Extinction
All life on earth, from single-celled microbes and simple fungus to dinosaurs and mammals, is compelled to adapt to changes in surroundings. If any species cannot adapt, it will simply die out, becoming extinct, in other words, totally eliminated. Those changes that may lead to extinction can be gradual background extinctions or relatively sudden mass extinctions.
Background extinctions
Gradual changes may lead to the extinction of an entire species. Extinctions caused by gradual changes are called background extinctions. These extinctions are caused by small changes in the environments: climate or habitat, depleted resources, inter- or intra-species competition; changes that require life forms to adapt and be flexible or become extinct. There is always a normal background rate of extinction, punctuated by mass extinctions. The fossil record provides strong evidence that of all the species which have ever existed, 99.9 % are now extinct.
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.
Theoretical examples of the causes of background extinction include change in available diet or a change in average temperature that would alter the sex ratio in hatchlings (a phenomena that is evident in crocodiles, turtles and tuatara for example[1]) [2][3][4][5][6]
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 species are eliminated.
- 3. Many types of species are eliminated.
- 4. The extinctions are clustered in a short amount of geological time (relatively speaking, a few million years is a short time in terms of geological time).
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. [2]
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. [2]
The Devonian Extinction (345 Ma) in which 85% of species became extinct. [2]
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. [2][7][8][9][10][11]
The end-Triassic Extinction (ca. 200 Ma) in which 76% of species became extinct. [2][7][8]
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. [2][9][11]
Theories of mass extinction causes
Sources
References
- ↑ Tuatara Wellington Zoo
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Extinction Thomas J. Herbert, Professor of Biology, College of Arts and Sciences, University of Miami
- ↑ Background extinction Kent Holsinger 2007-08-25, University of Connecticut
- ↑ Extinction Glossary of terms from Northern Arizona University
- ↑ Helen M. Regan, Richard Lupia, Andrew N. Drinnan, and Mark A. Burgman (2001). The Currency and Tempo of Extinction. The American Naturalist, volume 157 (2001), pages 1–10
- ↑ What Killed The Dinosaurs? The Great Mystery University of California Museum of Paleontology
- ↑ 7.0 7.1 Great extinction came in phases BBC April 1, 2005. Retrieved June 18, 2007
- ↑ 8.0 8.1 Boost to CO2 mass extinction idea Helen Briggs, BBC News science reporter Aug. 28, 2005. Retrieved June 18, 2005
- ↑ 9.0 9.1 Asteroid 'destroyed life 250m years ago' David Whitehouse, BBC News Online science editor February, 23, 2001. Retrieved June 18, 2007
- ↑ Boost to asteroid wipe-out theory Paul Rincon BBC News Online science staff. May 13, 2004. Retrieved June 18, 2007
- ↑ 11.0 11.1 Double whammy link to extinctions Paul Rincon, BBC News Online science staff. April 1, 2004. Retrieved June 18, 2007