The Origin and Evolution of Mammals - Moodle

290 THE ORIGIN AND EVOLUTION OF MAMMALS
a herbivore to shift from its preferred food plants to
others for which it is less well adapted. Guthrie
(2003) observed a decrease in mean body size of
horses in North America 12,000 years ago, which
suggests a climatic shift had occurred.
It is hard to believe that either human activity
or environmental change alone could be the whole
explanation, and Owen-Smith (1999), for example,
explicitly combined the two. He proposed
that early humans hunted the very large herbivores
at a greater rate than their slow reproductive rates
could replace. Removal of these key species caused
a shift in the habitat from mixed open grassland
and light forest to more dense, closed-canopied forest,
so in their turn the large numbers of open area
grazers and low browsers were affected. Others
have proposed that the extinction was indeed
caused by humans, but indirectly by their effect on
the environment from the use of fire to clear areas
eg. Miller et al. 1999.
The most recent approach to the question is the
construction of computer models to test hypotheses.
Mossiman and Martin (1975) created an early model,
which pointed towards an overkill explanation, but
whose assumptions were probably unrealistically
simple (Whittington and Dyke 1984). Using what he
took to be more appropriate and detailed assumptions
about population sizes and reproductive rates
of herbivores, human population sizes, and hunting
efficiency, Alroy (2001) also found that most of the
observed pattern of large mammal extinctions in
North America could be simulated without the need
to assume any environmental changes at all, which
he took as powerful corroboration of the overkill
hypothesis. However, like all such models this one is
very sensitive to some of the values attributed to the
parameters (Brook and Bowman 2002), and suffers
from the inevitable over-simplification compared to
the real thing.
Others have attempted to test the hypotheses by
making explicit predictions. Beck (1996) considered
the Blitzkrieg version of the overkill hypothesis for
North America. If the humans had entered the
southern part of the continent from the vicinity of
present day Edmonton, and then migrated like a
‘bow-wave’ from northwest to southeast, exterminating
species of large mammals on the way, there
should be a significantly higher number of last fossil
occurrences of species in the southeast than in
the northwest segment of USA. In fact, not only did
he not find this to be so, but to a small extent the
very reverse is true. However, it is another measure
of the very intractability of the problem that this
result can quite readily be explained other than by
rejecting the hypothesis. Perhaps the movement of
the human population was not a simple bow-wave,
but included advances along some lines such as the
Pacific coast and reflexions back northwards along
other lines. Perhaps there are reasons for a systematically
biased fossil record of last occurrences,
which are not necessarily the same as sites of final
extinction.
Apart from the science of this issue being difficult,
there are also contemporary socio-political overtones.
Was the end-Pleistocene megafaunal extinction
the last of the many acts of biotic reorganisation
by unbridled Nature, or the first of the many acts of
global devastation by unconstrained Humanity?