Encyclopedia of Evolution.pdf - Online Reading Center

population
Percent of Mammal Genera Becoming Extinct
upon Arrival of First Humans (from Barnosky)
Continent Extinction
Africa 18%
Australia 88%
Eurasia 36%
North America 72%
South America 83%
eradication of elephant birds and giant lemurs by the first
immigrants to Madagascar, and the eradication of passenger
pigeons and near-eradication of bison by white Americans.
Difficulties have been noted, and some responses formulated,
regarding this “Pleistocene overkill hypothesis”:
• In North America, the arrival of humans was not the only
thing that was happening 11,000 years ago. The most
recent Ice Age was also ending, accompanied by rapid climatic
changes. Defenders of the overkill hypothesis point
out that the climate was becoming warmer, which should
have encouraged the survival, not the extinction, of animals.
The abundance and diversity of animals during the
Ice Ages, however, suggests that a warmer climate was not
necessarily an improvement for these animals. Any climatic
change that would have altered the food base for these animals
may have undermined their populations.
• Some large mammal species were already in decline by the
time humans arrived, for example the large bison of North
America.
• Evidence is lacking for massive kills of large mammals.
Arrowheads have been found in mammoth bones, but only
occasionally, and mass grave sites of large animals are very
uncommon.
• Native Americans, on whom the Pleistocene overkill in
the New World is blamed, do not have a mass slaughter
approach to hunting. In fact, some Native American traditions
are famous for spiritual preparation and gratitude
before killing an animal, as well as careful use of the animal.
However, the Native Americans that the Europeans
encountered beginning just before the 16th century were
perhaps very different from those that arrived in America
at the end of the last Ice Age. In particular, Native Americans
may have developed a conservation ethic in response
to the collapse of the hunting stock caused by their careless
ancestors.
Paleontologist Anthony Barnosky performed what may
be the most thorough analysis of the Pleistocene extinctions
to date. He confirmed that large mammals died in disproportionately
large numbers, and at times that corresponded
(within a thousand years) to human arrival. He concluded
that neither climate change nor overhunting could by themselves
have caused the extinctions. Together, however, they
could have produced the Pleistocene extinctions. It would
not have been necessary for either climate or hunters to have
killed every last animal. Once a population becomes sufficiently
small, it becomes vulnerable to extinction. Overhunted
animal populations were vulnerable to extinction
from climate changes, and climate changes may have reduced
animal populations to the point that hunting could send them
spiraling to oblivion.
As the Earth enters into what many scientists consider
the sixth of the mass extinctions as a result of human activity,
the lessons of the Pleistocene extinctions may be essential.
Now, as then, it is not necessary to kill many animals in
order to drive them to extinction. At the end of the Pleistocene,
climate change reduced their habitats and made them
vulnerable to hunting. Today, habitat destruction for agriculture
and human habitation is the principal threat to the survival
of most threatened species (see extinction).
Further Reading
Barnosky, Anthony D. “Assessing the causes of late Pleistocene
extinctions on the continents.” Science 306 (2004): 70–75.
Martin, Paul S. Twilight of the Mammoths: Extinctions and the Rewilding
of America. Berkeley: University of California Press, 2005.
———, and R. G. Klein. Quaternary Extinctions: A Prehistoric Revolution.
Tempe: University of Arizona Press, 1984.
Miller, Gifford H., et al. “Ecosystem collapse in Pleistocene Australia
and a human role in megafaunal extinction.” Science 309 (2005):
287–290.
Mosimann, J. E., and Paul S. Martin. “Simulating overkill by Paleoindians.”
American Scientist 63 (1975): 304–313.
Pielou, E. C. After the Ice Age: The Return of Life to Glaciated
North America. Chicago: University of Chicago Press, 1992.
population All of the members of one species within a
defined area, such as a pond, a forest, a nation, or the world,
constitute a population. Within this area, the individuals of
the population are potentially able to interbreed, which is
generally considered to define species membership. This is the
biological species concept (see speciation).
Diversity in Populations
Being members of the same species is the only thing that
individuals in a population can be sure to have in common.
Populations, whether of Streptococcus pyogenes bacteria or
Homo sapiens humans, contain great diversity:
• Individuals within a population differ in age and size.
• Individuals within a population differ in sexual characteristics.
Most animals are either male or female; in some
animal species, however, individuals have both male and
female organs. Most plants have reproductive structures
(such as flowers) that contain both male and female components;
however, in some plant species, there are separate
male and female reproductive structures on the same individual,
and in some other species, there are separate male
and female plants. In fungus and many protist species,
there are many different mating types that cannot be easily
designated as male or female.