What killed the mammoths?

I've recently started poking around on PLoS (Public Library of Science) Biology open access journal. There are some really good articles there. Today I found an intriguing article about What Killed the Mammoths.

Over the years I've read several papers about what killed off megafauna (large animals like mammoths, giant ground sloths, etc.) during the Pliestocene epoch. There are quite a few theories, including climate change, overhunting by Native Americans, or disease. I never totally agreed with any of the theories I read, although I found parts of them very intriguing. One of the most well-known theories is from Paul Martin who came up with the Pliestocene Overkill Hypothesis in an article titled The Discovery of America. In the article, he suggests that as humans crossed the Bering Straight, they encountered huge animals not used to human hunters; humans were able to easily kill the large animals. He suggests that humans followed the large game, fanning out across both North and South America, and killing off all of the large animals as they went. By the time the new colonists reached Tierra del Fuego, all of the large game had been hunted to extinction. His theory is rather compelling, but I just have a hard time believing that humans could have killed off all of some large animals, but not other species that did not go extinct.

A new paper in PLoS presents a new and interesting take on this theory.

The author, Caitlin Sedwick, suggests that a combination of global warming, habitat loss, and overhunting by humans is what finally drove mammoths extinct.

To examine the likely factors contributing to the mammoth's demise, the authors performed quantitative modeling of the climatic conditions inhabited by the mammoth in several periods of the last ice age. Their model related the fossil record—showing the distribution and age of mammoth remains—with simulated maps of the mean highest temperature, the mean lowest temperature, and average rainfall conditions on the Eurasian supercontinent for three time points during the last glacial advance in the Pleistocene (42,000, 30,000, and 21,000 years ago) and to a point in the previous interglacial middle of the Holocene (6,000 years ago). Next, they aligned their climatic models on the Eurasian supercontinent 126,000 years ago (the previous time the planet had warmed between glacial advances). Together, these data allowed the group to estimate the characteristics and extent of the animals' favored habitat at the various time points studied.

The authors' findings demonstrate that mammoths experienced a catastrophic loss of habitat: as the last glaciers retreated and the planet warmed, 90% of the animals' former habitat disappeared. Prime mammoth habitat progressively shrank from 7.7 million square kilometers 42,000 years ago (in the midst of the last glacial advance) until just 0.8 million square kilometers remained 6,000 years ago. The animals were restricted to isolated tracts spotted across Eurasia and tiny patches squeezed up against the northern coastal edges.

As adequate habitat disappeared, the populations dwindled. The author proposes that this same scenario occurred during past warming events, and mammoths crept dangerously close to extinction, but managed to rebound when the climate again cooled. The difference between past ice ages and the most recent one is that mammoths now had human hunters to contend with. Due to restricted habitat and already reduced numbers, it didn't take much hunting to wipe out that last of the mammoths

This is the most compelling case I've read for why mammoths disappeared. It's also an interesting tie to today's ecological problems. Even though in the distant past animals were able to weather dramatic shifts in climate, I doubt that many life forms today could escape from human interference.