Overexploitation is the overuse by humans of a population of organisms to an extent that threatens the viability of the population or radically alters the natural community in which it lives. There is a tendency to think that overexploitation is a relatively new phenomenon. However, that view is a bit naïve, as we will see in some examples of past overexploitation. After the most recent glaciation, which was at its height between about 20,000 and 14,000 years ago, the grasslands in central North America harbored an extraordinary array of large animals. The diversity of antelope, horses, cheetahs, giant ground sloths, mammoths, mastodons, and other animals easily rivaled that of the large animal fauna of Africa today. However, about 11,000 years ago, at the end of the Pleistocene epoch, many disappeared; 34 genera of large mammals became extinct in fewer than 1,000 years, while 40 more became extinct in South America. This was a massive die-off when you consider that only 20 large mammal genera had become extinct in North America over the previous three million years. Is it a coincidence that so many large mammals became extinct shortly after the time that humans, crossing from Siberia to Alaska, probably first arrived in the Western Hemisphere? Paul Martin, an anthropologist, thinks not and has argued in many articles and books that overhunting was primarily responsible for the extinctions. Martin's critics have argued that the extinctions were primarily the result of significant climate change. If climate change was responsible, we would expect many small mammals, primarily rodents, to have become extinct in the region, too; however, only four North American genera of small animals became extinct at the end of the Pleistocene, compared with 46 genera over the preceding three million years. On the other hand, perhaps small mammals are less susceptible to climate change than large mammals are. We cannot definitely say which answer is correct, and the truth may lie in the middle, but many scientists believe that the end of the Pleistocene saw a massive overkill by the first human inhabitants of the Americas. The best evidence that overhunting by early people eliminated some species comes from islands. On many remote islands, birds evolved in the absence of mammalian predators, sometimes losing their ability to fly in the process. When people arrived on these islands, they found easy prey. For example, when Polynesians, now known as Maori, arrived in New Zealand about A.D. 1200, the islands had 11 species of moas: flightless birds that ranged in size from as small as a turkey to larger than an ostrich. By the time Europeans colonized the islands in the 1700s, the moas were gone, along with five species of rail and six waterfowl species. The demise of the moas and other birds undoubtedly was hastened by forest clearing and other changes brought about by the Maori, but the abundance of moa remains at Maori village sites makes it clear that hunting was a major factor. On small islands throughout the Pacific, scores of birds are known to have become extinct after the arrival of Polynesians. In the Hawaiian Islands, 44 species of endemic land birds out of 82 became extinct between the arrival of Polynesians and the arrival of Europeans. Again, habitat changes were undoubtedly important, but it is likely that overhunting was a major problem, especially for various species of flightless geese, ibis, and rail. On Madagascar, the loss was not limited to birds. The arrival of people 1,500 to 2,000 years ago caused the extinctions of two giant tortoises, a bear-sized giant lemur, a small species of hippopotamus, many other mammals, and elephant birds, some of which rivaled the largest moas in size. Currently, the worst overexploitation may be happening through global overfishing. This is partially disguised by the fact that we are still able to harvest huge quantities of marine species; only on closer inspection does one notice that the predatory fish that used to dominate catches are being replaced by species further down the food chain.