The Earth is losing species much faster than normal, or than new ones are evolving. The rate of loss may be the fastest since the aftermath of the asteroid that ended the reign of the dinosaurs, 65 million years ago. This has led some scientists to claim we are in a mass extinction; others have pushed back, and the debate has been running for at least three decades. Professor Alisa Bokulich of Boston University used the pandemic to convene meetings over Zoom to consider this question, among other weighty issues in the philosophy of science.
The results have now been accepted for publication, and the authors confirm the question does not lend itself to simple answers. Nevertheless, on balance the authors conclude we are not in a mass extinction. That does not mean, however, we are not in danger of reaching one soon.
A major obstacle to answering the question is that no universally agreed-on definition of a mass extinction exists. Five events are recognized as meeting the criteria, at the ends of the Ordovician, Permian, Triassic and Cretaceous periods, and during the Devonian.
However, another event – or possibly two – not long before the end-Permian extinction may have seen the disappearance of a higher proportion of the Earth’s species than four of these. Despite the pleas of some palaeontologists, this event is currently not classified as a mass extinction. Another candidate occurred before multicellular life emerged. The spottiness of the fossil record hinders attempts to estimate the severity of each of these events.
In the absence of agreement on what it takes to make an event a mass extinction, Bokulich and co-authors consider historical definitions and why some events have been given the classification and others so far have not. Then, they consider modern conditions in the same light.
Part of the disagreement is because conservation biologists and palaeontologists look at extinctions differently. Biologists and environmentalists count the loss of each individual species, since once gone we shall not see their like again. Ancient extinctions, on the other hand, have the loss of an entire genus as their unit of measurement. As the only surviving members of the genus Homo, the difference may not be applicable to us, but Bokulich gives the example of Panthera, whose surviving members include lions, tigers, jaguars, and two species of leopards. All five would need to go extinct before a palaeontologist would count it.
“If we were in a sixth mass extinction today, we would expect hundreds of genera to be going extinct, but we hardly know of any genera being lost in modern times, so the numbers don’t quite compare,” Bokulich said in a statement.
Rates of extinction are probably accelerating, so it might be argued we will be in a mass extinction soon if we can’t turn things around. The paper provides ideas on how to improve comparisons between the current situation and the “Big Five” past extinctions if this occurs. However, it also questions whether this is the best way to think about the current crisis, given what we are learning about the diversity in mass extinctions’ causes and the length of time over which they occurred.