Colloquium paper: Megafauna biomass tradeoff as a driver of Quaternary and future extinctions

Abstract

Earth's most recent major extinction episode, the Quaternary Megafauna Extinction, claimed two-thirds of mammal genera and one-half of species that weighed >44 kg between approximately 50,000 and 3,000 years ago. Estimates of megafauna biomass (including humans as a megafauna species) for before, during, and after the extinction episode suggest that growth of human biomass largely matched the loss of non-human megafauna biomass until approximately 12,000 years ago. Then, total megafauna biomass crashed, because many non-human megafauna species suddenly disappeared, whereas human biomass continued to rise. After the crash, the global ecosystem gradually recovered into a new state where megafauna biomass was concentrated around one species, humans, instead of being distributed across many species. Precrash biomass levels were finally reached just before the Industrial Revolution began, then skyrocketed above the precrash baseline as humans augmented the energy available to the global ecosystem by mining fossil fuels. Implications include (i) an increase in human biomass (with attendant hunting and other impacts) intersected with climate change to cause the Quaternary Megafauna Extinction and an ecological threshold event, after which humans became dominant in the global ecosystem; (ii) with continued growth of human biomass and today's unprecedented global warming, only extraordinary and stepped-up conservation efforts will prevent a new round of extinctions in most body-size and taxonomic spectra; and (iii) a near-future biomass crash that will unfavorably impact humans and their domesticates and other species is unavoidable unless alternative energy sources are developed to replace dwindling supplies of fossil fuels.

Fig. 1.

Body-size distribution of mammals in North America. The black bars illustrate the distribution of species that went extinct in the QME. Note that humans are at the lower end of the distribution for species that went extinct. Illustration modified from ref. ; see that source for similar distributions of fauna from other continents.

Fig. 2.

Number of non-human megafauna species that went extinct through time plotted against estimated population growth of humans.

Fig. 3.

Estimated biomass of humans plotted against the estimated biomass of non-human megafauna. See Methods for parameters used.

Fig. 4.

Change in the sum of human and non-human wild megafauna biomass through time. The brackets indicate when extinction pulses hit the respective geographic areas. See Methods for parameters used.

Fig. 5.

Semilog plot of the sum of human and non-human wild megafauna (dots) and the sum of human, wild, and domestic megafauna (triangles connected by line). Yellow bar indicates the timing of the YD-Holocene climatic event that led into the current interglacial. See Methods for parameters used.