The frugivorous Rufous-bellied Thrush (Turdus rufiventris) is also an active seed disperser.
Photo: © Mathias Pires

Tropical forests can't recover naturally without fruit-eating birds

Natural forest regeneration is hailed as a cost-effective way to restore biodiversity and sequester carbon. However, the fragmentation of tropical forests has restricted the movement of large birds limiting their capacity to disperse seeds and restore healthy forests.

New research from the Crowther Lab at ETH Zurich in Switzerland illustrates a critical barrier to natural regeneration of tropical forests. Their models – from ground-based data gathered in the Atlantic Forest of Brazil– show that when wild tropical birds move freely across forest landscapes, they can increase the carbon storage of regenerating tropical forests by up to 38 per cent.

Fruit eating birds such as the red-legged honeycreeper, the palm tanager or the rufous-bellied thrush play a vital role in forest ecosystems by consuming, excreting and spreading seeds as they move throughout a forested landscape. Between 70 and 90 per cent of the tree species in tropical forests are dependent on animal seed dispersal. This initial process is essential for allowing forests to grow and function.

While earlier studies have established that birds are important for forest biodiversity, researchers at the Crowther Lab now have a quantitative understanding of how they contribute to forest restoration.

The new study, published in the journal Nature Climate Change, provides evidence of the important contribution of wild birds (frugivores) in forest regeneration. Researchers compared the carbon storage potential that could be recovered in landscapes with limited fragmentation with that of highly fragmented landscapes. Their data shows that highly fragmented landscapes restrict the movement of birds, thereby reducing the potential of carbon recovery by up to 38 per cent. Across the Atlantic Forest region in Brazil, the researchers found that it is critical to maintain a minimum of 40 per cent forest cover. They also observed that a distance of 133 metres or less between forested areas ensures that birds can continue to move throughout the landscape and facilitate ecological recovery.

Furthermore, the study also found that different bird species have different impacts in terms of seed dispersal. Smaller birds disperse more seeds, but they can only spread small seeds from trees with lower carbon storage potential. In contrast, larger birds such as the toco toucan or the curl-crested jay disperse the seeds of trees with a higher carbon storage potential. The problem is that the larger birds are less likely to move across highly fragmented landscapes. "This crucial information enables us to pinpoint active restoration efforts – like tree planting – in landscapes falling below this forest cover threshold, where assisted restoration is most urgent and effective," says Daisy Dent, a lead scientist in the Crowther Lab at ETH Zurich.

Restoring functioning ecosystem services

Based on current data, this study advances the research from previous ground studies conducted by the authors in the Atlantic Forest in Brazil. The forest is one of the most biologically diverse regions in the world, but it is also one of the most fragmented, with only twelve per cent of the original forest remaining, chiefly mainly in small areas.

The forest is also one of the most important regions on the planet for large-scale ecological restoration, with twelve million hectares of land targeted for restoration and natural recovery under the Atlantic Forest Restoration Pact. The research shows that increasing forest cover beyond 40 per cent may be critical not only to maintain species diversity, as previously evidenced, but also to maintain and restore the functioning of ecosystem services, such as seed dispersal and carbon storage, to maximise the success of the massive-scale restoration initiative in this region.

Strategies for recovering tropical forests

Earlier research suggests that recovering forests could capture more than 2.3 billion tonnes of carbon in the Atlantic Forest region, and that natural regeneration is likely to be more cost-effective – as much as 77 per cent less in implementation costs – than active planting.

"By identifying the thresholds of forest cover in the surrounding landscape that allow seed dispersal, we can identify areas where natural regeneration is possible, as well as areas where we need to actively plant trees, allowing us to maximise the cost-effectiveness of forest restoration,” says Danielle Ramos, a co-author of the paper affiliated with the University of Exeter, UK and Universidade Estadual Paulista, Rio Claro, São Paulo, Brazil.



Bello C, Crowther TW, Ramos DL, Morán-López T, Pizo MA, Dent DH: Frugivores enhance potential carbon recovery in fragmented landscapes, Nature Climate Change, 15th April 2024

More information:

Link to Crowther Lab- University of Zurich

Link to Atlantic Forest Resorqtion Pact

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