Environmental Engineering Reference
In-Depth Information
speculation translated into values whose errors must remain unknown. What is clear
in this specii c case is that Barnosky (2008) had substantially underestimated the
total mass of megafauna during the past millennium while overestimating the mass
of humanity (by assuming average body weight of 67 kg during the past 500 years).
Similarly uncertain is the attempt by Doughty and Field (2010) to estimate the total
mass of vegetation that was liberated by the extinction of Pleistocene megaherbi-
vores and then gradually appropriated by humans: their calculation shows about
1.4 Gt C (put with an unlikely accuracy at 2.5% of the global terrestrial net primary
production, or NPP) and they estimated that humans began to harvest that much
phytomass only sometime during the seventeenth century.
Alroy (2001) limited his simulation to the Pleistocene-Holocene time as he
assumed slow human population growth rates, random hunting, and low maximum
hunting effort to model megafaunal extinction; his prediction of its median occur-
rence was about 1,200 years after the initial invasion by humans, a period roughly
corresponding to the dates for the earliest Clovis artifacts and the demise of the
North American megafauna. But how would his simulation fare when taking into
account the pre-Clovis migrants, and how applicable would it be in Africa, where
our species arose, where capable hunters were killing animals for millennia before
the settling of Americas—and where no megafaunal extinctions took place during
the late Pleistocene or, for that matter, for nearly another 10,000 years afterward?
Paying attention to specii cs (focusing on individual species, continents, and regions)
thus leads to a more complex understanding that does not i t any simplistic precon-
ceived model.
Some large species never made it into the late Pleistocene to be hunted down by
humans. The straight-tusked elephant was extinct in Europe perhaps as early as
40,000-50,000 years ago, and no later than about 33,000 years (Stuart 2005). Some
25,000 years of genetic decline preceded the extinction of the cave bear: neither
climate change nor hunting alone was responsible (Stiller et al. 2010). Other species
were under stress long before the late Pleistocene: Webb (2008) concluded that
Australia's increasing mid- to late Quaternary aridity had changed the distribution
of large mammals and reduced their numbers and, as a result, by the time of human
colonization the Australian megafauna had reached a precarious stage that made it
vulnerable to any level of hunting.
Ancient DNA analyses exclude humans as the drivers of musk ox decline in
Greenland (Campos et al. 2010), and the latest research on megafaunal extinctions
in Eurasia shows a remarkable degree of consensus, with little inclination to see
hunting as a decisive factor. Pushkina and Raia (2007) found that Eurasian hunters
