Is a sixth mass extinction already happening?

While new research suggests that a sixth mass extinction is underway and even more severe than was previously thought, other scientists say that alarmism won’t help conservation efforts. Tim Wallace reports.

A person riding an elephant silhouetted against bright sunlight.
Big-name endangered species like the elephant may be the tip of the extinction iceberg.
Aroon Phukeed

Are we living in an era of “biological annihilation”? Determining whether our planet is entering its sixth mass extinction depends on clearly demonstrating current extinction rates are far above the “background” rates that have prevailed between the five previous mass extinctions.

So wrote the authors of an influential 2015 paper that concluded a sixth mass extinction is indeed underway, based on “extremely conservative estimates” of the average rate of vertebrate species extinction over the past century. That extinction rate, they estimated, was up to 100 times higher than the background rate.

Gloomy conclusion notwithstanding, they sounded a small hopeful note: “Averting a dramatic decay of biodiversity and the subsequent loss of ecosystem services is still possible through intensified conservation efforts, but that window of opportunity is rapidly closing.”

Now two years on, two of the ecologists behind that paper – Gerardo Ceballos, of the National Autonomous University of Mexico, and Paul Ehrlich of Stanford University – have made a more urgent call, based on an assessment co-authored with Ehrlich’s Stanford colleague Rodolfo Dirzo.

The true severity of current situation has been underestimated by just focusing on species extinction, argues the new paper, published in Proceedings of the National Academy of Sciences, because it overlooks the accelerating extinction of populations.

“Population extinctions today are orders of magnitude more frequent than species extinctions,” the three ecologists write. “Population extinctions, however, are a prelude to species extinctions, so Earth’s sixth mass extinction episode has proceeded further than most assume. The massive loss of populations is already damaging the services ecosystems provide to civilisation.”

Their conclusion is based on calculations of population declines using a sample of 27,6000 vertebrate species – nearly half of all known vertebrate species – and more detailed analysis of 177 mammal species. The authors report “extremely high” rates of population loss even in species “of low concern” to the International Union for Conservation of Nature (the body that oversees the respected Red List of Threatened Species). Of the 177 mammals for which detailed data was available, all had lost 30% or more of their geographic ranges, with more than 40% having experienced population declines of more than 80%.

“as much as 50% of the number of animal individuals that once shared Earth with us are already gone.”

The new research includes spatial analysis that splits the surface of the globe into 10,000-km2 pieces and maps the global loss of vertebrate numbers in each. This mapping indicates, perhaps not surprisingly, that the species-rich areas of the tropics have larger numbers of decreasing populations, however temperate regions show similar or greater proportions of species decline.

“When public mention is made of the extinction crisis, it usually focuses on a few animal species (hundreds out of millions) known to have gone extinct, and projecting many more extinctions in the future,” write Ceballos, Ehrlich and Dirzo. “But a glance at our maps presents a much more realistic picture: they suggest that as much as 50% of the number of animal individuals that once shared Earth with us are already gone, as are billions of populations.”

Compelling as these figures are, not all agree the current state of events warrants being called the sixth mass extinction, elevated to a status alongside the planet’s five past mass extinction events. One notable dissenter is Douglas Erwin, Curator of Paleozoic Invertebrates at the Smithsonian Institution.

“Many of those making facile comparisons between the current situation and past mass extinctions don’t have a clue about the difference in the nature of the data, much less how truly awful the mass extinctions recorded in the marine fossil record actually were,” he told The Atlantic’s Peter Brannen recently.

Erwin emphasised he was in no way seeking to suggest that humans haven’t done great damage to marine and terrestrial ecosystems; he absolutely agreed that many extinctions had occurred and more would certainly occur in the near future as a result of human impositions on the biosphere.

But scientists had a responsibility to be accurate about such comparisons, he argued. “People who claim we’re in the sixth mass extinction don’t understand enough about mass extinctions to understand the logical flaw in their argument,” he said. “To a certain extent they’re claiming it as a way of frightening people into action, when in fact, if it’s actually true we’re in a sixth mass extinction, then there’s no point in conservation biology.”

Paul Ehrlich may be vulnerable to such a criticism: he first came to prominence in the late 1960s with his cataclysmic predictions of societal collapse due to overpopulation. “The battle to feed all of humanity is over,” he gloomily stated in the original edition of his 1968 best-seller The Population Bomb. “In the 1970s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now. At this late date nothing can prevent a substantial increase in the world death rate.”

prophecies of doom have limited currency in changing both attitudes and behaviour

That over-egged prediction has dogged Ehrlich since, though it hasn’t averted him from continuing to bang the drum about the ecological apocalypse posed by increasing human population. The “ultimate drivers” of the biotic destruction causing species population decline and extinction, as this new paper puts it, are “human overpopulation and continued population growth, and overconsumption, especially by the rich”.

The new paper, despite its dire conclusion of a “a dismal picture of the future of life, including human life”, isn’t quite as fatalistic about that future as Ehrlich might once have been. Echoing the glimmer of hope of the previous paper, it suggests there is still some chance to salvage the situation, though “the window for effective action is very short, probably two or three decades at most”.

If there’s one big lesson we can take from Ehrlich’s career of ringing alarm bells, it’s that prophecies of doom have limited currency in changing both attitudes and behaviour. How we respond to problems is shaped by the appeal of the solutions on offer. Research has shown, also, that attempts to frighten people into action, particularly when they involve non-immediate, distant concerns, are usually not only ineffective but can also be counterproductive.

So perhaps there’s something to be learnt from the slightly less pessimistic perspective of evolutionary biologist Chris Thomas, of the University of York. “There is no doubt that the rate at which species are dying out is very high, and we could well be in for a ‘Big Sixth’ mass extinction,” Thomas wrote recently. “Yet, we also know that the Big Five mass extinctions of the past half billion years ultimately led to increases in diversity.”

“This does not let us off the hook – species are genuinely dying out – but it does mean that we should not regard change per se as negative. We should perhaps think of ourselves as inmates and moulders of a dynamic, changing world, rather than as despoilers of a formerly pristine land.”

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