When we think about losing a species to extinction, we rarely think about the millions of years of evolution that went into creating the species in the first place. A study published yesterday (Oct. 15) in the Proceedings of the National Academy of Sciences set out to calculate that measure, figuring out exactly how many years of evolution have been negated by the mammalian species that have gone extinct since the rise of humans. The research puts the sixth mass extinction—and the devastation humanity has wrought—in clear terms.
The researchers, led by Matt Davis at Aarhus University in Denmark, calculated that humans, in just the 130,000 years we’ve existed, have managed to saw off 2.5 billion years-worth of evolutionary development among some 300 million mammal species that have gone extinct since the last ice age.
Some species represent far more evolutionary history—also known as “phylogenetic diversity”—than others. As Davis put it to the Guardian, hundreds of species of shrew exist, for example. Losing a shrew species to extinction would not be a happy development, but it would be less devastating to phylogenetic diversity than losing one of the only two remaining elephant species, both of which carry with them massive tracts of evolutionary history. According to the Guardian, Davis compared losing elephants to “chopping a large branch off the tree of life…whereas losing a shrew species would be like trimming off a small twig.”
According to the researchers’ calculations, if current extinction trends continue for the next 50 years—i.e., at some point in about 50 years humans either suddenly cease contributing to mammal extinctions or to go extinct—it would take mammals between three million and seven million years of evolution to regain the phylogenetic diversity the class of animals has lost in the last 130,000 years.
That timeline could be shortened with concerted conservation efforts that prioritize phylogenetic diversity, the researchers write. For example, if humans ceased contributing to extinctions right now, species diversity could recover within 500,000 years.
But barring that, the current rate of extinctions “puts us on the same scale as previous mass extinctions,” Davis told the Atlantic. “What we are going through now could have as big an impact as the asteroid” that led to the demise of the dinosaurs, he said.
Meanwhile, insects in tropical forests are being decimated by climate change—another environmental catastrophe caused by humans. That’s leading to the collapse of the tropical-forest food web. In a paper also published in the Proceedings of the National Academy of Sciences on Monday, researchers from Rensselaer Polytechnic University and the Universidad Nacional Autónoma de México found the biomass—or net weight—of insects and other arthropods living in the Luquillo rainforest in Puerto Rico has fallen between 10-60 times from what it was in the 1970s.
Animals that eat those insects are now themselves vulnerable to collapse as well—along with the animals that eat those animals, and so on. “Our analyses revealed synchronous declines in the lizards, frogs, and birds that eat arthropods,” the scientists write. “If supported by further research, the impact of climate change on tropical ecosystems may be much greater than currently anticipated.”
Since the 1970s, the average annual temperature in the Luquillo rainforest rose by 2°C.
Climate warming, the researchers write, “is the major driver of reductions in arthropod abundance, indirectly precipitating a bottom-up trophic cascade and consequent collapse of the forest food web.”