Many scientists believe that a sixth mass extinction is underway, placing three-quarters of the Earth’s species at risk of “biological annihilation.” And to avoid this catastrophic future, the only hope may be learning what species face the greatest threats and why. So a team of international researchers tackled the question, analyzing how the threat of extinction relates to animal body size. As Helen Briggs reports for the BBC, the results of their study suggest that the risk of extinction is highest among the world’s largest and smallest creatures.

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The authors examined at 27, 647 vertebrate species of the 44,694 assessed by the International Union for the Conservation of Nature's Red List of Threatened Species, which contains evaluation of conservation status for a wide range of both plants and animals. Among the animals analyzed were mammals, birds, reptiles, amphibians, bony fishes, and cartilaginous fishes like sharks and rays. ​Of the selected species, approximately 4,000 are endangered.

The team’s findings, published in the Proceedings of the National Academy of Sciences, show that animals with the highest and lowest body mass faced greater threat of extinction than species in the “Goldilocks zone,” as an Oregon State University press release puts it, referring to animals that are “neither too big, nor too small.”

Further analysis showed that the greatest threat facing large vertebrates is “harvesting,” or deliberate killing by humans. “Harvesting of these larger animals takes a variety of forms,” the authors of the study write, “including regulated and unregulated fishing, hunting and trapping for meat consumption, the use of body parts as medicine and killing due to unintentional bycatch.”

Smaller species, by contrast, are primarily threatened by habitat degradation, which is caused by factors like cropping, logging, development and pollution. As Peter Hannam explains for the Sydney Morning Herald, small animals have a relatively limited geographic range, so habitat degradation often eradicates the only ecosystems they can thrive in. Among the little critters at risk, according to the Oregon State press release, are Clarke's banana frogs, sapphire-bellied hummingbirds, gray geckos, hog-nosed bats and waterfall climbing cave fish.

The study’s findings are significant for a number of reasons. First, large animals tend to attract more attention, often receiving a greater amount of funding for conservation, according to the authors. But the study shows that the tiny animals of the world are also in dire need of protection.

The study also suggests that different conservation approaches are required at opposite ends of the spectrum. For large species, the authors write, it is imperative to curb harvesting practices by implementing “community tolerance” programs, controlling harvesting in unprotected areas, and reducing the amount of wild meat consumed worldwide. Smaller species are better served by the implementation of protected areas, which can stave off the habitat destruction that puts them at risk.

These conservation methods need to be put in place quickly for the world’s threatened vertebrates, be they big or small. As things stand, the authors of the study write, we are “poised to chop off both the head and tail of the size distribution of life.”