of Biological Diversity
The stresses of urbanization and habitat intrusion by farming and industry has resulted in a decline in genetic,
species and ecosystem diversity. Extinction, popuilation decreases and degradation of ecosystems reduces biological
diversity on the Earth.
Extinction is the disappearance of every individual of a species from the eentire planet. It is a natural
part of the Earth's history. Scientists estimate that 99% of species that have ever existed on the Earth are now
extinct (many by mass extinction - sudden environmental change, like the Ice Age). Most extinctions take place
over long periods of time, but the rate of extinctions is rising, and this is reducing the biological diversity
of our planet.
Extirpation is a local extinction, or the disappearance of a species from a particular area.
The General Status of Alberta Wild Species 2000 (pdf format)
Natural Causes of Extinction and Extirpation
Natural selection is a slow process. Even if there is a lot of variation within a species, sometimes the changes
in the environment are so drastic that and so quick, that none of the individuals within a species can survive.
Most extinctions, in the past, were due to:
- catastrophic events (volcanic erutions, earthquakes, floods,
- lack of food (due to overpopulation)
Not all extinctions happened millions of years ago. Diseases
and natural events occur all the time and when they do, a species, within a particular area, can be extirpated
Sometimes organisms have adaptations that suit them only
to a very narrow set of environmental conditions. This usually occurs in a relatively stable area, where the environment
does not change for a very long period of time. This is called overspecialization and it
is another cause of extinction. The giant panda is a species that is overspecialized, because it relies on bamboo,
making it vulnerble to extinction, when the bamboo is scarce.
Human Causes of Extinctions and Extirpations
Most extinctions and extirpations today are caused by human
Habitat destruction - as a result of -
- agricultural development
- Damming of rivers
- Pesticides, Herbicides and Fertilizers
Introduction of Non-Native Species
When introduced species use the same resources, as the native species, the competition will cause a decline in
the numbers of native species, simply because there is less to go around. The introduced species will have no natural
predators to limit its population and will, in time, take over from the native species.
This was the major cause of the decline and eventual extirpation of the plains Bison, as well as the extinction
of the passenger pigeon. Sometimes species are hunted to deliberately extirpate them. The
black-tailed prairie dogs were onsidered a pest in the 1930's and were hunted to reduce their
Effects of Extinctions and Extirpations
Extinctions and extirpations reduce biological diversity.
When an organism disappears locally or globally, many other species are affected. The cycle of life is adversely