Why did so many large animal species go extinct in "near time", in the last twenty thousand years or so? In End of the Megafauna Ross MacPhee explores this question, centering Paul Martin's "overkill" hypothesis, that the extinctions were the result of hunting by humans, and climate change as an alternative. But he also looks at possible explanations such as food web disruptions, bolide impacts, changed fire regimes, the spread of disease, and combinations of all these — and the likelihood that their effects varied geographically. It is a lively account, capturing the to-and-fro of scientific debate and the uncertainties of the paleontological and archaeological record.

"I found that many ecologists and most conservation biologists have no doubt at all that Paul Martin was correct in perceiving humanity's cloven hoofprints stamped all over the record of prehistoric extinctions. That does not mean that they necessarily accept overhunting as the exclusive kill mechanism. Indeed few do, preferring instead to lay the blame on contributory activities, such as overexploitation, environmental damage, and the introduction of exotic species. ...

By contrast, I find that many paleontologists and archeologists are reluctant to implicate human practices, arguing that factors other than overhunting must have been at work during Near Time ..."

End of the Megafauna has extensive, gorgeous illustrations by Peter Schouten, including many full page panoramas. The captions to these include a fair bit of general ecology and biology, helping give a feel for the diversity of lost animals (mostly mammals), which included some strange and unusual species as well as larger versions of existing ones.

"MALTA PANORAMA: Many Mediterranean islands supported native species of mammals and birds that have now completely disappeared. Elephants, deer, and hippos were frequently present in these faunas, bearing witness to their ability to colonize islands. As elsewhere, large mammals tended to undergo strong selection for downsizing: the dwarf Maltese elephant illustrated here was no larger than a small pony; the tiny hippo was the size of a large pig. By contrast, island-adapted birds often grew much larger than their mainland ancestors, in part because they no longer needed to retain small body sizes in order to fly efficiently. The giant Maltese swan, for example, was probably about 25 percent larger than the North American trumpeter swan (Cygnus buccinator), the largest living waterfowl. Although the dwarf elephant and hippo lasted until the end of the Pleistocene, the swan is thought to have died out much earlier."

MacPhee largely evaluates "overkill" against a "climate change" alternative, but he mixes this up by also comparing "overkill" to other anthropogenic mechanisms, in a way that seems to me to conflate different questions. Given so much depends on the dating, most obviously on the ordering of and delay between human arrival and extinctions, it seems to me that a better approach would be to first evaluate the relative merits of anthropogenic and non-anthropogenic explanations, and then to consider specific mechanisms. (On the evidence MacPhee presents — most notably the lack of extinctions at anything like the same rate for millions of years previously, despite periods of rapid climate change — the weight of evidence seems to me heavily in favour of anthropogenic causes, even if the mechanisms remain unclear.)

September 2020

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