Scientific misunderstanding about the nature and consequences of the sixth mass extinction has led to confusion among policy-makers and the public. Scientists agree that there have been five mass extinctions in the past 600 million years (1). Although scientists also agree that Earth is now suffering the sixth mass extinction, they disagree about its consequences. Mass extinctions are defined as the loss of the majority of species in a relatively short geological time, caused by a catastrophic natural event (2). Some scientists argue that there is no reason for concern about the sixth mass extinction because extinction is normal, simply an inevitable consequence of the process of evolution (3, 4). This misunderstanding ignores some critical issues. First, the rate of species extinction is now as much as 100 times that of the “normal rate” throughout geological time (5, 6). Second, like the past mass extinctions, the current episode is not an inevitable consequence of the process of evolution. Rather, it is the result of a rare event changing the environment so quickly that many organisms cannot evolve in response to it.
In theory, evolution on Earth could proceed as long as conditions permitted with no mass extinction events. That has been the case for vast stretches of geological time between occasional encounters with unusual environmental circumstances. Extinctions did occur, but not suddenly and nearly universally, as is happening now (7, 8). The rate and extent of current extinctions is similar to those of past mass extinctions, not the intervals between them (9, 10). If past mass extinctions are any guide to the rate at which usual evolutionary diversification processes could restore a reasonable level of biodiversity and ecosystem services, the wait is likely to be millions, or even tens of millions of years (8, 9).
At the time of the past mass extinctions, there was no industrialized human population of almost 8 billion people utterly dependent on the ecosystem services biodiversity helps provide, such as pollination, pest control, and climate amelioration (7, 8, 11). Scientists who deny that the current mass extinction has dire consequences, and policy-makers who listen to them, fail to appreciate the penalties human civilization will suffer for continuing on society's business-as-usual course (2–5). Moreover, beyond the consequences to humans, exterminating most of the only known living things with which we share the universe is clearly wrong (5–8, 12). The future of life on Earth, and human well-being, depends on the actions that we take to reduce the extinction of populations and species in the next two decades (8). It is irresponsible and unethical not to act despite the overwhelming scientific evidence indicating the severity of the current mass extinction event.
References (see Google Scholar for all)
1. W. J. Ripple et al., Bioscience 67, 197 (2017). 2. A. Hallam, P. B. Wignall, Mass Extinctions and Their Aftermath (Oxford University Press, UK. 1997). 3. S. Brand, “Rethinking extinction” (2015); https://aeon.co/essays/we-are-not-edging-up-to-a-mass-extinction. 4. C. D. Thomas, Inheritors of the Earth (Hachette, UK, 2017). 5. S. L. Pimm et al., Science, 344, 1246752 (2014). 6. G. Ceballos et al., Sci. Adv. 1, e1400253 (2015). 7. R. Dirzo et al., Science 345, 401 (2014). 8. G. Ceballos et al., The Annihilation of Nature: Human Extinction of Birds and Mammals (JHU Press, 2015). 9. D. Jablonski, Evol. Biol. 44, 451 (2017). 10.A. D. Barnosky et al., Nature 471, 51 (2011). 11.C. A. Hallmann, PLOS One 12, e0185809 (2017).C 12.P. R. Ehrlich, A. H. Ehrlich, Proc. R. Soc. B 280, 20122845 (2013).