epictions of animals in ancient Egyptian artifacts have helped scientists assemble a detailed record of the large mammals that lived in the Nile Valley over the past 6,000 years. A new analysis of this record shows that species extinctions, probably caused by a drying climate and growing human population in the region, have made the ecosystem progressively less stable.
The study, published September 8 in Proceedings of the National Academy of Sciences (PNAS), found that local extinctions of mammal species led to a steady decline in the stability of the animal communities in the Nile Valley. When there were many species in the community, the loss of any one species had relatively little impact on the functioning of the ecosystem, whereas it is now much more sensitive to perturbations, according to first author Justin Yeakel, who worked on the study as a graduate student at the University of California, Santa Cruz, and is now a postdoctoral fellow at the Santa Fe Institute.
Around six millennia ago, there were 37 species of large-bodied mammals in Egypt, but only eight species remain today. Among the species recorded in artwork from the late Predynastic Period (before 3100 BC) but no longer found in Egypt are lions, wild dogs, elephants, oryx, hartebeest, and giraffe.
"What was once a rich and diverse mammalian community is very different now," Yeakel said. "As the number of species declined, one of the primary things that was lost was the ecological redundancy of the system. There were multiple species of gazelles and other small herbivores, which are important because so many different predators prey on them. When there are fewer of those small herbivores, the loss of any one species has a much greater effect on the stability of the system and can lead to additional extinctions."
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Depictions of animals in ancient Egyptian artifacts have helped scientists assemble a detailed record of the large mammals that lived in the Nile Valley over the past 6,000 years. A new analysis of this record shows that species extinctions, probably caused by a drying climate and growing human population in the region, have made the ecosystem progressively less stable.
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"Sometimes extinction happens naturally. Other times humans are to blame. Given the many millions of plant and animal species that have ever existed, it’s tough to know exactly how to assign responsibility. But new research indicates that we have an alarmingly large role.
Humans are wiping out species at least 1,000 times faster than nature is creating new species, according to a new study in Conservation Biology (paid access only). And it’s getting much worse. In the future, plants and animal species will go extinct at 10,000 times the rate at which new species emerge, the researchers assert.
Looking at both fossils and genetic variation, the study found that nature snuffs out its own creations much more slowly than we’d realized—at a rate of only one species per every 10 million. Past estimates put the “normal background extinction rate”—the rate at which species would go extinct without human interference—at about 10 yearly extinctions for every 10 million species.
Since mankind hit the scene, however, more than 1,000 out of every 10 million species have been dying out each year. “We’ve known for 20 years that current rates of species extinctions are exceptionally high,” said Stuart Pimm, one of the co-authors and president of the nonprofit organization SavingSpecies. “This new study comes up with a better estimate of the normal background rate—how fast species would go extinct were it not for human actions. It’s lower than we thought, meaning that the current extinction crisis is much worse by comparison.”
Overall species’ diversity grows exponentially richer over time, as branches of news species diverge. The authors liken this to a person’s bank account. Think of your income as the number of new species, while your spending is those that go extinct. Every month when you get paid, your net worth jumps for a while, before spending whittles it down again. If your spending is constant, that monthly spike will rise over time as your salary increases—just as the number of new species should also rise over time. But the authors saw no such increase, implying that extinction is happening far too fast for the pace of new species creation to keep up.
Take birds, for instance. There are 10,000 species of birds, as Pimm explains in a blogpost. At nature’s rate of one extinction per 10 million species, the disappearance of a single bird species should therefore be a once-in-a-millennium event. However, since the year 1500, at least 140 birds have disappeared—including 13 species we only identified after they went extinct."