Large fruit-eating mammals and birds play a role in fighting climate change, and if we drive them to extinction, we will make a hotter world in the process, new research suggests.
It might sound like the end of "Avatar," but fruit-eating animals are guerrilla warriors (or maybe gorilla warriors?) against global warming. Their size enables them to consume and disperse large seeds, particularly those that come surrounded by juicy fruit.
Although “mighty oaks from little acorns grow”, large hardwood trees that store the most carbon tend to have bigger fruit. If forests are to continue absorbing carbon from the atmosphere these trees need a mechanism to disperse their seeds.
Not only do these furred and feathered Johnny Appleseeds distribute fruits to places far from the parental tree, they often provide essential processing for the seeds to sprout. Many fruits cannot grow until they have passed through the digestive tract of a large animal – sometimes depending on a specific species.
Professor Carlos Peres of the University of East Anglia joined a Brazilian study of the effects on seed dispersal in more than 2,000 tree species when animals become locally extinct. The results, published in Science Advances, reveal just how important this natural seed bombing is to the health of the planet.
"Large birds and mammals provide almost all the seed dispersal services for large-seeded plants. Several large vertebrates are threatened by hunting, illegal trade, and habitat loss. But the steep decline of the megafauna in overhunted tropical forest ecosystems can bring about large unforeseen impacts,” Peres said in a statement. "We show that the decline and extinction of large animals will over time induce a decline in large hardwood trees. This in turn negatively affects the capacity of tropical forests to store carbon and therefore their potential to counter climate change."
How larger animals spread fruit in a healthy forest. Credit: Peres/University of East Anglia
Smaller birds and bats also disperse seeds, but unsurprisingly only small ones, with a maximum of 12 millimeters (0.5 inches) identified in the paper. The authors found that when hunters target larger animals the consequences for long-term forest regeneration are dire. They note: “Unsustainable hunting is a worldwide problem that has increased in the last few decades over tropical forests.”
Plants with seeds above the 12mm limit represented 21 percent of the specimens studied, but hold the majority of the stored carbon in Brazil's Atlantic rainforests. “We observed a greater loss of carbon as the percentage of removed large-seeded tree species increases, as a consequence of defaunation of large frugivores,” the authors report.
“Intergovernmental policies to reduce carbon emissions from tropical countries have primarily focused on deforestation, and to a lesser extent on forest degradation resulting from timber extraction and wildfires,” said Peres.” But our research shows that a decline in large vertebrate populations and the loss of key ecological interactions also poses a serious risk for the maintenance of tropical forest carbon storage.”
As the paper points out: “Tropical forests store 40 percent of the world's terrestrial carbon and their deforestation contributes to 7 to 17 percent of global carbon emissions.”