Environmental Engineering Reference
In-Depth Information
a swift and massive killing of large animals by an advancing wave of migrating
hunters. The rarity of megafaunal kill sites (just 14 of them have been found in
North America) is explained by an extremely low probability of the bones being
preserved in unprotected upland environments (Fiedel and Haynes 2004). If humans
were solely responsible for this rapid and massive extinction, it would have been
the most remarkable case of preagricultural destruction of biomass, comparable
only to the effects of deliberately used i re.
The principal arguments for the overkill hypothesis are the well-known effects
of even relatively low but persistent rates of killing of slow-breeding animals that
give birth to a single offspring after a long gestation (8 months for elks, 11 months
for horses, and 22 months for elephants). Fiedel (2005) also argues that dogs intro-
duced during the Paleoindian colonization of the Americas had more than one role
to play in megafaunal extinction. Genetic evidence indicates that wolves were not
domesticated i rst in sedentary Mesolithic societies of the Middle East but less than
16,300 years ago from several hundred wolves in southern China (Pang et al. 2009).
The two main competing explanations of megafaunal extinction invoke various
environmental hypotheses, ranging from catastrophic encounters with extrater-
restrial bodies and rapid changes in temperature to gradual fragmentation or a
complete loss of preferred habitats, usually attributable to climate change, and a
hypervirulent paleodisease. Ascribing the extinctions to a “hyperdisease” (MacPhee
and Marx 1997; Lyons et al. 2004) is not convincing. Such a pathogen would have
to survive in a carrier species without harming it; it would have to be sufi ciently
virulent to infect large numbers of animals; it would have to cause very high (more
than 50%) death rates; and the epizootic would have to able to infect species as
different as mammoths and tigers without ever causing a universal epizootic. No
virus, bacterium, or fungus can meet all of these requirements. And the proponents
of the overkill hypothesis also dismiss climate change as the main cause of extinc-
tion because such a major species loss did not take place during one of the many
similar climatic reversals that marked the Pleistocene.
After more than half a century of debate, the “overkill” controversy remains
unresolved. Brook and Bowman's (2004) simulations of megafaunal extinction on
a medium-sized land mass found that median times to extinction were less than 800
years, coni rming that the late Pleistocene human colonizations had “almost cer-
tainly” triggered a “blitzkrieg” of killing. But Waters and Stafford (2007) revised
the Clovis time range to a surprisingly brief span between 13,300 and 12,800 years
ago that overlaps non-Clovis sites in both Americas and implies that humans inhab-
ited America before Clovis. This conclusion was buttressed by Thomas et al. (2008).
