Big animals have big impacts on tropical forest carbon storage

Over-hunting is driving several species towards extinction. But the decline in wildlife populations affects more than just the animals in question. It can alter the carbon storage potential of tropical forests, a new study has found.

Trees in tropical forests often depend on wild animals to disperse their seeds. Animals such as elephants, tapirs, and monkeys eat and chew on fruits, defecating the seeds away from the original tree. Sometimes, seeds simply stick to an animal’s body and spread across the forest. In fact, the fate of many tree species is intricately linked to that of the wild animals in the forest.

So the loss of big animals can drive the decline of tree species that depend on them for dispersal of their seeds, in turn changing a forest’s composition. Since trees pull out carbon dioxide from the atmosphere during photosynthesis, locking the carbon into their wood, a shift in tree species (such as larger trees getting replaced by smaller trees) can then affect how much carbon the forest can store.

A new study published in Nature Communications has found that the loss of large wildlife — called “defaunation” — can reduce the above-ground carbon storage of many tropical forests in the world.

“Overhunting and the loss of fauna impact forests in many ways,” William Laurance, a Distinguished Research Professor at James Cook University, told Mongabay. “We’re not just losing pretty animals; we’re changing the forest itself in pervasive ways and degrading one of its most important ecosystem services—its capacity to store lots of carbon and thereby reduce the effects of global warming.”

To find out how defaunation affects the carbon-storage potential of tropical forests, an international team of researchers examined a dataset of over 25,000 trees belonging to about 2,500 species.

Unlike previous studies that have analyzed the impact of defaunation on carbon storage in specific countries like South America, this study analyzes a pan-tropical dataset, lead-author Anand Osuri of National Centre for Biological Sciences (NCBS) in Bangalore, India, told Mongabay. This includes sites in Tanzania, Cameroon and Congo in Africa, India in South Asia, Peru, Panama and Costa Rica in the Americas, Indonesia and Malaysia in Southeast Asia, and Australia.

For each site, the team looked at different scenarios of defaunation. They simulated the removal of 25 to 100 percent of large-seeded, animal-dispersed tree species, and replaced them with trees that depend on other modes of dispersal such as smaller animals or abiotic agents (like wind or water).

The team then compared changes in the size and wood densities of trees in the defaunated forests with that of trees in the original forests, and assessed how these changes translated to the loss or gain of carbon in the trees.

“A study of this nature can only be achieved after many years of data collection on the biology of plant species and their seed dispersers,” Laurance said. “The authors have done a very nice job of integrating a great deal of earlier research—and without that earlier research they could never have pulled off such a sweeping and impressive analysis.”

The analysis revealed some surprising results, team members say.

In African, American and South Asian tropical forests, which are dominated by tree species that depend on animals to disperse their seeds, the loss of large wildlife led to a marked reduction in the forests’ carbon storage potential, the study found.

For example, the removal of 50 percent or half of all large-seeded, animal-dispersed tree species in these forests resulted in an average 2 percent decrease in the above-ground carbon storage. The amount of carbon that would get released into the atmosphere due to the decline of these trees is equivalent to 14 years’ worth of Amazonian deforestation, the authors write.

In contrast, the team found that in Southeast Asian and Australian tropical forests, which are dominated by wind-dispersed tree species, declines in large wildlife did not reduce the amount of carbon stored by the forests significantly, even increasing carbon-storage in some places.

“Seeing these differences play out in this manner brings the message home sharply: all tropical forests are not the same,” co-author Jayshree Ratnam of NCBS told Mongabay. “While we all know and love tropical forests for their astonishing diversity, studies like these remind us that functional responses of these forests may often be driven by a few key species.”

The difference in carbon storage, the team found, is driven by the species that replace the large-seeded, animal-dispersed species when they are removed from the forests.

When large-seeded, animal-dispersed species decline in tropical forests of Americas, Africa and South Asia, which have fewer abiotically dispersed species, they get replaced by smaller trees. This in turn drives declines in stand volume and carbon stocks in the forest, the authors write.

But in regions like southeast Asia and Australia where large, abiotically dispersed species are more abundant (such as the Dipterocarp forests of Southeast Asia), loss of large-seeded, animal-dispersed species can further increase the abundance of these large, wind or gravity-dispersed species. This can potentially maintain or even increase stand volumes and carbon storage, the authors add.

“It is important to understand these inter-regional differences, which could then be used to frame more nuanced policies, rather than treat all tropical forests alike,” co-author Varun Varma of NCBS said.

Protecting only the forests, and not the seed-dispersing animals that inhabit the forests, can reduce the potential for tropical forests to regulate climate, he added. “Conserving seed dispersing animals should go hand-in-hand with conserving tropical forests per se.”

Citation:

  • Osuri AM, et al (2016) Contrasting effects of defaunation on aboveground carbon storage across the global tropics.Nature Communications. DOI: 10.1038/ncomms11351.

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