Page 308 - Earth's Climate Past and Future

284 PART V • Historical and Future Climate Change

15-6 Did Humans Cause Megafaunal Extinctions? south of the ice sheets by high summer insolation, reduc-
tion of habitat in the cooler north by the slow retreat of
Populations of large mammals, called megafauna, the ice sheet, and unusual (no-analog) mixtures of vege-
decreased drastically late in the most recent glacial oscil- tation that developed as forests and grasslands shifted
lation. Prior to 50,000 years ago, more than 150 genera from their glacial positions to their modern locations.
of mammals larger than 45 kg (~100 pounds) existed. By Critics of this climatic hypothesis note that no
10,000 years ago, 50 or fewer genera were left. This comparable pulse of extinction occurred in any of the
interval of extinctions was unprecedented compared to 50 or so earlier deglaciations. In fact, the number of
millions of years of prior history. species that went extinct near 12,500 years ago exceeds
During the most recent glaciation, near 50,000 years the total during all of the previous 2.75 million years.
ago (within the uncertainties of the dating methods), The same basic combination of climatic conditions—
many of the larger marsupials in Australia became high summer insolation, rising CO levels, and rapidly
2
extinct within a few millennia, including various kinds melting ice sheets—had occurred during earlier deglacia-
of kangaroos and wombats and a lion, as well as non- tions without causing pulses of extinction. So the critics
marsupials such as giant tortoises and flightless birds. ask why the extinctions occurred only during this one
Just before this time, humans had first entered Australia deglaciation and not the others.
from southeast Asia, helped by the exposure of land Another vulnerability of the climatic hypothesis is
masses by lowered sea level. These people, who used the fact that so many mammals throughout the Ameri-
“fire sticks” to burn grasslands and drive game, have cas, living in environments that ranged from semiarid
been proposed as the cause of these extinctions. grasslands to rain forests, suffered the same fate. Because
The megafaunal population of the Americas was these different environments followed different climatic
very diverse until the late stages of the most recent paths during deglaciation, climate change cannot possi-
deglaciation, with a rich array of mammals in North bly explain all the extinctions.
America (Figure 15–12). Then, within an interval of a A second explanation, called the overkill hypothe-
few thousand years centered on 12,500 years ago, over sis, put forward by the paleoecologist Paul Martin, is
half of the large mammal species living in both North that human hunting caused this extinction pulse. The
and South America became extinct. The list in North immediate cause of the extinctions could have been
America includes giant mammoths and mastodons either the first arrival of humans in the Americas or the
(larger than modern elephants), horses the size of mod- first appearance of a new hunting technology or strat-
ern Clydesdales, camels, giant ground sloths, saber- egy among the people already present.
toothed tigers, and beaver as large as modern bears. Both the origin and time of arrival of the first
One explanation for this rapid pulse of extinction is humans in the Americas were once thought to have
that major climate changes at the end of the glacial maxi- been resolved. They supposedly came by land from Asia
mum created new environmental combinations to which near 12,500 years ago, during the late stages of the last
many mammals were unable to adapt. These conditions deglaciation. They crossed into Alaska over a land
included strong summer warming and drying of the land bridge in the Bering Strait exposed by the lower glacial

FIGURE 15–12 Mammals of the glacial
maximum A rich array of large mammals
lived on the North American plains prior
to the most recent deglaciation, including
several forms that became extinct: woolly
mammoths, saber-toothed tigers, and
giant ground sloths. (Courtesy of
Smithsonian Institution, Washington, DC;
painted by Jay Matternes.)