Contrary to the established idea that large predators are particularly at risk of extinction, a study of more than 24,000 species of birds, mammals, and reptiles reveals that herbivores, especially the big-bodied varieties, live a more perilous existence. The report appears today (August 5) in Science Advances.
“This paper raises our attention that herbivores, who play a fundamental role in nutrient cycling, forest regeneration, and other key ecosystem services, are extremely vulnerable to human activities,” ecologist Mauro Galetti of the University of Miami who was not involved in the research writes in an email to The Scientist.
There is a sobering number of animal species that have been driven to extinction, or close to it, by human activities such as hunting and habitat destruction. A generally held view in the field of conservation biology is that top predators, such as tigers and polar bears, might be particularly at risk, says ecologist Trisha Atwood of Utah State University who led the work. Indeed, Atwood herself works on such predators but, she says, she came to realize that the evidence supporting this notion was flimsy.
There were a few papers on sharks or specific terrestrial predators, she explains, and “everybody just took those papers as gospel . . . and bought into the idea that it was predators [that were at risk].” But a broad analysis of the types of land animals most under threat was lacking. “That really motivated us” to investigate, she says.
The thrill of the paper is that it is claiming to overturn some fairly classically presumed truths: that it’s the predators of this world—the sharp-toothed, sharp-clawed creatures—that are the most threatened.—Dave Hodgson, University of Exeter
The team gathered data from the International Union for the Conservation of Nature (IUCN), the global authority on species extinction risk. The organization lists species that have recently disappeared and has made risk assessments for all living species of birds and mammals and for 70 percent of living reptiles. The IUCN also has risk assessments for fish, amphibian, and insect species, but for these the lists are far from complete, says Atwood, so the team excluded this data from their study.
To see whether predators really were the terrestrial animals most at risk of extinction, the team compared the proportions of living herbivore, omnivore, and predator species that were deemed at risk by the IUCN with the labels “vulnerable,” “endangered,” or “critically endangered.” To their surprise, it was the herbivores, with approximately 25 percent of species at risk, that came out on top—approximately 8 percent higher than the proportion of omnivores, and 10 percent higher than predators.
“We were quite shocked when we got the results back,” says Atwood. But, no matter how they dissected the data—breaking them down by taxonomic group (birds, mammals, reptiles), geographical area, or habitat—herbivores, in almost all cases, were the most vulnerable.
Even when the team looked at historical extinctions from the last 500 years listed by the IUCN and from the late Pleistocene documented in the literature they found herbivores were more likely to have disappeared than were omnivores or predators.
Atwood and colleagues went on to investigate drivers of extinction that might explain herbivores’ particular susceptibility. They found that body size positively correlated with extinction risk for all mammals and birds regardless of trophic group, as well as for herbivorous and omnivorous reptiles. Although “we know that herbivores overall tend to be larger bodied,” says Atwood, “that didn’t explain everything.” So, the team also looked at the effects of human activities, including hunting, habitat alteration, and pollution, using species-specific anthropogenic-threat data from the IUCN. For the most part, they were unable to make statistically strong links. “I’m going to be honest with you. We were kinda stumped,” says Atwood.
The inability to find particular human-caused correlates “certainly does not let us off the hook,” says ecologist Dave Hodgson of the University of Exeter who did not take part in the study, explaining that a likely explanation is that human influences are so multifarious and interwoven that the effects of individual ones are impossible to detect.
The study may not ultimately explain why herbivores are vulnerable, but, says Hodgson, “the thrill of the paper is that it is claiming to overturn some fairly classically presumed truths: that it’s the predators of this world—the sharp-toothed, sharp-clawed creatures—that are the most threatened.” By challenging this preconception, he adds, “it hopefully will provoke people to study herbivore extinction more deeply.”
T.B. Atwood et al., “Herbivores at the highest risk of extinction among mammals, birds, and reptiles,” Sci Adv, 6:eabb8458, 2020.