CREDIT: ILLUSTRATION: OLIVER DAY, OREGON STATE UNIVERSITY

According to a new study in the Proceedings of the National Academy of Sciences, an animals’ size is a good indicator of its risk of extinction. Animals that are in the “not too big, not too small” (think Goldilocks: just the right size) face a much lower risk of extinction than larger or smaller animals.

“Knowing how animal body size correlates with the likelihood of a species being threatened provides us with a tool to assess extinction risk for the many species we know very little about,” said William Ripple, a professor of ecology at Oregon State University and lead author of the study, in a news release.

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Ripple and his team’s investigation was motivated by a desire to better understand and prevent extinction, a noble cause considering species are disappearing at least 1,000 times faster than normal. Over 20,000 plants and animals are on the brink of vanishing forever and we’ve already lost too many animals, including species of rhinos, leopards, tortoises and dolphins. Based on current extinction rates, scientists and conservationists have warned that we’re heading towards a massive extinction, the likes of which have not been seen for over 65 million years. If such an event were to occur, it would likely take anywhere from ten to thirty million years for life to recover to its current state, over 100 times as long as humans have been on this planet.

Luckily, scientists like Ripple are hard at work deciphering the mechanics behind human driven extinction. “Determining the drivers of extinction risk has been a key pursuit of conservation biology,” the authors of the study write. “Considering that body mass could be a strong predictor of extinction risk, we constructed a global database of body masses for 27,647 vertebrate species.” The study found that larger animals make up the bulk of threatened species, and are the ones most threatened by hunting and harvesting; “many of the larger species are being killed and consumed by humans,” explained Ripple. “And about 90 percent of all threatened species larger than 2.2 pounds (1 kilogram) in size are being threatened by harvesting.”

The list of large animals includes many of the charismatic species that we often hear about: tigers, rhinos, bears, elephants, dolphins, whales, and primates. But the study also found that smaller animals, which often get less attention, are also increasingly at risk. Animals weighing in at less than 3 ounces in body weight tend to be highly susceptible to habitat loss and environmental degradation, due to their “restricted geographic ranges.” The authors of the study stressed the importance of recognizing the risks that these smaller but lesser-known animals face, writing “the lightest vertebrates are most threatened by habitat loss and modification stemming especially from pollution, agricultural cropping, and logging.”

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Among this group of threatened smaller animals, amphibians stand out: scientists estimate that more than one-third of all amphibians are threatened with extinction, earning them the title of most endangered class in the animal kingdom. And it was a large-scale study of extinction among amphibians that first led scientists to theorize that the current extinction rates were so high that humans could in fact be causing the world’s next mass extinction. Mass extinction events have happened only five times in geological history; the most famous of this is the fifth mass extinction, which occurred about 66 million years ago and killed off the dinosaurs.

Ripple and his colleagues posit that if the current extinction crisis were left to continue, it would have reverberating impacts across our planet, “jeopardizing ecosystem services to humans, and generating cascading ecological and evolutionary effects on other species and processes.”

If we want to have a chance of halting this human asteroid of mass extinction, we must better understand the drivers of extinction. And Ripple’s study is just one more piece in the larger puzzle that hopefully we solve before it’s too late.

“Our results offer insight into halting the ongoing wave of vertebrate extinctions by revealing the vulnerability of large and small taxa, and identifying size-specific threats,” concludes the paper. “Moreover, they indicate that, without intervention, anthropogenic activities will soon precipitate a... [fundamental] reordering the structure of life on our planet.”