Florian Hartig

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Defaunation of large-bodied frugivores reduces carbon storage in a tropical forest of Southeast Asia

Wirong Chanthorn, Florian Hartig, Warren Y. Brockelman, Wacharapong Srisang, Anuttara Nathalang & Jantima Santon

Scientific Reports, 9(1), 10015 (2019)
Cite this
@article{chanthorn2019defaunation,
  author = {Wirong Chanthorn and Florian Hartig and Warren Y. Brockelman and Wacharapong Srisang and Anuttara Nathalang and Jantima Santon},
  title = {Defaunation of large-bodied frugivores reduces carbon storage in a tropical forest of Southeast Asia},
  journal = {Scientific Reports},
  volume = {9},
  number = {1},
  pages = {10015},
  year = {2019},
  doi = {10.1038/s41598-019-46399-y},
}

DOI: 10.1038/s41598-019-46399-y
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Funding

This study was supported through funding by National Science and Technology Development Agency (Thailand).

View article (DOI)

Abstract

Recent studies have suggested that defaunation of large-bodied frugivores reduces above-ground carbon storage in tropical forests of South America and Africa, but not, or less so, in Southeast Asian tropical forests. Here we analyze the issue using the seed dispersal network (data of interaction between trees and animal seed dispersers) and forest composition of a 30-ha forest dynamics plot in central Thailand, where an intact fauna of primates, ungulates, bears and birds of all sizes still exists. We simulate the effect of two defaunation scenarios on forest biomass: 1) only primates extirpated (a realistic possibility in near future), and 2) extirpation of all large-bodied frugivores (LBF) including gibbons, macaques, hornbills and terrestrial mammals, the main targets of poachers in this region. For each scenario, we varied the population size reduction of the LBF dispersed tree species from 20% to 100%. We find that tree species dependent on seed dispersal by large-bodied frugivores (LBF) account for nearly one-third of the total carbon biomass on the plot, and that the community turnover following a complete defaunation would result in a carbon reduction of 2.4% to 3.0%, depending on the defaunation scenario and the model assumptions. The reduction was always greater than 1% when the defaunation intensity was at least 40%. These effect sizes are comparable to values reported for Neotropical forests, suggesting that the impact of defaunation on carbon deficit is not necessarily lower in Southeast Asian forests. The problem of defaunation in Asia, and the mutual benefits between biodiversity conservation and climate change mitigation, should therefore not be neglected by global policies to reduce carbon emissions.

What the paper shows and why it matters

(AI-generated)

Hunting removes large animals across the tropics, and with the fruit-eaters that carry tree seeds a forest may lose the tree species that depend on them. Several earlier studies predicted that this lowers carbon storage, which the paper traces to such trees tending to have dense, carbon-rich wood, and drew consequences for climate policies such as REDD+ (the UN scheme linking forest carbon with biodiversity conservation). For Southeast Asia, earlier studies had found little such effect. A possible explanation is that dipterocarps, a common regional tree family with wind-dispersed seeds, could make up for the loss (Osuri et al. 2016). The paper takes on the assumption that this buffer holds for Southeast Asian forests in general, since not all are dominated by dipterocarps.

The study argues that the carbon loss from losing large fruit-eating animals is not necessarily smaller in Southeast Asian forests than in Neotropical ones, and suggests that a low share of dipterocarps, the dense-wooded, wind-dispersed trees that could compensate, may be a key factor. It examines the question in the 30-ha Mo Singto forest plot in Thailand, where hunting has been low for decades and dipterocarps make up only 13% of tree basal area (trunk cross-section). Tree species that depend on large-bodied fruit-eaters hold nearly one third of the plot’s above-ground carbon and have significantly higher wood density than other species. Simulating their loss at intensities from 20 to 100%, with the remaining species taking their place, the study finds that complete loss lowers above-ground carbon by roughly 2 to 3%, depending on scenario and model assumptions. It reads most of this as an effect of wood density and finds it similar in size to Neotropical estimates.

Figure 3. Maximum size and wood specific gravity: compared between trees species dispersed by all large-bodied frugivores (All large) versus the other agents (Others; left panels), and compared between primate-dispersed species (Primates) versus species dispersed by terrestrial mammals, birds and other large dispersers (TBO; right panels). From Chanthorn et al. (2019), Scientific Reports, licensed under CC BY 4.0. Click the image to enlarge.

The paper therefore advises that REDD+ and forest restoration projects take fauna and seed dispersal into account. It calls the assumption that the remaining species take over all roles of the lost ones rather optimistic, with stronger losses possible if they do not. Limits remain: the estimate comes from one plot and assumes a community in equilibrium, so how far it carries to other Asian forests is open.

References

  • Osuri et al. (2016). Contrasting effects of defaunation on aboveground carbon storage across the global tropics. Nature Communications 7, 11351. doi:10.1038/ncomms11351