Each year I teach a class on extinction, more specifically on how we know when a species is extinct and the uncertainties associated with this. After this year's class a student came up to me and during our discussions on various aspects of extinction they pointed out that often in conservation we talk about extinction without appreciating what it really means, as though it is just a binary state or something nebulous hanging over us. Michael Hannah's book, Extinctions: Living and Dying in the Margin of Error, is therefore a welcome antidote to this situation by providing a deeper understanding of the five mass extinction events (often referred to as the Big Five) and how they relate to the Anthropocene.
Hannah's book is carefully written, providing the reader with an understanding of the geological time scale, how the different spheres (atmosphere, hydrosphere, geosphere and biosphere) interact and how diversity in the fossil record is measured, before diving into how mass extinction events are identified and the potential causal factors. As there are, or have been, uncertainties around various aspects of mass extinctions, Hannah is careful to discuss all competing hypotheses before explaining on which side of the fence he falls (although sometimes he appears to be sitting on the fence). It is this level of detail, the building-up of the reader's understanding and the precision of the writing, all supported with numerous references for further reading, that makes this a standout book on extinction. Hannah's writing style reminds me of one of those teachers who captures their students’ attention and is always remembered for their engaging delivery. You know it is a good book when you have to put it down and text a friend to discuss a potential idea it has just sparked in you.
There are nevertheless a couple of points that I found frustrating. The first is rather minor: Hannah suggests there were more than five mass extinction events, in fact over a dozen, but he says little else about them. I was therefore left wondering: When did these events occur? Were they clustered in time or distributed throughout the history of life on Earth? What do we know or not know about them?
The second frustration is bigger. The book does well up until the last chapter or two, where Hannah applies the knowledge from the previous chapters to the current Anthropocene, starting off with the megafauna extinctions during the Pleistocene. The take-home message seems to be that although vast numbers of species became extinct during the Big Five, life itself survived and that will most likely also be the case with the Anthropocene. Not to finish on too positive a note, Hannah suggests that if we do not act swiftly to counter anthropogenic extinctions, we will have caused a mass extinction unlike any of the previous Big Five. To me, this appeared as though he wanted to use the Big Five as a warning but fell somewhat short of this goal. I think this is because the previous chapters consider mass extinctions at the geological time scale, something Hannah frequently emphasizes, whereas we are living day by day (not millennium by millennium) in the Anthropocene. Despite these shortcomings, I would recommend the book to those interested in conservation who wish to get a big picture—a really big picture—on extinction and start thinking about not only how human activities are defining the Anthropocene, but how the effects of these activities are unprecedented and how they fit within the history of life on Earth.
Open access
