|OVERVIEW OF EXTINCTION
For every species that is alive today, perhaps a thousand more have lived previously and become extinct. Most of these extinctions occurred before humans evolved, and the species are known to us only through fossils.
Extinctions are a natural part of evolutionary processes, but through most of the history of life on Earth, idiological diversity has been increasing.
Periodically, however, major changes in the conditions on Earth have caused the collapse of living systems, and large percentages of species a have become extinct. These species will never return. It takes millions of years for life forms to diversify again.
The current extinction crisis is unique, in that the loss of biodiversity is occurring very rapidly, and the causes of the crisis are the activities of a single species: human beings. Some scientists believe the current crisis began when humans and their domestic animals first began to colonize the various parts of the globe.
Others believe it began around 1600, when human population growth exploded, and the level of per capita resource consumption began to rise dramatically in some parts of the world.
Of the species that are best known, the so-called "higher animals," more than one percent have become extinct in the last 400 years and the overwhelming majority of these extinctions are anthropogenic. Many more species are in danger of becoming extinct if we do not act quickly to conserve them.
The background rate of extinction is the number of extinctions that would be occurring naturally in the absence of human influence. Estimates range from one to ten species per year for the past 600 million years. It is difficult to estimate this rate, in part because the number of species in existence is not known.
The background rate of extinctions establishes a baseline from which the severity of the current extinctions crisis can be measured.
The current rate of extinction appears to be hundreds, or perhaps even thousands, of times higher than the background rate. It is difficult to be precise because most of the disappearing species today have never been identified by scientists.
The background rate of extinction has been interrupted periodically in Earth's history by episodes of mass extinction, periods in which a large percentage of the existing species become extinct in a geologically short amount of time.
Mass extinction episodes represent major collapses of biodiversity and ecosystems, and they lead to fundamental changes in the make-up and distribution of life on Earth.
The species that are most likely to survive mass extinctions are widespread generalists such as cockroaches and weeds.There are five widely recognized major mass extinction episodes in the Earth's history, and many scientists believe that we have now entered the sixth.
However, there is a fundamental difference. In the past, mass extinctions have been caused by climate change, extreme geological activity, huge meteors colliding with the Earth or other natural factors.
These changes in the environment took tens of thousands or even millions of years to occur. The sixth great extinction episode has been precipitated by human activities, and it appears to be happening very quickly.
Types of Extinction
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.
Extinction is not limited to application at the species level. Extinctions in the ancient past frequently are described in terms of whole groups of related species, such as a genus or a family.
The farther back in time, the more difficult it is to distinguish different individual species from one another on the basis of fossils, and sometimes scientists can only tell when all the members of a genus or a family disappear. In contrast, it is often useful categorize extinctions in the recent past by distinctions that are finer than the species level, such as subspecies and poppulations.
Another important type of extinction is extinction in the wild. Members of a species may exist in captive breeding programs in zoos, but if there are no individuals living in their natural habitat, that species has become extinct in the wild.
Similarly, a species may be effectively extinct, if members of the species are still alive, but the species has no chance of reproducing. These cases include those in which all the remaining individuals are of a single sex.
The extinction case studies represent a broad geographical and temporal range of extinctions that were caused by humans or are believed to have been caused by humans. These past extinctions cannot be undone, but the future is unwritten. Human beings have created the factors that threaten so many species today, and human beings can create the solutions that will reverse the current extinctions crisis.
Words in bold italics can be found in the glossary.