The Biodiversity Crisis: Brown-Eyed, Milk-Giving… and Extinct: Losing Mammals Since A.D. 1500*
Part of the Biodiversity Crisis Curriculum Collection.
Ross D.E. MacPhee and Clare Flemming
* Although humans have a variety of eye colors, the overwhelming majority of all living mammals, and probably most extinct mammals as well, are or were brown-eyed.
Readers of this book would by now probably agree that an imminent upsurge in extinctions, widely predicted, should be one of our leading concerns. But exactly how severe is the extinction problem?
As mammalogists, we felt that assessing the scope and pattern of species loss among mammals would be a feasible—and revealing—project. Mammals are now, and probably were always, relatively scarce. Fewer than 5,000 exist today. (In comparison, there are millions of species of insects.) Because mammals leave behind bones and teeth, we can identify species long after they have disappeared. Scientists probably know nearly all the mammal species that have lived during the past ten thousand years.
To discover very recent extinction patterns, we used scientific literature as well as records and chronicles by early explorers and travelers to compile a list of mammal species that have definitely disappeared since A.D. 1500. Our count: ninety mammal species. (Download Figure 2 for the complete listing.)
However, up to fifty percent of our listed species differed from those cited on similar lists by others.
Why were there so many discrepancies? One source of divergence was honest disagreement about classification. Some biologists see subspecies (varieties within a single species) where others see full species, and vice versa. A good example is provided by the quagga, the peculiar half-striped zebra that disappeared from southern Africa in the 1880s. Many scientists had considered it a full species. Recent molecular evidence, however, shows that the quagga is best regarded as a subspecies of Burchell’s zebra, an animal still common in the region. Loss of subspecies, while regrettable, is different from the extinction of an entire species.
Incorrect extinction dates can explain other differences among lists. This was a particular problem with fossil species (those known only from their bones). Some, identified as modern extinctions, disappeared much earlier. For example, the extinct 300-pound Caribbean rodent Amblyrhiza is often listed as surviving into the sixteenth century; however, recent radiometric dating of some of its remains shows that this megarat actually died out 100,000 years ago. In contrast, many lists have excluded several species that survived into the modern era. In 1996, working in Jamaican caves with our colleague Donald McFarlane, we found remains of the extinct monkeyXenothrix mcgregori (Figure 1) mixed with the bones of European black rats; the black rat was introduced into the New World in Columbus’ time. This circumstantial evidence—reinforced by an intriguing mention in a 1725 work by the renowned early naturalist Hans Sloane—indicates that Xenothrix survived into modern times. Remarkably, this is the only monkey species known to have died out in the last 500 years.
Our list reveals some patterns that defy the conventional wisdom on extinctions. According to the available evidence, for instance, major mammal losses have not occurred in the Amazon rain forest or the clear-cut forests of the United States or on Africa’s Serengeti Plain. In fact, none of these regions—all on continental mainlands—has suffered a single, documented mammal extinction in the last 500 years! Australia is the only continent that ranks high on the mammalian extinction list. Continental Africa has lost only four species. The continental Americas, for all their biotic richness, show evidence of only one mammal species lost in the modern era. Globally, other vertebrate groups, such as birds, have suffered a much greater rate of loss in recent times.
Another puzzle: despite extraordinary levels of exploitation, only two marine mammal species have suffered extinction in the modern era: the Caribbean monk seal and the ten-ton Steller’s sea cow. Remarkably, not one species of cetacean (whales and dolphins) has become extinct in the last 500 years, even though bowheads and other great whales were reduced to tiny populations in the early twentieth century. The gray whale came close to extinction when its entire Atlantic population was wiped out by the mid-1600s. Today, the species survives robustly only in the Pacific.
As can be seen on our map, the vast majority of modern-era mammal extinctions have occurred on islands. Other groups—including birds, reptiles, mollusks, and plants—have also suffered disproportionately on islands, many of which have lost all, or nearly all, their original biodiversity.
Ninety mammal extinctions in the past five centuries is our working figure. We would not be very surprised if, in the next few years, the number was revised upward to 110 or 115 confirmed losses. That’s close to two percent of all mammal species on Earth. Does this sound like a little or a lot? Estimates of the “natural,” or background, extinction rate for mammals run from one species each million years to one every 400 years. Let’s choose the faster rate and do a quick calculation. Ninety species lost in five centuries represents a rate of one complete disappearance every five and one-half years—for a minimum 7,100 percent increase over the natural rate. Is this too high? Decide for yourself.
Excerpted from Natural History, April 1997.
This is an excerpt from THE BIODIVERSITY CRISIS: LOSING WHAT COUNTS, edited by Michael J. Novacek, a publication of the New Press. © 2000 American Museum of Natural History. To order the book, call 1-800-233-4830.