The Earth’s ongoing sixth mass extinction has claimed 73 genera of vertebrates, or entire branches of the tree of life, over the past 500 years, scientists said on Monday in a new study examining the pace of species loss.
Their analysis has shown that species loss has since AD 1500 proceeded at a rapid pace, which they say is “mutilating” the Earth’s tree of life, altering the trajectory of evolution, and damaging the ecosystems that make human civilisation possible.
The most vertebrate genera extinctions since AD 1500 have occurred among birds (217 species, 44 genera), followed by amphibians (182 species, 5 genera), mammals (115 species, 21 genera) and reptiles (90 species, 3 genera), the study found.
The extinct genera include elephant birds and sloth lemurs of Madagascar, the Tasmanian tiger, the flightless moa of New Zealand, the Yangtze river dolphin, the passenger pigeon of North America, and the saddle-backed giant tortoise of Rodriguez Island in the Indian Ocean.
The gharial crocodile found in India and Nepal, the king cobra of Asia, and the African elephant are among the genera currently under risk from the ongoing mass extinction that the scientists say is akin to a “biological annihilation”.
“In the long term, we’re putting a big dent in the evolution of life on the planet,” Gerardo Ceballos, a conservation scientist at the National Autonomous University of Mexico and the study’s coauthor, said in a media release. “But also, in this century, what we’re doing to the tree of life will cause suffering for humanity.”
Ceballos and his collaborator Paul Ehrlich, a senior biologist at Stanford University in the US, have calculated that the current rate of vertebrate genera extinction is 35-fold higher than the expected natural background extinction rate over millions of years before humans emerged.
“The genera lost in the last five centuries would have taken some 18,000 years to vanish in the absence of human beings,” the scientists said in their study published on Monday in the US journal Proceedings of the National Academy of Sciences.
Scientists expect the extinction of genera to hit ecosystems harder than the extinction of species. When a species goes extinct, Ceballos said, other species in the genus can fill at least part of its role in the local ecosystem. But when a genus disappears, it could take tens of millions of years for the gap to be covered through the evolutionary process of species emergence. The ecosystem gaps could lead to increased exposure of humans to diseases.
Scientists have attributed, for instance, the increasing incidence of the tick-borne Lyme disease to the loss of passenger pigeons in North America that allowed mouse populations to balloon and increase the risk of Lyme disease transmission to humans.
Environmental researchers believe that the large-scale destruction of forests and the resulting loss of biodiversity are among the factors that have contributed to the emergence of several potentially deadly infections, including ebola, Marburg, and hantavirus disease.
The extinction of genera could affect ecosystem functions and services, including productivity, biogeochemical cycles, and the myriad species interactions upon which human civilisation depends, the scientists said.
The extinctions may be changing the planet into a state in which “it may be impossible for our current civilisation to persist”, the scientists said in their paper, calling for “immediate political, economic, and social efforts of an unprecedented scale to prevent the extinctions and their societal impacts”.
Multiple extinctions have occurred throughout the Earth’s history, including five “major” ones between 2.5 billion years ago and 66 million years ago, all from natural causes — including volcanism and an asteroid hit — that changed the living conditions on the Earth. Scientists believe the ongoing sixth extinction is a human-driven phenomenon, the outcome of human population expansion and the loss of natural habitats.
