Geology Reference
In-Depth Information
Modern Extinctions and Biodiversity
In this section, I describe a significant extinction event in the age of man. After humans
evolved and began to spread across the globe, the large mammals of the Cenozoic era
(which I describe in Chapter 21) began to decline. In this section, I present possible ex-
planations for that decline, and I touch on ideas about how man's continuing impact on
our planet may affect biodiversity.
Hunting the megafauna
About 14,000 years ago, humans first entered the Americas, probably by way of a land
bridge connecting Siberia and Alaska. What they found was a land full of large mammals,
or
megafauna,
such as mammoths, mastodon, bison, horses, ground sloths, and rhino-
ceros to name a few. Shortly after the arrival of humans, the numbers of large mammal
species declined dramatically, leading some scientists to conclude that humans hunted
these animals into extinction.
The proposal that human hunting resulted in megafauna extinction is called the
prehis-
toric overkill hypothesis.
Supporters of this hypothesis claim that animals with no previ-
ous exposure to humans do not adapt quickly enough to the predatory skill of humans
and their ability to kill large numbers of animals at once.
A recent example of an overkill situation is the extinction of the dodo bird from the is-
land of Mauritius in the Indian Ocean. In the year 1600, sailors arrived on the island and
started hunting this large flightless bird and its eggs to eat. The dodo, with no natural
predators and no previous experience with humans, was completely wiped out by 1681
—only 80 years after it started co-existing with humans.
Supporters of the overkill hypothesis also point to similar events in Australia more than
40,000 years ago. When human populations first arrived in Australia, numerous species
of large
marsupials
(kangaroo-like mammals) went extinct. The cause of this extinction
— like the North American megafaunal extinctions — is still being debated. Some scient-
ists think it was a direct result of human migration to the continent and overhunting of
the animals. Other scientists suggest that human changes to the environment (through
the use of fire to clear vegetation) were a more important factor in causing the extinc-
tions.
However, other hypotheses have been proposed to explain the disappearance of these
large mammals:
