The "elephant-effect" on the rainforests of central Africa
Forest elephants are one of the largest terrestrial animals on the planet but are shrouded in mystery as they inhabit the impenetrable rainforests of central Africa. We described the key ecological role of forest elephants in shaping African rainforests and enhancing carbon stocks.
Our results show that forests elephants (Loxodonta cyclotis), by reducing the number of small trees, modify forest conditions allowing trees to grow larger and favoring "hard wood" trees which store more carbon. These results imply that large herbivores play an important role in long-term rainforest dynamics. Across central Africa, the “elephant-effect” enhances aboveground carbon stocks by 3 billion tonnes. Indirectly, elephants contribute to reduce atmospheric CO2, and help us combat global warming. However, the collapse of forest elephant populations, mainly caused by ivory poaching, is depriving us of a formidable ecosystem engineer, which is also important for distributing nutrients and planting seeds of future tree generations. The conservation of this species is partly hampered by its classification as a sub-species of the African elephant (Loxodonta africana). Our results bring further evidence of the important and unique role of forest elephants in central African rainforests.
The story behind the paper: When I arrived at the University of Tuscia for my PhD, word preceded me about the “elephant guy”, and it was not because of my weight 😊. The University, situated in Viterbo near Rome, was probably the last place on earth where someone would go to study forest elephants. The PhD program coordinator strongly suggested to “forget about the elephants” and work on another departmental project which was fully-funded. I politely refused, causing a complicated internal reshuffle that resulted in me having not one, but two PhD supervisors, who were kind enough to solve the problem. Not the smoothest start for my PhD…
My initial interest was on the long-term effects of seed dispersal by forest elephants. Having limited funding, I couldn’t go to the Congo to observe elephants and collect data, so I had to find some alternatives. Luckily, Stephen Blake and François Bretagnolle, who had studied elephants in different sites in the Congo Basin, provided their precious data, but I could not find how to use it for my research question. After I visited François in France, I was still struggling and had to finish my PhD thesis in less than a year. My gut instinct was telling me to follow an idea Christopher Doughty and I discussed much earlier in Oxford...
Christopher told me about the differences between Amazonian and African rainforests, the latter having fewer but larger trees, and on average higher aboveground biomass. The hypothesis was that large herbivores, which have disappeared from Amazonia more than 10,000 years ago but still present in Africa, are contributing to these differences by removing small trees and reducing the competition for resources among plants. Could we test if this disturbance caused by large animals would allow the remaining trees to grow larger and hold more biomass? The data I had acquired was better suited for this question as it included sites with and without elephants, so I had a contrasting gradient of elephant disturbance. However, I needed to use a forest model to simulate the long-term effects (>100-1000 years) of elephant disturbance. An opportunity came up to spend a few months in Brazil with Marcos Longo, one of the main developers of the Ecosystem Demography forest model I wanted to use. Marcos and I developed a method to simulate elephant disturbance in Ecosystem Demography and obtained exciting results. We than used our field data to corroborate and validate the model results. This was not without some difficulties, as we had some heated (but respectful!) internal discussions, which caused some anxiety for the PhD student (i.e., me), but ultimately improved the paper. This is what happens when you throw in the same room field ecologists with ecological modelers!
Cover-photo credit: Markus Lilje, some rights reserved (CC BY-NC-ND). Source: iNaturalist, https://www.inaturalist.org/photos/8194565