Pleistocene Overkill

Around 11,000 years ago, near the end of the Pleistocene era (2.6 million to 11,700 years ago), most megafauna- Hlarge, mostly herbivorous (plant-eating) land animals- -went extinct in North America. Their disappearance has been the topic of much debate, but considerable evidence supports a hypothesis known as “Pleistocene overkill.” The idea is that, as humans spread across the continent, they preyed upon large herbivores, such as woolly mammoths, ground sloths, and tortoises, and wiped them out. As originally formulated by the American geoscientist Paul Martin, the large mammals were driven to extinction in a few hundred years in a massive, fast-moving event. A newer version of the hypothesis posits that the extinctions were more gradual, based on evidence that, in some areas, humans and large animals coexisted in the same habitats for long periods of time, despite hunting. However, the end result was the same: extinction of the megafauna. Large animals are more vulnerable to extinction than smaller ones because they cannot hide easily from human predators and because they reproduce quite slowly. It is possible that the large animals were also relatively unafraid of human beings, since they would have evolved for hundreds of thousands of years without humans present. In addition, there is some indication that a rapid shift in climate reduced the habitats of many of the giant herbivores, making them more vulnerable to human predation. Likewise, Australian biologist Tim Flannery suggests humans may have changed the environment through their actions, especially by increasing the frequency of fires.

Not unexpectedly, when the large herbivores disappeared, their natural predators, such as saber-toothed tigers and short-nosed bears, became extinct as well. The large scavenger birds, which had adapted to eating the remains of large animals, also followed them into extinction. The California condor may have held on because it had access to the carcasses of large marine mammals such as whales and sea lions, which did not go extinct at this time. The loss of these giant animals also impacted the diversity of smaller animals. Because abundant large animals (such as mammoths and tapirs) alter plant communities by the way they graze (feed on plants), their disappearance would have caused a shift in the plant communities, resulting in the extinction of many smaller species that depended on the habitats maintained by the large grazers. In fact, there existed a grassland ecosystem in Alaska called the mammoth steppe that disappeared entirely once the woolly mammoth went extinct in that region.

The idea of the Pleistocene overkill is quite controversial, yet the principal alternative to explain the rapid loss of all these giant animals is a drastic climate change that occurred when the last ice age ended near the end of the Pleistocene. Recent fossil data and archeological discoveries increasingly support the idea that the early native peoples were responsible for the extinction of many species. One of the early groups (but not the first) to colonize North America was the Clovis people. At Clovis archeological sites, researchers have found distinctive, beautifully made stone spearheads that would have been well-suited to killing large herbivores. These same Clovis spearheads have been found imbedded in the skeletons of large animals, which strongly suggests that these animals were hunted by the Clovis people. Clovis culture rapidly spread throughout North America, and then abruptly disappeared after about 300 years- -a disappearance that seems to coincide with the extinction of many (but not all) of the large animals of interior North America.

Additional support for the overkill idea comes from the growth rings of mammoth tusks at the time of the mammoths’ extinction, which indicate that the animals were eating well and not experiencing the starvation that would normally accompany climate-driven extinction. Also, many of the extinct species of megafauna had already survived several other glacial climate cycles during the previous 100,000 years, and so presumably they could have survived one more. It is worth noting that a similar event occurred in Australia, with early human societies apparently wiping out many species of giant marsupials, as well as on islands throughout the world. If we accept some version of the Pleistocene overkill hypothesis, then we also have to accept the idea that early hunter-gatherer societies were capable of having large and permanent effects on their environment.

1.Around 11,000 years ago, near the end of the Pleistocene era (2.6 million to 11,700 years ago), most megafauna- Hlarge, mostly herbivorous (plant-eating) land animals- -went extinct in North America. Their disappearance has been the topic of much debate, but considerable evidence supports a hypothesis known as “Pleistocene overkill.” The idea is that, as humans spread across the continent, they preyed upon large herbivores, such as woolly mammoths, ground sloths, and tortoises, and wiped them out. As originally formulated by the American geoscientist Paul Martin, the large mammals were driven to extinction in a few hundred years in a massive, fast-moving event. A newer version of the hypothesis posits that the extinctions were more gradual, based on evidence that, in some areas, humans and large animals coexisted in the same habitats for long periods of time, despite hunting. However, the end result was the same: extinction of the megafauna. Large animals are more vulnerable to extinction than smaller ones because they cannot hide easily from human predators and because they reproduce quite slowly. It is possible that the large animals were also relatively unafraid of human beings, since they would have evolved for hundreds of thousands of years without humans present. In addition, there is some indication that a rapid shift in climate reduced the habitats of many of the giant herbivores, making them more vulnerable to human predation. Likewise, Australian biologist Tim Flannery suggests humans may have changed the environment through their actions, especially by increasing the frequency of fires.