It seems strikingly obvious that modern humans are a pretty big deal. In Pat Shipman’s The Invaders she argues that H. sapiens can be thought of as a top predator which is so efficient that it rearranges the whole ecosystem, wreaking havoc with the conventional trophic cascades. We can see this in the archaeological record. Humans arrive in Australia, and all sorts of cool marsupial species disappear. A similar phenomenon is attested for the New World. The more recent extinctions on islands such as Madagascar and New Zealand are well attested.

Nevertheless, many people still argue that the pattern of extinctions which we see over the Pleistocene and Holocene is not the outcome of human expansion, but climate change. In other words, they are not anthropogenic. Don’t believe me? Here’s a paper from a few years ago, Serial population extinctions in a small mammal indicate Late Pleistocene ecosystem instability:

Examination of an under-exploited source of ancient DNA—small-mammal remains—identified previously unreported and unprecedented temporal population structuring of a species within Europe during the end-Pleistocene. That we identify a series of population extinctions throughout the Pleistocene from a small-mammal species demonstrates an extensive and prolonged diversity loss and suggests a nonsize-biased reduction in ecological stability during the last glaciation, a pattern consistent with climatic and environmental change as key drivers for changes in Late Pleistocene biodiversity.

You can find similar arguments for other particular regions and areas. For example, in North America. The mysterious hand of climate is everywhere. To some extent it reminds me of arguments about the Indo-European languages, and their origin. “Out of India” proponents make points which would be just as valid for the Greeks, e.g. the early Indians and Greeks did not have a memory of being from anywhere else. Obviously the Indo-European languages are unlikely to both originate in India and Greece, but when examining just one area the arguments can seem persuasive. What needs to happen when assessing probabilities though is to get a sense of the broader framework of prior information. The same applies to the mass extinctions of the past few hundred thousand years. A new preprint on bioRxiv tries to do this, Historic and prehistoric human-driven extinctions have reshaped global mammal diversity patterns:

…Results: We find that current diversity patterns have been drastically modified by humans, mostly due to global extinctions and regional to local extirpations. Current and natural diversities exhibit marked deviations virtually everywhere outside sub-Saharan Africa. These differences are strongest for terrestrial megafauna, but also important for all mammals combined. The human-induced changes led to biases in estimates of environmental diversity drivers, especially for terrestrial megafauna, but also for all mammals combined. Main conclusions: Our results show that fundamental diversity patterns have been reshaped by human-driven extinctions and extirpations, highlighting humans as a major force in the Earth system. We thereby emphasize that estimating natural distributions and diversities is important to improve our understanding of the evolutionary and ecologically drivers of diversity as well as for providing a benchmark for conservation.

It’s a preprint, you can read the whole thing. Of course I’m broadly persuaded, since it only confirms rigorously what I already believed impressionistically.

Citation: Historic and prehistoric human-driven extinctions have reshaped global mammal diversity patterns, Søren Faurby , Jens-Christian Svenning, doi: http://dx.doi.org/10.1101/017368