The “Too-Few Elephant Problem”: Quantitative Analysis of Historical Elephant Tree-Toppling Rates Across Africa

Abstract

Elephants were a major driver of tree stem mortality in Africa, but their impacts are now spatially constrained and they no longer topple trees across the continent. Thus, most African landscapes lack a key ecological agent that helped maintain diversity and functioning. Even ecologists living in Africa fail to anticipate the rampant destruction of trees that can occur when elephants live in and move through landscapes. If we can build in a consensus on the degree of above-ground tree turnover that is inherent in different contexts, it would inform restoration projects and ecological definitions of degradation. We collated empirical data on treefall in ecosystems with and without elephants and predicted past tree loss spatially. The metrics used (%trees or #trees ha −1 ) reveal assumptions about the processes controlling elephant damage. On average elephants knock down 5.4 [IQR 3.0–8.3] % of the trees in an ecosystem per annum. Mapping this into places that no longer contain elephants reveals the above-ground biomass turnover that would have been the norm for millennia before elephants were widely exterminated. On average, this is ~ 1/3 of woody biomass production but at a local scale it can often exceed production. There is an urgent need for better theory on elephant tree interactions, and restoration projects should embrace the rapid turnover dynamics these ecosystems evolved with.

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