In the 1960s a brown snake from New Guinea and nearby islands turned up in growing numbers on Guam, a Pacific island with no native snakes. Within a few decades the island's forests had fallen almost silent. That story is a small model of a larger crisis, but it begins with a surprisingly recent discovery: that species can vanish at all.

Discovering extinction

Before about 1800 many naturalists doubted that a species could disappear. Thomas Jefferson hoped that mammoths might still roam the unexplored American West. In 1796 the French anatomist Georges Cuvier compared fossil mammoth bones with the skeletons of living elephants and showed that they belonged to different species. Cuvier's work established that species can go extinct, and over the following decades the fossil record revealed a long history of loss. Most estimates hold that more than 99 percent of all species that have ever lived are now extinct.

Background and mass extinctions

Species are always disappearing at a slow natural pace, known as the background rate. Several times in Earth's history that pace has jumped. A mass extinction is the loss of roughly three quarters or more of species in a geologically short time. Paleontologists recognize five: at the end of the Ordovician period about 445 million years ago, in the Late Devonian, at the end of the Permian about 252 million years ago, at the end of the Triassic, and at the end of the Cretaceous 66 million years ago, when the large dinosaurs died. The end-Permian event, linked to huge volcanic eruptions in Siberia, was the worst, wiping out most marine species. After each one, life took millions of years to rebuild its diversity, often through groups that had been minor players before, as mammals were before the dinosaurs fell.

Today's losses

Since 1500, scientists have documented the extinction of several hundred animal species and many plants, from the dodo, last reliably seen in the 1660s, to the passenger pigeon, whose last member died in the Cincinnati Zoo in 1914. Many more have surely vanished unrecorded. Researchers estimate that current extinction rates are tens to hundreds of times higher than the background rate. In 2019 the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, known as IPBES, estimated that around a million species are threatened with extinction.

Are we in a sixth mass extinction? Some scientists say yes, pointing to rates that match those of past mass extinctions and to trends that are accelerating. Others urge care with the label. They note that recorded extinctions remain far below the mass extinction threshold of three quarters of species, so the term describes a possible future more than the present.

What drives the losses

The 2019 IPBES assessment ranked the direct causes of nature's decline. Changes in how land and sea are used ranked first, mainly the conversion of forests, grasslands and wetlands to farms, plantations and cities. Direct exploitation, such as hunting, logging and fishing, came second, followed by climate change, pollution and invasive species. Losses are often delayed. An extinction debt arises when species survive for decades after their habitat is lost before finally dying out, so today's damage may produce tomorrow's extinctions. Most recorded extinctions have struck islands.

Invasive species

Humans have moved plants and animals around the world on purpose and by accident. Most introduced species fail to establish, and most of those that do cause little trouble. An invasive species is an introduced species that spreads and causes harm to native species, economies or health. Why do some succeed so dramatically? The enemy release hypothesis holds that invaders often thrive because they have left behind the predators, parasites and diseases that held them in check at home. Cane toads, brought to Australia in 1935 to control beetles in sugarcane, failed at that job but spread across the north, poisoning native predators that tried to eat them.

Some ecologists argue that conservation should judge species by their effects, not their origins, since many introduced species are harmless or even useful. Others answer that harm often appears only after a long lag, by which time eradication is impossible, so caution is justified. Eradication sometimes works: South Georgia, in the South Atlantic, was declared free of rats in 2018 after a long campaign, and its seabirds are returning.

Worked example: the brown tree snake on Guam

The brown tree snake probably reached Guam as a stowaway in military cargo shortly after the Second World War. By the 1960s it was spreading across the island, and native birds began to disappear. At first, scientists blamed pesticides or disease. In the 1980s the biologist Julie Savidge showed that the snake was the cause. Guam's birds had evolved without snakes and did not recognize the danger, while the snake had escaped its natural enemies, a case of enemy release. Most of the island's native forest birds were lost from the wild; the Guam rail and the Guam kingfisher survived only because some were taken into captivity. The losses kept spreading. With few birds left to eat spiders, the forests filled with webs, and with few birds to carry seeds, tree seedlings no longer spread the way they once did. The Guam rail has since been released on nearby islands, and is no longer classed as extinct in the wild.