Aug 17, 2015
The Sixth Extinction is an eloquent look at our world, now out in paperback.
Science writer Elizabeth Kolbert won a Pulitzer Prize for this book’s fine explanation of what extinction is and how we are participating in it. “Right now,” she writes, “we are deciding, without quite meaning to, which evolutionary pathways will remain open and which will be forever closed.”
She explains how humans began to understand that some animals that had once existed no longer did so. Extinction became a scientific concept only in the late 18th and early 19th century, thanks to the work of French naturalist Georges Cuvier. He recognized a fossil as a tooth – large as a brick, but still a tooth. It was the remains of an animal no longer seen on earth. This period also saw new understanding of earth’s layers and age, thanks to the work of scientists like Charles Lyell, and it is also the period when Charles Darwin took his voyage to South America and the Pacific Ocean, after which he formulated his theory of change by natural selection. An extinction well-known to scientists was that of the great auk, a large flightless bird eaten in many colder countries. It was hunted to extinction by 1844.
In recent decades scientists have found the proof that there were five mass extinctions in which thousands of species of plants and animals died. The most important for us was the loss of the dinosaurs some 65 million years ago, after a catastrophic collision between an asteroid and the earth. Extinction usually takes place at a slow pace, as one paleontologist put it, so that “long periods of boredom [are] interrupted occasionally by panic.” Very little could live on parts of the earth after that collision, thanks to a lack of sunlight, extreme cold, and the loss of about 50% of the plants and animals that provided food.
Kolbert discusses whether we are living in the age of the sixth mass extinction. The problems we face include rising temperatures, loss of trees in urban areas, carbon dioxide from use of fossil fuels, acidification of the ocean, evaporation of fresh water, erosion of soil. She asks, can the earth provide food for seven or eight billion people, and still remain home to millions of other species?
She visits Central America, where the last of its frogs reside in sterile containers. She visits Vermont, where a fungus has killed off the bats. She visits the coral reefs off the eastern shore of Australia, which have declined by 50% over the last 30 years.
Her explanation of the loss of large mammals is clear and interesting: their rate of reproduction is quite slow, thanks to a lack of predators they have had to avoid. So hunters can wipe out their numbers quickly. Hunters in North America killed off the buffalo (bison). In the past few decades, the numbers of chimps, and other great apes, are down by half. There are fewer than 100 Sumatran rhinos still in existence. African elephants have declined by 50%. Six of the eight species of bears in the world are “endangered.” Tigers, lions, cheetahs, jaguars - all are declining more rapidly than they can be replaced by their offspring.
Insect species number in the millions, and not all have been identified. So we don’t know how rapidly some of them are dying out. In the plant world, the American chestnut, now gone, once dominated half the continent. In 50 years a fungus killed off four billion trees.
When the dinosaurs were wiped out, a scrawny species of mammal with very little strength and low numbers managed to grow and spread all over the earth. This mammal eventually evolved into Homo sapiens. Time will tell how it survives as the seas rise, other species decline and the air, water and land are poisoned by its activities.