Sixth time unlucky

There’s something sobering – terrifying is the more apt word – about a peer-reviewed paper that contains, under the heading “Significance”, these words:

200603 vertebrates
Terrestrial vertebrates on the brink include (a) the Sumatran rhino (b) Clarion island wren (c) Española Giant Tortoise (d) Harlequin frog. Credit: (a) Rhett A Butler (b) Claudio Contreras Koob (c) and (d) Gerardo Ceballos

“The ongoing sixth mass extinction may be the most serious environmental threat to the persistence of civilisation, because it is irreversible.

“Thousands of populations of critically endangered vertebrate animal species have been lost in a century, indicating that the sixth mass extinction is human caused and accelerating. The acceleration of the extinction crisis is certain because of the still fast growth in human numbers and consumption rates.

“In addition, species are links in ecosystems, and, as they fall out, the species they interact with are likely to go also. In the regions where disappearing species are concentrated, regional biodiversity collapses are likely occurring. Our results re-emphasise the extreme urgency of taking massive global actions to save humanity’s crucial life-support systems.”

The research article this statement precedes – just published in Proceedings Of The National Academy Of Sciences – is titled “Vertebrates on the brink as indicators of biological annihilation and the sixth mass extinction”. Biological annihilation – the authors’ words.

More than 500 terrestrial vertebrate species, the study says, are on the brink of extinction. They include icons such as the Sumatran rhino (Dicerorhinus sumatrensis) and Española Giant Tortoise (Chelonoidis hoodensis) and their decline is likely driven in large part by human activities in biodiversity hotspots.

To assess the risks faced by terrestrial vertebrates and reach their conclusions, Gerardo Ceballos, of the National Autonomous University of Mexico, and colleagues looked at data from the IUCN Red List of Threatened Species and from Birdlife International.

The researchers found that 1.7% – or 515 out of 29,400 – of the evaluated terrestrial vertebrate species are on the brink of extinction, which they define as having fewer than 1000 remaining individuals.

The authors report that species at the brink of extinction have lost most of their populations and individuals. They compared the historic and current distributions of 48 species of mammals and 29 species of birds (mammals and birds are the only groups for which such data are available) to learn about the extent and significance of population extinction.

200603 species map
Historic (top) and current (bottom) range of 48 mammal and 29 bird terrestrial species on the brink.

Their comparisons indicated huge reductions of the historic geographic ranges of those species, which represented a massive loss of populations.

The data suggest, say the authors, that during the past 200 years, roughly 3600 populations of the 48 mammal species and 2930 populations of the 29 bird species have disappeared.

Those mammal and bird species have lost an average of 95% and 94% of their geographic range since 1900. If we assume a similar reduction of the historic range of all of the 515 vertebrate species on the brink, then 237,000 of their populations have disappeared since 1900.

Vertebrates on the brink of extinction are located primarily in tropical and subtropical regions, concentrated in areas that are heavily affected by human activities.

Adding to the agony, about 84% of the 388 terrestrial vertebrate species that have fewer than 5000 remaining individuals are located in the same geographical regions as species on the brink. They may therefore soon face a similar risk due to the human-driven collapse of regional biodiversity.

The authors suggest that the findings underscore the need for global action to prevent further loss of terrestrial vertebrate species.

In their conclusions, they note: “The extinction crisis, like the toxification and climate crises to which it is tied, poses an existential threat to civilisation. Although it is more immediate than climate disruption, its magnitude and likely impacts on human well-being are largely unknown by governments, the private sector, and civil society. It is, therefore, a scientific and moral imperative for scientists to take whatever actions they can to stop extinction.

“There is time, but the window of opportunity is almost closed. We must save what we can, or lose the opportunity to do so forever. There is no doubt, for example, that there will be more pandemics if we continue destroying habitats and trading wildlife for human consumption as food and traditional medicines.

“It is something that humanity cannot permit, as it may be a tipping point for the collapse of civilisation. What is at stake is the fate of humanity and most living species. Future generations deserve better from us.”

200603 species chart
Population size of species on the brink. Most are especially close to extinction because they consist of fewer than 250 individuals, usually scattered through several small populations.

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