Sixth Mass Extinction Is Cutting Branches From the Tree of Life

Human activity is severing whole limbs from the tree of life, much like the comet that struck the dinosaurs - only more gradually, and with consequences that are equally lethal, a new study confirms.

“It is changing the trajectory of evolution globally and destroying the conditions that make human life possible,” ecologists warn in their new paper.

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

The sixth mass extinction is becoming visible

During the past several 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. Entire penguin populations failed to breed last year, while researchers have spent years examining a worrying decline in insect life.

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

A genus is the taxonomic rank immediately above species, bringing together the closest-related organisms - rather like siblings on a family tree.

This pace is 35 times greater than earlier genus-level extinction rates. In the absence of human impacts, the same number of genera would have taken 18,000 years to disappear. Other research has likewise identified comparably high 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,” explain the researchers.

Lost genera mean lost ecological functions

The biosphere is profoundly interconnected, meaning that the disappearance of groups of species with particular roles in this living network can 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 the disappearance of whole ecological functions carried out by species groups affects people directly as well.

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

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

The species at greatest risk are often the planet’s most distinctive, yet least noticed. Their disappearance will erase millions of years of evolutionary history that cannot be recreated, alongside the vital roles they played in keeping surrounding biological cycles functioning smoothly.

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

Climate change compounds the extinction crisis

Climate change alone is severely destabilising these systems, disrupting the timing of essential ecosystem services such as pollination, reducing the range of species present and making it easier for new species to invade.

A separate recent study recorded precisely these processes in a dryland stream in Arizona between 1985 and 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 immense, interconnected catastrophe of climate change alone.

From plastics and pesticides to habitat destruction and poaching, human activity is giving the life around us no respite.

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

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

“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.