I’ve just read a well-planned and lateral-thinking paper in Nature Communications that I think readers of CB.com ought to appreciate. The study is a simulation of a complex ecosystem service that would be nigh impossible to examine experimentally. Being a self-diagnosed fanatic of simulation studies for just such purposes, I took particular delight in the results.
In many ways, the results of the paper by Osuri and colleagues are intuitive, but that should never be a reason to avoid empirical demonstration of a suspected phenomenon because intuition rarely equals fact. The idea itself is straightforward, but takes more than a few logical steps to describe:
- Tropical plant species, and mainly trees, depend heavily on animals to disperse their seeds.
- The biggest tree species tend to have the biggest seeds.
- Animals tend to prefer bigger seeds.
- The loss of animals (= ‘defaunation’) through hunting or by deforestation/fragmentation can weaken seed dispersal.
- Thus, defaunation can cause tree recruitment to shift towards smaller-seeded, animal dispersed or abiotically (e.g., wind) dispersed species.
- The volume of wood in a forest therefore declines as defaunation progresses.
- As wood volume declines, so to does its carbon content.
- Thus, defaunation reduces the amount of carbon a forest can sequester, thus exacerbating anthropogenic climate change.
Simple — yes. Easy to demonstrate — no.
You can appreciate that to show this phenomenon experimentally would be very challenging, especially at scales relevant to the global carbon cycle. However, Osuri and colleagues solved the problem through a clever simulation study that mimicked the change in tree species composition following putative animal declines.
Another cool finding was that because the proportion of animal-dispersed species varies from tropical forest to tropical forest, the carbon sequestration decline from defaunation was in fact much higher in tropical forests of the Americas, Africa and South Asia, whereas because more species are abiotically seed-dispersed in Southeast Asia and Australia, the sequestration decline was much less (or even non-existent).
Most tropical forests are facing major defaunation as we speak from hunting, habitat fragmentation, selective logging and other human disturbances, so this is a real worry. In fact, the Americas, Africa and South Asia account for 3/4 of all tropical forests combined, meaning that defaunation alone (and the changing tree species composition this implies) could equate to 14 years’ worth of Amazon deforestation (in carbon terms) alone.
Another nail in the coffin of doubt that ecosystem erosion is bad for humanity.
CJA Bradshaw
