role of frugivorous animals in maintaining plant biodiversity is so central that efforts to restore and protect ecosystems are at risk of failing to meet their goals if the decline of seed dispersers is not mitigated.
A recent study published in the journal Science by some of the researchers who signed the alert showed that the worldwide loss of birds and mammals results in a 60% reduction in seed dispersal. "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 and understand, for example, which plants and ecosystems are most vulnerable to this loss. In addition, of course, we need to identify which strategies best restore seed dispersal," says the researcher.
Unknown heroes
When a dispersing animal eats a fruit, it becomes "contaminated" by the seed that passes through the digestive tract. The seed receives chemical treatment from gastric juices or mechanical treatment – in the case of birds, for example, the gizzard crushes the seed – which allows water to enter the seed, leaving it ready to germinate wherever the animal later deposits it when defecating.
"Therefore, seeds consumed by animals will germinate more, faster, and will establish themselves in safer places to grow. And if there's no animal to 'bruise' the seed and take it away from the mother plant, it won't germinate, and even if it germinates near the mother plant, it'll probably die because there'll be competition between them," says Galetti.
However, it is important to note that there is no standard. This interaction is different in every place in the world and for every species of tree and vertebrate animal. "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," says Galetti.
While birds, bats, monkeys, and tapirs are the main seed dispersers in the Atlantic Forest, fish play a crucial role in the Amazon and Pantanal. "Pacu and tambaqui fish, for example, travel long distances and eat large quantities of fruit, which makes them super dispersers of different species in riparian forests," says the researcher.
Ecosystem services
Like bees and other pollinators, frugivorous animals play a crucial role in plant reproduction. However, despite being threatened by similar factors, such as land use changes and direct exploitation, the two groups respond differently to these impacts. Pollinators are more affected by pesticides, while seed dispersers are more affected by habitat loss and hunting.
Another difference is that the decline of pollinators has received more public and political attention because their absence directly affects food production. The impacts of seed disperser loss, on the other hand, are more difficult to measure and influence biodiversity and carbon storage over time.
"Both are important and should be taken into account in restoration and conservation projects. 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," Galetti explained to Agência FAPESP.
The scientist says that the economic costs of the decline of seed dispersers – such as the loss of carbon storage, the reduced supply of forest products, and the decline in natural resilience to extreme environmental events – have not yet been quantified globally. "Restoration isn't just about planting trees; you have to consider who will maintain the future of that forest, which are the dispersing animals. 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," he says.
In the article, the researchers highlight that new syntheses and data models are capturing large-scale functional changes and helping to reveal long-term impacts, such as impaired recovery from forest fires and degraded animal habitats. "Addressing the decline of seed dispersers is critical to preserving animal biodiversity, ensuring forest connectivity, and balancing plant communities," says Galetti.
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