Estimating Non Human Megafauna Biomass

In principle, megafauna biomass for a given species is calculated by multiplying the average body mass by the number of individual animals. To estimate this and produce the figures in this chapter, I used the following parameters. Average body mass values were taken from a recent compilation (Smith et al., 2003). For the few species not listed in that compilation, I used values for similarly sized animals that were listed. Number of individual animals per species was estimated in the following way. First, there is a correlation between body mass and population density, that is, individuals per km2 (Damuth, 1993; Silva and Downing, 1995; White et al., 2007). To estimate density, I used regressions from Silva and Downing (1995): for large herbivores, density = -0.44 x log(kg body mass) + 1.01; and for large carnivores, density = -1.31 x log(kg body mass) + 1.22. Second, megafauna species typically have geographic range sizes that average between 7% and 9% of the area of the continent on which they live. For Australia, I estimated the geographic range size of each megafauna species to be 7.8% of the continental area, or «600,000 km2 (Murray and Dickman, 2000). For Africa, Eurasia, North America, and South America, geographic ranges sizes were set to 8.6%, 8.1%, 8.2%, and 7.2% of the respective continental areas (Letcher and Harvey, 1994; Smith et al., 1994; Brown, 1995). For each species, estimated density was multiplied by estimated geographic area to give an approximate number of individuals, which was then multiplied by estimated mass per individual.

Continental area was not constant through the time spanned by the QME, because during glacial times, nearly one-half of North America, about one-tenth of Europe and northern Asia, and a small percentage of South America were covered by glaciers. This loss of land was offset only to a very small extent by the exposure of currently submerged land with the lower sea level of glacial times. To account for varying continental area in the estimates of geographic range size, continental area during glacial times was considered to be 50% of its current size for North America, 90% of its current size for Europe and northern Asia, and 95% for South America. For the transitional time «10 kyr B.P., area for these continents was set at the intermediate values of 75%, 0.95%, and 98% of their current respective sizes.

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