Extinction

All life on earth, from single-celled microbes and simple fungus to dinosaurs and mammals, is compelled to adapt to changes in their environment, which includes efficient competition with individuals of other species. If any species cannot adapt, it will die out, becoming extinct, in other words, totally eliminated.

The fossil record provides strong evidence that of all the species which have ever existed, 99.9 % are now extinct. Millions of species extinguished during the earth's history through two fundamentally different processes: background extinctions or mass extinctions.

There has always been, throughout earth's history, a normal background rate of extinction, punctuated by few mass extinctions.

Background extinctions

Extinctions caused by moderate environmental changes or normal biological interaction are called background extinctions. Extinctions have been a normal part of the life cycle and are occurring throughout time. The normal background rate of extinctions is about two to five families of marine invertebrates and vertebrates per million years.

These extinctions are caused by small changes in the environment, climate or habitat (a change in average temperature for example, would alter the sex ratio in hatchlings as happens with crocodiles, turtles and tuatara ), depleted resources (e.g. a change in available diet) or by inter- or intra-species competition. An example of species inter-competition is the accidental introduction of a poison snake, Boiga irregularis, into Guam island. This resulted in the extinction of 13 endemic species of birds - a case of background extinction caused by biological (predator-prey) interaction.

Mass extinction

Punctuating the process of background rate extinctions are occasional mass extinctions that result in the total elimination of a large number of taxa (groups of life forms collectively categorized taxonomically as kingdom, phylum, class, order, family, genus, or species). Mass extinctions are, relatively speaking, sudden. They result in the global decrease in the number and diversity of life forms. These extinctions have taken place from time to time throughout the existence of life on Earth. Mass extinctions are defined by the four criteria:

• 1. Extinctions occur all over the world.
• 2. A large number of taxa disappear.
• 3. Species belonging to different ecological groups (e.g., terrestrial and marine; benthic and pelagic) disappear.
• 4. The extinctions are clustered in a short amount of geological time (usually, less than one million years).

Major mass extinction events

The Precambrian and Vendian Mass Extinctions

• Precambrian period (4.6 billion to 523 million years ago)
• Vendian period (523-543 million years ago)
• Both Precambrian and Vendian periods were host to at least one mass extinction each. The Precambrian about 250 million years ago and the Vendian about

Extinctions are proposed to have affected the earliest organisms on Earth. Approximately 650 million years ago, seventy percent of the dominant Precambrian flora and fauna perished in the first great extinction.

The Ordovician-Silurian Extinction (439 Ma) in which 85% of species became extinct.

• Ordovician period (510- 438 million years ago)
• Ordovician extinction (440-450 million years ago)

This extinction, in which an estimated 85% of all species became extinct, is the second only to the Precambrian extinction in devastation. It resulted in the elimination of one third of all brachiopod and bryozoan families, many groups of conodonts, trilobites, and graptolites and ended a large part of the reef-building fauna. In total, more than one hundred families of marine invertebrates ceased to exist in this extinction.

The Devonian Extinction (345 Ma) in which 85% of species became extinct.

The end-Permian Extinction (ca. 250 Ma) at the end of the Permian Period and the beginning of the Triassic in which most notably, trilobites became extinct (an estimated 15,000 species). More than ninety percent of all species became extinct including about fifty percent of all animal families.

The end-Triassic Extinction (ca. 200 Ma) in which 76% of species became extinct.

The end-Cretaceous Extinction (65 Ma) in which 20% of the families of plants and animals on land (50% in the sea) and 85% of all species became extinct. All dinosaurs became extinct.

Theories of mass extinction causes

Approximately 439 million years ago at the termination of the termination of the Ordovician and the beginniing of the Silurian a mass extinction took place. One probable cause was brought on by the drop in sea levels as glaciers formed followed by rising sea levels as the glaciers melted resulting in the extinction of twenty-five percent of marine families and sixty percent of marine genera.