There have been five mass extinctions in the history of life on Earth, during which vast numbers of animals and plants died out. So far, after each big die off, new species and families have evolved, filling the world with a new, but just as diverse array of life forms as before the disaster
The general intuition is that a mass extinction creates an impoverished, less diverse collection of species. The survivors who weather the disaster are the founders of the great radiation of new diversity. (This pattern is seen at a smaller scale in local disasters, such as volcanic eruption that obliterates almost all life.)
This intuition is often applied to our own age, which we recognize as the beginning of the sixth great extinction.. We see many specialized species reduced and wiped out, while other robust “generalists”, such as cockroaches or rats, thrive and spread. Presumably, 100,000 years from now, there may be a vast radiation of new species of rodents, expanding into the empty niches of the post human Earth.
But is this process really what has happened in the past extinctions?
This month, David J. Button and colleagues publish a report of their study of “faunal cosmopolitanism” among 1046 early amniote species ranging from 315–170 M years ago. This period includes the Permian–Triassic and Triassic–Jurassic mass extinctions .
They take into account the relationships among the species, so that individuals from related but distinct species can reflect the geographical range of the group, even if only a few samples are available.
The basic finding supports the common intuition: there is a sharp rise in their index of cosmopolitanism (phylogenetic biogeographic connectedness) after the Permian–Triassic extinction, followed by a decrease (i.e., more geographic specialization through the Triassic, and another spike after the Triassic–Jurassic extinction.
Furthermore, they find evidence that “the increases in pBC following each extinction were primarily driven by the opportunistic radiation of novel taxa to generate cosmopolitan ‘disaster faunas’, rather than being due to preferential extinction of endemic taxa .” (p. 4) I.e., new “cosmopolitan” species emerge, rather than a old species survives to spread over the world. (This is bad news for cockroaches and rats, I’m afraid.)
These results certainly indicate the importance of unique events in the history of life, such as mass extinctions. They also suggest that mass extinctions have a predictable effect, at least at a global level.
- David J Button, Graeme T. Lloyd, Martin D.Ezcurra, and Richard J. Butler, Mass extinctions drove increased global faunal cosmopolitanism on the supercontinent Pangaea. Nature Communications, 8 (1):733, 2017/10/10 2017. https://doi.org/10.1038/s41467-017-00827-7