Abstract
Understanding extinction requires an inherently interdisciplinary approach. Ecological research is needed to study the population dynamics of declining species in real time, and their responses to different threat processes and management interventions, in order to develop effective policies for combating today’s catastrophic biodiversity loss. A knowledge of broad-scale patterns of biodiversity turnover across the fossil record is also crucial, to determine the long-term dynamics and trajectory of biotic responses to environmental change. This second area of research has developed considerably in recent years into the applied discipline of conservation palaeontology. However, these two approaches require fundamentally different perspectives and methodologies, and are separated by an ‘epistemological gap’ (Kemp, 1999) that can be challenging for researchers to traverse. As a result, overviews of extinction will typically approach their subject from one of these two contrasting starting points, each of which comes with both strengths and limitations in the insights they can make.
Although Michael Hannah’s book Extinctions has a generalised title and a cover that features both a Cambrian fossil and a polar bear, suggesting an integrated overview of extinction both past and present, it approaches the topic very much from a deep-time perspective (as its quirky subtitle reveals, and as explained in the book’s preface). This in itself is no bad thing. Slightly more frustratingly (but maybe inevitably), the book focusses largely upon the famous mass extinctions documented throughout Earth’s history, with little attention given to background extinction rates and what the fossil record might teach us about the process of extinction under ‘normal’ conditions. Indeed, sometimes the book runs the risk of lapsing into just presenting the well-trodden story of the evolutionary history of life through time, and contains some rather detailed and lengthy tangents about geological field trips that do not add much to the main narrative (although it is healthy to be reminded that most of the history of life consisted of prokaryotic cells and microbial mats, and I am always happy to learn the latest opinions on what Ediacaran fossils actually were).
Where Hannah’s book succeeds is in its portrayal of the geological history of life helping to establish and maintain an interconnected Earth System, comprising an atmosphere, hydrosphere, geosphere and biosphere that are all linked by self-regulatory feedback loops. Although mass extinctions have had different specific ‘kill mechanisms’, in this model they can all be seen as disruptions that overwhelmed the Earth System’s homoeostatic balance, forcing it past a tipping point and resulting in sudden, catastrophic loss of biodiversity. In the case of the end-Permian mass extinction, the worst of the lot, this led to an estimated disappearance of 95% of all marine species – the question here is how the Earth System managed to recover at all, instead of flipping to an alternative stable state that was permanently inimical to life. Hannah makes a convincing argument that each mass extinction might have comprised a fateful combination of proximate and ultimate events, making them ‘accidents waiting to happen’: ‘[. . .] a build-up of longer-term ecological stress pushing life towards extinction but requiring a further trigger to deliver the final blow [. . .]’ (p. 158). Life on Earth might therefore be best described by analogy to the life of a soldier: ‘[. . .] long periods of boredom separated by short intervals of terror [. . .]’ (p. 47). Hannah contextualises this journey through the history of mass extinctions with another crucial cautionary lesson, repeated as a helpful refrain throughout the book: it remains extremely difficult to determine how much biodiversity was lost in each mass extinction, how long these events lasted or even what triggered them, because the fossil record is so incomplete and biased that we cannot read it at face value (it is memorably compared to a chain-link fence: strong, durable and full of holes). Comparing data from the present and the past can thus be like trying to compare apples and oranges, something that modern-day ecologists still often do not fully appreciate.
The flipside is that Hannah’s book is unfortunately rather weak on the ecology of extinction, and falls short of offering any real predictive insights about why some species have clearly been more vulnerable than others, both to mass extinctions in the past and to anthropogenic threats today. This is a considerable shortfall when trying to use the ‘big picture’ of the geological past to identify potential resilient solutions for the future. In the book’s top-down ‘Earth Systems’ perspective, biodiversity is presented as broadly equivalent when it comes to extinction risk, which we know is not the case. Some ecological attributes are inherently riskier than others when conditions change, which leads to extinction risk across species being phylogenetically clustered rather than random, because related species tend to share similar attributes. For example, the basic ecological reasons why the Pleistocene megafauna was intrinsically more vulnerable to either natural or human-caused extinction are not touched upon (big animals have lower population densities, require more resources and are slower to recover their populations compared to small animals). Even some of the ecological factors associated with winning or losing in older mass extinctions can be identified in retrospect, with huge significance for predicting potential vulnerability to different kinds of future environmental change (e.g. larger geographic ranges or greater investment in offspring seem to have been key determinants across ammonoids and nautiloids; Staaf, 2017).
So, if we are on the cusp of a new, human-caused mass extinction, what does the future hold? The bad news is that, although life in some form will continue, global biodiversity will likely take a million years or more to recover to pre-extinction levels (and maybe much longer: coral reefs took 27 million years to become re-established after the end-Permian extinction). The good news is that mass extinction is not yet inevitable; the Earth System can be surprisingly resilient before reaching an irreversible tipping point, as demonstrated by the lack of a noticeable extinction event associated with the rapid and severe global temperature increase at the Palaeocene-Eocene boundary. However, this does not provide room for complacency. So, what is to be done? Societal solutions to this global emergency require a range of practical policy and behavioural changes, but mitigative actions must be evidence-based and grounded in good science. Unfortunately, although the book is framed around the premise that we can learn lessons from the past that can change the future, it ends with limited consideration about how the fossil record might play a helpful role in this process, with few suggestions other than ‘[. . .] to fully document our biota to help us make environmentally sensible decisions [. . .]’ (p. 207) (sadly easier said than done, as funding and training for taxonomy continue to dwindle; Britz et al., 2020).
Despite the global urgency of understanding how extinction ‘works’, books about the topic tend to be fairly limited in scope, typically dealing either with specific prehistoric mass extinctions or with specific recent species losses (including my own effort; Turvey, 2009). Accessible overviews that synthesise wider patterns and processes of extinction are still surprisingly thin on the ground, and so Hannah’s contribution is to be welcomed. I hope we see a continued increase in books about the science of this topic that attempt to balance deep-time and near-time insights, as a much-needed resource for decision-makers trying to prevent extinctions from continuing to escalate in the world around us.