3. Extinction in the past

Despite the less-than-perfect nature of the fossil record, it still provides a unique window on the history of life, and reveals that there have been dramatic fluctuations in extinction intensities since complex life evolved around 600 million years ago. ‘Extinction in the past’ considers Jack Sepkoski’s database compiled in the 1980s, and his series of highly informative charts showing both diversity and extinction rates since the start of the Cambrian Period 541 million years ago. The calculation of extinction rates and the improved dating of extinction events are discussed, along with the extinction trends that can be observed. Fossils also provide valuable evidence on the nature of selection during extinction.

4. The great catastrophes

What is a mass extinction? Mass extinction events are geologically short intervals of time (always under a million years), marked by dramatic increases of extinction rates in a broad range of environments around the world. In essence they are global catastrophes that left no environment unaffected and that have fundamentally changed the trajectory of life. ‘The great catastrophes’ describes the big five mass extinctions—the end-Ordovician 445 million years ago, the Late Devonian 374 million years ago, the Permo-Triassic 252 million years ago, the end-Triassic 201 million years ago, and Cretaceous-Paleogene sixty-six million years ago—and thoughts on their likely causes, along with other important extinction events identified at the start of the Cambrian and in the Early Jurassic.

2. Extinction today and efforts to stop it

The fossil record shows that life has experienced five major mass extinctions. A sixth catastrophe may be underway. Past mass extinctions were geologically short-lived intense crises that affected animals and plants in all environments. They removed the dominant and abundant species, leaving ecological voids to be filled by groups that were often rare or insignificant beforehand. Uniquely, the big five also saw the collapse of the base of the food chain in the oceans. The current extinction crisis does not yet have any of these attributes, but there are concerns over rising species extinction rates. ‘Extinction today and efforts to stop it’ compares current extinction rates with those of the past, and considers different terrestrial and marine conservation approaches.

6. What happened to the Ice Age megafauna?

The Ice Age or Pleistocene Period, from 2.6 million to 11,650 years ago, was a time when the climate cycled from glacial to interglacial states every 100,000 years or so, resulting in significant sea-level changes. During the last glacial maximum, numerous large, terrestrial animals disappeared, now termed the Pleistocene megafauna extinction. Its cause has been one of the most highly contested topics in palaeontology in recent years. ‘What happened to the Ice Age megafauna?’ describes how although the losses overall were trivial compared to the scale experienced during the big five mass extinctions, they were highly selective and very similar on all continents. The two main culprits proposed have been climate change and humans.

1. Why extinctions happen

‘Why extinctions happen’ looks at what we currently know about causes and styles of extinction today and in the past. This knowledge lies at the heart of our understanding about the way the Earth’s environment works and what happens at times of extremes. The importance of the species–area relationship is discussed and it is shown that species with large and widely dispersed populations are at risk of extinction as well as those with small and narrowly dispersed populations. Extinction today is being demonstrably caused by human activities, such as habitat destruction, the introduction of invasive species, and over-exploitation of natural resources. Future climate change is an additional potential cause of extinction.

5. How to kill nearly everything

‘How to kill nearly everything’ considers the proposed causes of mass extinctions, often called the kill mechanisms. An almost bewildering array of ideas have been put forward as likely bringers of death, but most of the debates have concentrated on just a few culprits, notably volcanism and meteorite impact, because their timing is closely coincidental with the extinctions. However, it is important to note that while they may be the ultimate cause of a crisis, it is their consequences that likely lead to proximate (or direct) causes of extinction. The main causes discussed are large igneous provinces, hyperthermals, ocean anoxia, ocean acidification, ozone depletion, ice ages, sea-level change, and meteorite and comet impacts.