The Sixth Extinction: An Unnatural History by Elizabeth Kolbert argues that we are currently in the middle of a sixth mass extinction on Earth. Under normal conditions, extinctions happen fairly rarely, but they do occur, and scientists describe this as a "background extinction rate." Such a rate is low. For example, the background extinction rate for mammals is estimated to be around one species every seven hundred years. For amphibians, it's one species every thousand years.
In a mass extinction, the rate of extinction skyrockets. One scientist has defined a mass extinction as one that eliminates a "significant proportion of the world's biota in a geologically insignificant amount of time." There have been five mass extinctions so far: the end of the Ordovician Period (450 - 440 million years ago), the late Devonian Period (375 - 360 mya), the end of the Permian Period (252 mya), the late Triassic Period (200 mya) and the end of the Cretaceous Period (65 mya). The last one, of course, wiped out the non-avian dinosaurs, and the Permian extinction, which is also known as the "Great Dying" nearly wiped out life on Earth. 90 to 96% of the world's species perished -- within 200,000 years.
One astonishing aspect of extinction is how recent the concept is. For most of human history, people believed that the animals around them had always existed. Then, in the 18th century, a French explorer found mastodon fossils in the United States. The scholars of the day identified them as elephant bones -- in a country that did not have elephants. Thomas Jefferson, then President of the United States, actually hoped that Lewis and Clark would find live mastodons during their expedition. It was the French scientist, Georges Cuvier (1769 -1832), who first grasped the truth: The "American Elephant" was an extinct species. During his career, he discovered other extinct animals including the Megatherium (giant ground sloth) and a species of pterodactyl.
Cuvier also speculated as to what caused animals to go extinct and concluded the answer had to be some sort of catastrophe. While this isn't necessarily the case with background extinctions, it is true of mass extinctions. As most people know, the dinosaurs were wiped out by an asteroid impact at the end of the Cretaceous. That impact caused a dust cloud that blocked sunlight resulting in a multi-season "impact winter," that killed many of the plants. No land animal bigger than a house cat survived. Marine ecosystems were similarly devastated.
Climate change was involved in at least two of the mass extinctions. The Ordovician Period saw a drop in temperature, resulting in glaciation, while the Permian Period saw extreme global warming. Ocean temperatures skyrocketed by as much as 18 degrees, which resulted in increased acidification and reduced oxygen.
In each chapter, Kolbert spotlights an extinct species (the mastodon) or one currently facing a severe crisis. The little brown bat is actually very common, but white-nose syndrome has wreaked havoc on the population. Kolbert also talks to the scientists studying a given animal and they describe the causes of that species' extinction or decline. In too many cases, there's a human cause. The chytrid fungus that has been devastating frog populations all over the world may have been introduced by humans transporting African clawed frogs or North American bullfrogs to various places. Both of these frogs are known carriers of the fungus, but aren't otherwise affected by it. Unfortunately, they've been infecting other frogs that are vulnerable to the fungus.
Elizabeth Kolbert is a staff writer at The New Yorker who specializes in environmentalism, and she has also written Field Notes from a Catastrophe: Man, Nature, and Climate Change. Her book is for the general reader, which means there is little jargon. It can be grim reading, and she includes a warning at the end: If we keep wiping out other species, we'll eventually render ourselves extinct.