Extinction is the complete demise
of a species. It takes place when all individuals of a species die
out. Extinction has occurred throughout the history of life on Earth.
It is the ultimate fate of all species. In fact, it has been estimated
that 99.9% of all species that have ever lived on Earth are now
extinct. The causes of extinction are many and highly variable.
They vary from environmental changes brought about by events such
as floods, volcanic eruptions, or meteorite impacts, to fluctuations
in sea level or climate, to competition between species. In the
past 25,000 years humans have become a significant additional cause
of extinction for many species.
Species are constantly going extinct,
often for reasons that are not particularly obvious, and at other
times for reasons that are peculiar to the species in question.
The processes that lead to this pattern of constant background extinction
occur continuously, so that at any given time, while some species
are going extinct, others are making their appearance for the first
time. Over time, this process of continual turnover produces great
changes in the species composition of the Earth’s ecosystems.
However, there have been times in the past when rates of extinction
have been significantly higher than normal background rates. These
are referred to as mass extinctions. During such events, vast numbers
of species disappear in a relatively short period of time.
In the history of Earth, there have been
five mass extinctions. The best known and most recent occurred at
the end of the Cretaceous period, 65 million years ago. At that
time, many kinds of organisms, plants and animals, marine and terrestrial,
went extinct, the best known being the dinosaurs. A total of 85%
of all species on Earth disappeared. Most scientists believe that
this mass extinction was caused by the impact of an asteroid, which
threw vast amounts of dust into the atmosphere, blocking sunlight
and lowering global temperatures for many months. Study of this
Four other mass extinctions are known to have occurred:
- Late Triassic (210 million years ago):
extinction of 76% of species including many sponges, gastropods,
bivalves, cephalopods, brachiopods, insects, and vertebrates;
possibly due to glaciation or an extraterrestrial bolide impact.
- Permian (250 million years ago): extinction
of 96% of marine species, including all trilobites and many terrestrial
animals—Earth’s biggest extinction. Among the explanations
are global cooling, glaciation, lowered sea levels, and ash clouds
produced by volcanic eruptions.
- Late Devonian (365 million years ago):
85% of species went extinct, including many corals, brachiopods,
and single-celled foraminiferans, probably because of global cooling
- Ordovician (440 million years ago):
85% of species disappeared, including trilobites, graptolites,
and conodonts, as a result of global cooling, glaciation, and
lowering of sea levels.
During the last ice age, about 15,000
to 25,000 years ago, many large mammals became extinct in North
America. These included mammoths, mastodons, glyptodonts, horses,
and ground sloths. These extinctions may have been caused by overhunting
on the part of humans, who had reached North America from Asia at
about this time. Climate change may also have played a role in the
demise of these animals. Similar extinctions occurred in Australia
about 18,000 years ago, also timed to the arrival of humans on that
Since the ice age, humans have become
a dominating factor in the extinctions of many species through hunting,
habitat destruction, or competition caused by the introduction of
non-native species. One of the most famous examples of recent extinction
is the dodo, a flightless pigeon that lived on the island of Mauritius,
in the Indian Ocean. After Europeans reached Mauritius they introduced
non-native species such as pigs and cats, which eradicated the defenseless
dodo. Human overhunting may have doomed other flightless birds,
such as the giant moas of New Zealand and the elephant birds of
Madagascar. Today, several species of whales are near extinction
from overhunting as well. Habitat destruction threatens countless
plants and insects of the Amazon, while the introduction of the
brown tree snake into Guam has exterminated most of that island’s