Extinction risk picks on vertebrates big and small

At first glance, little animals like grey geckos and big ones like whale sharks don’t seem to have much in common. Researchers examining extensive data on body mass and threat status have recently discovered, however, that larger and smaller vertebrate species worldwide are more likely to go extinct than those in between, mostly due to human impacts.

“In addition to the largest animals being highly threatened, the smallest vertebrates were also highly threatened,” said William Ripple, first author on the paper published in Proceedings of the National Academy of Sciences.

Ripple, a distinguished professor with Oregon State University, had come across a couple bird and mammal studies that suggested bigger species faced greater extinction risk. To test whether this pattern held true across vertebrates, his team compiled the most comprehensive existing global dataset on vertebrate body mass, covering over 27,000 species of mammals, birds, amphibians, reptiles, cartilaginous fish and bony fish. Combining these numbers with endangerment classifications for each species from the International Union for the Conservation of Nature, the scientists formulated models to investigate the extinction probability looming over vertebrates based on their size.

“The breakpoint for all vertebrates is 0.035 kilograms,” Ripple said. “From that point, as you go larger, species become more threatened. And as you go smaller, species become more threatened.”

Large-bodied vertebrates may be highly imperiled in part because they have slow life history traits and require vast habitat, Ripple said, but also because humans hunt, fish and trap them for meat and other purposes. Smaller animals, on the other hand, have limited geographic ranges and suffer from habitat degradation and loss driven by human activities.

Ripple urged careful harvest management and landscape conservation for large species and habitat protection for small species, particularly terrestrial and freshwater systems.