Poaching reduces the dispersal of tree seeds in tropical forests by animals, which adversely affects the climate regulation function of these forests. Without animals, many of these forests store less carbon over the long term. This was the main conclusion of a study by an international team of scientists, including Patrick Jansen, published in Nature Communications. Jansen refers to the creation of ‘empty forests’.
The loss of carbon storage is an indirect consequence of the loss of seed dispersal by larger animals, such as monkeys, which have fallen prey to poaching. Most of the tropical forests have many species of trees with large seeds that are dependent on such animals for dispersal. Without seed dispersal these trees have poor reproductive success.
The space left open by the large-seeded species is then occupied by small-seeded species that are dispersed by smaller animals such as birds and bats, and by species that are dispersed by wind. The seed dissemination of the latter species is obviously not affected by poaching. But small-seeded species produce much smaller trees on average. Forests dominated by small-seeded species therefore have less wood volume on average and store less carbon.
“We have long predicted that the loss of animals will change the composition of tropical forests dramatically,” says Jansen. “This study shows that such a change will have a negative impact on the amount of carbon that is stored in tropical forests. That is detrimental to the international climate goals.”
The researchers compared the characteristics of ten tropical forests worldwide and calculated how the aboveground biomass would change if large-seeded, animal-dispersed tree species were gradually replaced by other species. The predicted carbon loss was as high as 12%, and occurred primarily in South America, Africa and South Asia, where the forests are dominated by large-seeded, animal-dispersed tree species. In Southeast Asia and Australia, in contrast, there were few changes; these forests are already dominated by species that are dispersed by wind.
Jansen suspects there is another unexplored route along which poaching may reduce carbon storage. “Lianas also benefit indirectly from the loss of animals, because the seeds of most liana species are dispersed by the wind. But lianas produce little wood and therefore store much less carbon. If lianas become more dominant due to poaching, the carbon storage in the forests will decline sharply.” Jansen would like to investigate whether this prediction, which he made previously in Science, is correct.
According to Jansen it is sensible not only to protect trees with a view to climate regulation, but also to protect the fauna of tropical forests. “Many protected forests in the tropics seem undamaged, but in reality they are badly damaged because the fauna has been eradicated. These ‘empty forests’ will gradually transform entirely.”
It will take a long time before the transformation is completed, as many tree species in tropical forests can live for hundreds of years. “These transformations are so slow that they are detectable only by long-term monitoring,” explains Jansen, who is also involved in global forest monitoring projects of the Smithsonian Tropical Research Institute. “But this slowness makes the transformations no less important”.