Quick Study: The sixth mass extinction

Scientists say Earth is undergoing a sixth mass extinction.
Scientists say Earth is undergoing a sixth mass extinction.
The decline of numerous animal and plant species has led scientists to conclude that the sixth mass extinction in Earth’s history is underway.

    The study, in the Proceedings of the National Academy of Sciences, states that Earth is undergoing a “massive anthropogenic erosion of biodiversity and of the ecosystem services essential to civilization. This ‘biological annihilation’ underlines the seriousness for humanity of Earth’s ongoing sixth mass extinction event.”
    Here is an overview:

When were the first five mass extinction events?  Planet Earth can be a dangerous place. To understand, read about the other mass extinction events on Phys.org:

  • Ordovician extinction, about 445 million years ago. About 60 to 70 percent of species were lost due to an ice age.
  • Devonian extinction, 375 million to 360 million years ago. Up to 75 percent of species were lost due to oxygen depletion in the ocean.
  • Permian extinction, about 252 million years ago. Roughly 95 percent of species were lost. The possible causes were asteroid impact or volcanic activity.
  • Triassic extinction, about 200 million years ago. About 70 to 80 percent of species were lost. The cause of this extinction is hotly debated, but it may have been brought on by “climate change and rising sea levels resulting from the sudden release of large amounts of carbon dioxide,” according to Encyclopedia Britannica online. That carbon dioxide resulted from “widespread volcanic activity associated with the rifting of the super-continent Pangea, where eastern North America met northwestern Africa."
  • Cretaceous extinction, about 66 million years ago. An estimated 75 percent of species were lost. The likely cause was an asteroid strike. This extinction event eliminated dinosaurs.

 So why do scientists believe that we are undergoing a sixth extinction? They studied a sample of 27,600 vertebrate species, along with a detailed analysis documenting the population extinctions between 1900 and 2015 in 177 mammal species. In the detailed analysis, they found an “extremely high degree of population decay in vertebrates, even in common species.”  Indeed, of the 177 mammals, “all have lost 30 percent or more of their geographic ranges and more than 40 percent of the species have experienced severe population declines.”
     The loss is particularly noticeable with some species. The African lion population has dropped 43 percent since 1993, the paper noted.

Yes, but doesn’t this happen naturally? After all, there were five other extinction events. But this is not necessarily the natural way of things. The authors explain that “conservatively almost 200 species of vertebrates have gone extinct in the last 100 years. These represent the loss of about two species per year. Few realize, however, that if subjected to the estimated ‘background’ or ‘normal’ extinction rate prevailing in the last 2 million years, the 200 vertebrate species losses would have taken not a century, but up to 10,000 years to disappear, depending on the animal group analyzed.”

Why is this happening? The authors sum it up this way: “In the last few decades, habitat loss, over-exploitation, invasive organisms, pollution, toxification, and more recently, climate disruption, as well as the interactions among these factors, have led to the catastrophic declines in both the numbers and sizes of populations of both common and rare vertebrate species.”

The title of the study is: "Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines," published in the Proceedings of the National Academy of Sciences. The lead author is Gerardo Ceballos of the Institute of Ecology, National Autonomous University of Mexico, along with Paul R. Ehrlich and Rodolfo Dirzo, both with the biology department at Stanford University.


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