When mammoths roamed

Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America:

Although the North American megafaunal extinctions and the formation of novel plant communities are well-known features of the last deglaciation, the causal relationships between these phenomena are unclear. Using the dung fungus Sporormiella and other paleoecological proxies from Appleman Lake, Indiana, and several New York sites, we established that the megafaunal decline closely preceded enhanced fire regimes and the development of plant communities that have no modern analogs. The loss of keystone megaherbivores may thus have altered ecosystem structure and function by the release of palatable hardwoods from herbivory pressure and by fuel accumulation. Megafaunal populations collapsed from 14,800 to 13,700 years ago, well before the final extinctions and during the Bølling-Allerød warm period. Human impacts remain plausible, but the decline predates Younger Dryas cooling and the extraterrestrial impact event proposed to have occurred 12,900 years ago.

Ed Yong has the bases covered:

What about humans, those pesky slayers of animals? Some scientists believed that North America's Clovis people specialised in hunting big mammals, causing a "blitzkrieg" of spear-throwing that drove many species to extinction. But these hunters only arrive in North America between 13,300 and 12,900 years ago, around a thousand years after the population crashes had begun.

If people were responsible, they must have been pre-Clovis settlers. There's growing evidence that such humans were around, but they weren't common or specialised. They may have contributed to the beasts' downfall, while Clovis hunting technology delivered a coup de grace to already faltering populati0ons.

By analysing the sediment at Appleman lake - spores, pollen, charcoal and all - Gill has replayed the history of the site, spanning the last 17,000 years. Her data rule out a few theories, but as she says, they "[do] not conclusively resolve the debate" about climate causes versus human ones. It's possible that similar studies at different sites and other continents will help to provide more clues.

A complex story like this is perhaps more common than an event such as the extinction of the passenger pigeons. Populations of organisms often go through cycles in census size, whether due to environmental variation or coevolutionary dynamics with parasites. Consider the example of the Tasmania devil, the disease which it is susceptible to is not the doing of humans, but the introduction of dingos (probably by humans) mean that the species is restricted to Tasmania. Additionally, humans have laid claim to much of the habitat of the devil (or what was the habitat of the devil). When a virulent disease hits, the devil has a much smaller margin of error than it had before. It could be that recent megafaunal extinctions are ultimately due to humans, even if they are not always proximately due to humans.