CLASSROOM IN THE WILD

The word "extinction" can refer to several different phenomena. Most of the world's extinctions have been true extinctions, when a species completely dies out and leaves no descendants. A few have been pseudoextinctions, when the original or ancestral species has become transformed by evolution into another species.

All species living today, including ourselves, evolved from another species.True extinctions and pseudoextinctions are both a type of global extinction.

Global extinction is the complete elimination of a particular species everywhere in the world. Many endemic species have a limited georgraphic range, such as a single island. No matter how small that area is, their disappearance from it is a global extinction if the species is not found anywhere else.A local extinction is the extirpation of a species from a portion of its geographic range.

Local extinctions mean the loss of the genetic diversity represented by that population and the removal of that species' contribution to the local ecosystem. Because members of the species still exist in other locations, local extinctions can be reversed if the original causes are addressed, and the species can recolonize or can be reintroduced into the area.

Unfortunately, local extinction is often the precursor to global extinction.

Mass extinctions are episodes in the history of life on Earth during which unusually large numbers of species die off. They stand in contrast to the background rate of extinction, which occurs even when the diversity of life is increasing.

Scientists recognize five major mass extinctions in the Earth’s history. The extinctions are measured in terms of large groups of related species, called families.

The five mass extinction episodes occurred because of major changes in the prevailing ecological conditions brought about by climate change, cataclysmic volcanic eruptions, or collisions with giant meteors.

The sixth mass extinction appears to be in progress now, and the main cause is environmental change brought about by human activities.