Mass extinctions and biodiversity loss

Key points

• During mass extinctions, 20-90% of species were wiped out; these include a broad range of organisms, and the events appear to have happened rapidly.

• It is difficult to study mass extinctions in the Precambrian, but there seems to have been a Neoproterozoic event between the Ediacaran and Early Cambrian faunas.

• The "big five" Phanerozoic mass extinctions occurred in the end-Ordovician, the Late Devonian, the end of the Permian, the end of the Triassic and the end of the Cretaceous. Of these, the Late Devonian and end-Triassic events seem to have lasted some time and involved depressed origination as much as heightened extinction.

• The end-Permian mass extinction was the largest of all time, and probably caused by a series of Earth-bound causes that began with massive volcanic eruptions, leading to acid rain and global anoxia.

• The end-Cretaceous mass extinction has been most studied, and it was probably caused by a major impact on the Earth.

• Smaller-scale extinction events include the loss of mammals at the end of the Pleistocene, perhaps the result of climate change and human hunting.

• Recovery from mass extinctions can take a long time; first on the scene may be some unusual disaster taxa that cope well in harsh conditions; they give way to the longer-lived taxa that rebuild normal ecosystems.

• Extinction is a major concern today, with calculated species loss as high as during any mass extinction of the past. The severity of the current extinction episode is still debated.

The Dodo never had a chance. He seems to have been invented for the sole purpose of becoming extinct and that was all he was good for.

Will Cuppy (1941) How to Become Extinct

Extinction, long studied by paleontologists to inform them of the past, is now a key theme in discussions about the future. Will Cuppy, the famous American humorist, was able to talk about the extinction of dinosaurs, plesio-saurs, the woolly mammoth and the dodo, all of them icons of obsolescence and failure. The dodo is perhaps the most iconic of icons (Fig. 7.1), and it used to be held up as a moral tale for children: here was a large friendly bird, but it was simply too friendly and stupid to survive. The message was: be careful, take care, and don't be as improvident as the dodo! The dodo is now an icon of human carelessness rather than of avian extinction.

The most spectacular extinctions are known as mass extinctions, times when a large cross-section of species died out rather rapidly. There may have been only five or six mass extinctions throughout the known history of life, although there were many extinction events, smaller-scale losses of species, often in

Permian Extinction

Figure 7.1 An image of a dodo from another era. Lewis Carroll introduced the dodo as a kindly and wise old gentleman in Alice Through the Looking Glass, although at the time most people probably regarded the dodo as rather foolish. Driven to extinction in the 17th century by overhunting, the dodo is now an image of human thoughtlessness.

Figure 7.1 An image of a dodo from another era. Lewis Carroll introduced the dodo as a kindly and wise old gentleman in Alice Through the Looking Glass, although at the time most people probably regarded the dodo as rather foolish. Driven to extinction in the 17th century by overhunting, the dodo is now an image of human thoughtlessness.

a particular region or involving species with a particular shared ecology.

The serious study of mass extinctions is a relatively new research field, dating only from the 1980s onwards, and it has wide interdisciplinary links across stratigraphy, geochemistry, climate modeling, ecology, conservation and even astronomy. The study of mass extinctions involves careful hypothesis testing (see p. 4) at all levels, from the broadest scale ("Was there a mass extinction at this time? Was it caused by a meteorite impact or a volcanic eruption?") to the narrowest ("How many brachiopod genera died out in my field section? Does their extinction coincide with a negative carbon isotope anomaly? Do the sediments record any evidence for climate change across this interval?"). The excitement of studies of mass extinctions, and smaller extinction events, is that these events were hugely important in the history of life, and yet they are unique paleontological phenomena that cannot be predicted from the modern-day standpoint. In practical terms, the field involves such a broad array of disciplines that research involves teamwork, often groups of five or 10 specialists who pool their expertise and resources to carry out a study.

In this chapter, we will explore what we mean by extinctions and mass extinctions, and whether there are any general features shared by these times of crisis. We shall then explore the two most heavily studied events, the Permo-Triassic mass extinction of 251 million years ago, and the Cretaceous-Tertiary mass extinction of 65 million years ago, in most detail. Finally, it is important to consider how paleobiology informs the current heated debates about extinctions now and in the future.

MASS EXTINCTIONS Definition

Extinction happens all the time. Species have a natural duration of anything from a few thousand years to a few million, and so they live for a time and then disappear. This means that there is a pattern of normal or background extinction that happens without any broad-scale cause. In any segment of time, perhaps 5-10% of species may disappear every million years. In fact, more species have died out during normal times than during the more spectacular mass extinctions.

Nonetheless, mass extinctions fascinate paleontologists and the public because these were times of concentrated misery, and represent perhaps unusually intense environmental catastrophes. But how is a mass extinction to be defined? All mass extinctions share certain features in common, but differ in others. The common features are:

1 Many species became extinct, perhaps more than 30% of plants and animals of the time.

2 The extinct organisms spanned a broad range of ecologies, and typically include marine and non-marine forms, plants and animals, microscopic and large forms.

3 The extinctions were worldwide, covering most continents and ocean basins.

4 The extinctions all happened within a relatively short time, and hence relate to a single cause, or cluster of interlinked causes.

5 The level of extinction stands out as considerably higher than the background extinction level.

It is hard to define these terms more precisely, first because each mass extinction seems to have been unique, and second because it is sometimes hard to pin down exactly the timing and scale of events.

Paleontologists commonly talk about the "big five" mass extinctions of the last 540 myr, the Phanerozoic, and the current extinction crisis is sometimes called the "sixth extinction". The five mass extinctions (Fig. 7.2) are the end-Ordovician, Late Devonian, end-Permian, end-Triassic, and Cretaceous-Tertiary (KT) events. Study of the Neoproterozoic reveals a further one or two possible mass extinctions, before and after the Ediacaran (see p. 242) so perhaps we should refer to the "big six" or the "big seven" such events.

The notion of five somewhat similar mass extinctions throughout the Phanerozoic has been questioned, however. In a careful statistical survey, Bambach (2006) has shown that there were perhaps only three real mass extinctions, the end-Ordovician, the end-Permian and the KT events. The Late Devonian and end-Triassic events do not stand out so clearly above background extinction rates at those

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