“Biological annihilation”: extinctions change the trajectory of evolution

Scientists believe entire genera – the evolutionary classification that groups separate but genetically distinct species – are disappearing as other organisms buckle under pressure from human activity across the planet.

In research published in the Proceedings of the National Academy of Sciences, ecologists Professor Gerardo Ceballos and Paul Ehrlich suggest vertebrates – backboned animals – are not merely disappearing at the species level, but also at higher taxonomical grouping of species.

It points to the thylacine (Thylacinus cynocephalus), baiji (Lipotes vexillifer, the Yangtze River dolphin) and passenger pigeon as recent examples of species that have been declared extinct, representing both the demise of the species and genera.

They calculated the background extinction rate – the pace with which a species could be expected to go extinct without human influence – at about 1 vertebrate genera every 250 years, based on there being 5,418 currently described groupings at this taxonomic level.

But they found the actual rate is much higher – in the past 500 years, 72 vertebrate genera have disappeared. In effect, a genus disappears every 7 years.

They describe this process as a “mutilation of the tree of life” – with individual branches essentially pruned by human pressure.  While non-vertebrate species like insects, molluscs, fungi and plants are not included in their study, other research suggests similar rates of decline are being experienced by these groups.

“As scientists, we have to be careful not to be alarmist,” Ceballos says

“[But] we would be unethical not to explain the magnitude of the problem, since we and other scientists are alarmed.”

One such scientist is Professor Corey Bradshaw, the director of Flinders University’s Global Ecology Laboratory and a chief investigator of the ARC Centre for Excellence for Australian Biodiversity and Heritage.

While he wasn’t involved in the Ceballos-Ehrlich study, he says the findings are, if anything, conservative.

“Surprisingly conservative,” Bradshaw says. “I think that given the really poor data on actual extinctions – we usually have it for only the most charismatic and largest species – these estimates are actually surprisingly low.”

Bradshaw says climate change’s complex impacts and the effects of co-extinctions – species disappearances that occur as part of a ‘cascade’ from the wipeout of another – are underrepresented in models. His recent work suggests co-extinctions could contribute 30% on top of primary extinctions by the end of the century due to climate and land use change.

Sixth mass extinction underway

Evolutionary scientists warn that the world is likely experiencing a new “mass extinction event.”

Such events – this is believed to be the sixth – see species disappear over massive timescales.

The risk for ecological systems is the collapse of important functions provided by key organisms and organism families.

In their study, Ceballos and Ehrlich warn of biosphere-wide transformations through taxonomic losses that could make it “impossible for … civilisation to persist.

“In the long term, we’re putting a big dent in the evolution of life on the planet,” Ceballos says. “But also, in this century, what we’re doing to the Tree of Life will cause a lot of suffering for humanity.”

The loss of taxa leaving holes in what they describe as the tree of life’s ‘canopy’ will not be readily replaced, though similar vertebrates could temporarily occupy the void left by a missing group.

They point to the unusual gastric-brooding frogs (Rheobatrachus) of Queensland, which swallow their eggs (rather than releasing them into their environment) and develop tadpoles inside their stomachs – effectively converting them into uteruses – as a lost opportunity for human study.

Similarly, the demise of once-widespread passenger pigeons limits diets in northeastern North America for predators, and allows rodent species they competed with for food to flourish. The knock-on for humans is less food to eat and more disease borne by rodents.

Bradshaw describes the current ecological situation as a “perfect storm”, and warns the limited research on non-vertebrate extinctions is a cause for concern.

“Systems are already highly compromised through habitat conversion, pollution, invasive species and all the other things we’ve done, so you add climate change into the mix and you get this catastrophic drop in the Earth’s carrying capacity for every species,” Bradshaw says.

“And as you get natural systems collapsing around us, our demise will be guaranteed as a result.

“It’s quite a concern, we’ve taken out all of the elasticity left in the system and we’re continuing to push it… the ‘balloon’ is bursting.”

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