Environmental Engineering Reference
In-Depth Information
This has not prevented Smith, Elliott, and Lyons (2010) from assuming that about
100 million North American herbivores were eradicated by hunting. But their esti-
mate was made without any reference to the total size of the continent's human
population. Thanks to genetic mapping, our estimates of prehistoric populations
have become better constrained, but the specii c numbers of people that inhabited
a given region at any particular prehistoric time—and much more so the fractions
of these populations that were highly dependent on hunting large animals more than
10,000 years ago—remain only poorly constrained guesses. And if hunting was only
a contributory factor, then we would have to apportion the relative importance of
individual causes whose contribution hardly remained static for hundreds or thou-
sands of years of the period under consideration: this is a task that our knowledge
makes impossible, unless it is a pure guess. Nevertheless, some order-of-magnitude
calculations give an indication of plausible impacts (box 7.2).
As the Pleistocene diet was dei nitely far from purely carnivorous, and as a non-
negligible share of animal food came not only from smaller megafaunal species but
also from small animals, the total number of large mammals killed annually by
hunters could have been less than half that number—or twice that number if the
body mass composition of kills was bimodal (including larger numbers of smaller
animals). For comparison, the recent slaughter of cattle, the world's most common
domesticated megafaunal species, has been about 300 million animals annually;
with an average body mass of some 350 kg, this amounts to just over 100 Mt of
fresh weight and about 60 Mt of carcass weight.
But even if late Pleistocene hunting had been an important (or, for the sake of
argument, dominant) cause of megafaunal extinctions, its overall impact on the
productivity of the biosphere would have been overwhelmingly positive. The disap-
pearance of so many species of megafauna, including herbivores more massive than
the surviving elephants (mostly between 5 and 8 t for adult mammoths, vs. 3-5 t
for most adult African elephants), had profound consequences for the composition
and primary productivity of affected ecosystems. As attested by studies of extant
African megafauna, both grazers (animals that eat grasses and forbs) and browsers
(those that eat primarily woody plants) profoundly affect plant structure, with
elephant browsing having a particularly strong negative impact on the total tree
cover, and this effect appears to increase synergistically in the presence of i re
(Guldemond and Van Aarde 2008; Holdo, Holt, and Fryxell 2009).
Pleistocene megaherbivore browsers, feeding on woody phytomass and invulner-
able to predation, created open, grassy landscapes by destroying trees and suppress-
ing their regrowth (Zimov et al. 1995). The removal of trees by massive browsers
