pigs, goats, sheep, and cattle were first domesticated (Pedrosa et al., 2005; Beja-Pereira et al., 2006; Chen et al., 2006; Fernandez et al., 2006; Larson et al., 2007). For time slices up to 6 kyr B.P., only pigs, goats, and cattle were included in the domestic livestock count. More recent time slices also included horses, buffalo, camels, chickens, ducks, turkeys, and catfish. Clearly for prehistoric times, this method provides an overestimate of domestic stock biomass, because no one would argue that the first ranchers had as many domestic stock per person as is the case presently. However, because the purpose of this part of the analysis was to see whether domestic stock compensated for a reduction in wild megafauna, the overestimation actually makes the conclusions more robust.
Sensitivity tests were conducted to assess how robust the general trends were to varying assumptions about density and geographic range size, on which the calculated biomass value for each species depends. Calculated density was varied by applying the regression equation for large carnivores (Silva and Downing, 1995) to the whole dataset at one extreme (results in least biomass), by applying the regression equation for large herbivores (Silva and Downing, 1995) to the whole dataset at the other extreme (results in most biomass), and by applying an average density equation to all species, density = −0.77 × log(g body mass) + 3.98 (Damuth, 1993). One test also assumed a 10% increase in density of the megafauna that survived after the QME. Assumed geographic range size for each species was variably set between ≈9% and 5% of the area of the continent on which the species lived. Varying these parameters does not alter the general trend of biomass change through the QME. Varying them does affect the absolute values calculated for biomass and the amount of time indicated for biomass recovery but not in ways that obviate the main conclusions of this chapter.
Methods used here are intended to give simply an order-of-magnitude indication of how biomass changed through time and identify times of major biomass crash and recovery. The calculations are necessarily coarse. Exact values change given different inputs to the estimations, but the sensitivity tests make it seem unlikely that the important trends are simply estimation artifacts. Additional refinements would be desirable but are beyond the scope of this initial work. Such refinements ideally would include body mass vs. density regressions tailored to each species, refining the geographic range estimates for each species through niche model-