Introduction to mass extinction.

In most simple terms a "mass extinction event" is the phenomena where a large number of species become extinct in a relatively short period of time. In more scientific complexity this refers to a large number of different types of organisms occupying a range of diverse and widespread environments, with the rate of disappearance of species largely exceeding the natural extinction rate. This rate as determined from the fossil record has been calculated at 2-5 taxonomic families of marine invertebrates and invertebrates every million years. During a mass extinction event 20-50% of all genera on Earth may become extinct in less than a million years.

Five major mass global extinction events have occurred during the past 542 million years as shown in the fossil record. There have been numerous extinction events, but these five have been identified as the most significant by Raup and Seposki (1982).

(Graph showing timeline of mass extinctions and number of families lost.)

1/ Ordovician Crisis (444 mya)

2/ Devonian Crisis (360 mya)

3/ Permian Crisis (251 mya)

4/ Triassic Crisis (200 mya)

5/ Cretaceous Crisis (65 mya)

The graphs also includes the Jurassic Crisis, which I will not be including as a mass extinction event.

The causes of these extinctions are numerous and diverse. Scientists often differ on what they believe caused different extinction events, something which I will discuss in a later post. The one factor which all past mass extinctions share is that the extinctions all had natural causes, something which the potential sixth mass extinction may differ from.

Within the last 2000 years a large number of species have become extinct in ways which can be linked to human dispersal or activity. This has led many scientists to believe that a sixth mass extinction is underway, defined as the Holocene extinction event. In 1998 a survey by the American Museum of Natural History found that 70% of biologists view the present era as part of a mass extinction event. 

This blog will try to identify the causes of this extinction event, whether it is the first mass extinction with a biotic cause. Focusing upon hard and soft evidence for species extinctions over the past 2000 years. I will also look back at the causes and rates of past extinction events, and see if any comparisons can be made, and look forward using modelling predictions to see what the future holds. 

My next blog post will give an introduction to the sixth mass extinction.

References: New World Encyclopedia, http://mac122.icu.ac.jp/biobk/biobookpaleo3.html, Raup and Seposki (1982) - Mass Extinctions in the Marine Fossil Record, Science 215: 1501-1503.