How fruit-eating birds could help regrow tropical forests

Tropical fruit-eating birds are so much more than just eye candy. These wildly colored avians are also a vital part of regenerating tropical forests. Data gathered on the ground in the Atlantic Forest of Brazil indicates that if birds like the red-legged honeycreeper, palm tanager, and toco toucan can move around more freely, carbon storage can increase by up to 38 percent. The findings are detailed in a study published April 15 in the journal Nature Climate Change

A crucial, but fragmented landscape

The Atlantic Forest in Brazil is one of the most biologically diverse regions in the world. It’s home to nearly seven percent of the world’s plant species and five percent of all vertebrates. This region is also one of Earth’s most fragmented tropical forests, due to deforestation, agriculture, and other human activities. Roughly 88 percent of its vegetation has been lost in the last 500 years, with only 12 percent of the original forest remaining in a patchwork of micro-forests. Many of these widely scattered forests are too far apart from one another to support bird movement.

Wild birds that eat a variety of fruits–or frugivores–can play a vital role in forest ecosystems by eating, excreting, and spreading seeds as they move around. Between 70 to 90 percent of the tree species living in tropical forests depend on seed dispersal from animals, as it allows the forests to grow and function.  

To combat this, 12 million hectares of land are targeted for restoration and natural recovery under the Atlantic Forest Restoration Pact. New data from this study is helpful for determining how to proceed. 

[Related: Three nations pledge to reverse decades of destruction in the rainforest.]

“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,” Daisy Dent, a study co-author and naturalist at ETH Zurich, a public research university in Switzerland, said in a statement. 

Bigger birds, bigger seeds

In the study, the team compared the carbon storage potential that could be recovered in landscapes with limited forest fragmentation to the more splintered landscapes. They found that the  more fragmented landscapes restricted the bird’s movement and more tree cover was needed.

According to the team, a minimum of 40 percent forest cover is critical across the Atlantic Forest region for species diversity and also maintain and restore ecosystem services needed to maximize forest restoration efforts. These ecosystem services include carbon storage and seed dispersal.

Different bird species also have differing impacts in terms of seed dispersal. 

The smaller birds can spread more seeds around, but they can only carry the smaller seeds that have lower carbon storage potential. 

Larger larger birds like the toco toucan or the curl-crested jay can disperse the seeds of bigger trees with a higher carbon storage potential. However, the larger birds are less likely to move across more highly fragmented landscapes.

“Allowing larger frugivores to move freely across forest landscapes is critical for healthy tropical forest recovery,” study co-author and ETH Zurich ecologist and biologist Carolina Bello said in a statement. “This study demonstrates that especially in tropical ecosystems seed dispersal mediated by birds plays a fundamental role in determining the species that can regenerate.”

What can be done

Preventing the poaching of tropical birds is one strategy, as more birds flying around can translate into more trees.

“We have always known that birds are essential, but it is remarkable to discover the scale of those effects,” study co-author and ETH Zurich ecologist Thomas Crowther said in a statement. “If we can recover the complexity of life within these forests, their carbon storage potential would increase significantly.”

[Related: Songbirds near the equator really are hotter, color-wise.]

Earlier studies suggest that recovering these forests could capture more than 2.3 billion tonnes of carbon. Natural regeneration could also be more cost-effective than planting more trees, but both should be done. This enhances animal movement in the areas where a more passive restoration is more likely. In highly fragmented landscapes, active restoration like planting more trees is necessary. In order for these tree planting methods to be ecologically effective, ensuring that the trees actually belong in the area and not are not being planted in grasslands is important. 

[Related: When planting trees is bad for the planet]

“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 maximize the cost-effectiveness of forest restoration,” study co-author and nature based solutions project manager Danielle Ramos, said in a statement. Ramos is affiliated with the University of Exeter in the United Kingdom and the Universidade Estadual Paulista, in Rio Claro, São Paulo, Brazil.

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