Sixth Mass Extinction Is Erasing the Tree of Life

The sixth mass extinction is accelerating

Human activity is pruning whole limbs from the tree of life, much as the comet that struck the dinosaurs did - only more gradually, but with consequences that are no less lethal, a new study has confirmed.

Ecologists warn in their new paper: "It is changing the trajectory of evolution globally and destroying the conditions that make human life possible."

"It is an irreversible threat to the persistence of civilization and the livability of future environments for Homo sapiens."

During recent months, the sixth mass extinction has become alarmingly apparent.

There have been widespread seabird die-offs, coastlines strewn with large numbers of dead fish, and sea lions poisoned by algal blooms triggered by heat. Last year, entire penguin populations did not breed, while researchers have for years been examining a troubling decline in insect life.

Extinctions are erasing branches of the tree of life

Ecologist Gerardo Ceballos, of the National Autonomous University of Mexico, and Stanford University conservation biologist Paul Ehrlich examined species extinctions since 1500 CE, comparing them with those recorded across the previous 500 million years. Their analysis found that human activity has caused the extinction of 73 genera of vertebrates over the past 500 years.

A genus is the taxonomic rank immediately above species. It brings together the most closely related organisms, rather like siblings within a family tree.

This pace is 35 times greater than earlier genus-level extinction rates. In the absence of human influence, the same total number of genera would have taken 18,000 years to disappear. Separate research has likewise identified comparably elevated extinction rates among plants, fungi and invertebrates.

"[The sixth mass extinction] is causing rapid mutilation of the tree of life, where entire branches (collections of species, genera, families, and so on) and the functions they perform are being lost," the researchers explain.

The biosphere in which we exist is deeply interconnected. Losing groups of species with particular roles in this web of life can therefore trigger serious knock-on effects.

"We and all other species have evolved together thriving within a stable tree of life," Ceballos and Ehrlich say, meaning that the disappearance of ecological functions carried out by species groups affects humans directly as well.

For instance, the decline of frogs that eat mosquitoes has coincided with rising malaria infections in Central America.

Lost genera and destabilised ecosystems

Ceballos and Ehrlich calculate that the loss of genera is likely to speed up further. Should the present course continue and every currently endangered genus vanish by 2100, the equivalent loss across 300 years since 1800 would, at normal background extinction rates, have required 106,000 years.

The species most at risk are often the planet's most distinctive, yet they tend to be overlooked. Their disappearance will erase millions of years of evolutionary history that cannot be recreated, along with vital functions that helped the surrounding biological cycles operate like a well-oiled machine.

"[It] required millions of years for evolution to generate functional replacements for the extinct organisms," Ceballos and Ehrlich note.

Climate change by itself is already causing major disruption across these systems. It is shifting the crucial timing of ecosystem services such as pollination, reducing the range of species present and enabling new species to invade more easily.

A separate new study recorded precisely these processes in an Arizona dryland stream from 1985 to 2019.

"Our study provides evidence of climate change-induced modifications of mechanisms underpinning long-term community stability, resulting in an overall destabilizing effect," University of California ecologist Junna Wang and her team write.

However, the sixth mass extinction extends far beyond the vast, interconnected disaster of climate change alone. Plastics, pesticides, habitat destruction and poaching, among other pressures, are giving the life around us no opportunity to recover.

"Immediate political, economic, and social efforts of an unprecedented scale are essential if we are to prevent these extinctions and their societal impacts," Ceballos and Ehrlich say.

Unlike the comet that wiped out non-avian dinosaurs, humans are aware of what they are doing and can alter their direction.

"What happens in the next two decades will very likely define the future of biodiversity and H. sapiens," the team concludes.

The research was published in PNAS.