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biodiversity losses. The energetic constraints that underlie the biomass tradeoff mean that, as human biomass grows, the only way other species can persist is through conscious stepped-up efforts to save them, by such actions as setting aside reserves, enforced protection of existing reserves, and efficient and sustainable food-production practices. It is particularly urgent to act upon the knowledge that the high level of megafauna biomass today, which means humans, can be sustained only by developing alternative energy resources to replace the dwindling supply of fossil fuels.

METHODS

Dating Conventions

Unless otherwise noted, dates are expressed as calendar years before present (kyr B.P.).

Timing of Extinctions on Each Continent

I used supporting information table 1 in Koch and Barnosky (2006) to place the extinction of each megafauna species in one of the following temporal bins: <100 kyr B.P., 100–50 kyr B.P., 50–15.5 kyr B.P., 15.5–11.5 kyr B.P., and 11.5–0.5 kyr B.P. The latter bin is cut off at 500 years ago to exclude recent extinctions. As far as is known, before 500 years ago, the last megafauna species extinction was 3,000 years ago. Despite being somewhat coarse, these bins are adequate to examine the biomass tradeoff at the order-of-magnitude level of resolution to which the rest of the data are appropriate.

Estimating Human Biomass

Hern (1999) provided estimates of the numbers of hominins on Earth from the approximate first appearance of Homo habilis some 3 million years ago up to the number of H. sapiens projected to occur in approximately the year 2455. His estimates, based on calculating doubling times for hominin and human populations, were constrained by the fossil record of human evolution, by archaeological information, and by historical and demographic records up to 1999. I used his estimates for the numbers of people on Earth at a given point in time and multiplied that by the average weight of a person to estimate global human biomass for each time slice of interest. Following logic detailed in Hern (1999), average weight for a human was considered 50 kg up to ≈400 years ago, and 67 kg thereafter.



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