Herbivores at highest risk of extinction

A comprehensive analysis covering thousands of species of land and water animals has revealed that herbivores, not predators, have the highest risk of extinction – currently and historically.

This surprise discovery upends current thinking, according to Trisha Atwood from Utah State University, US.

“Our findings challenge a two-decade-long perception that meat-eating predators were the most likely group to meet the ire of Earth’s sixth mass extinction,” she says. “It also helps us better understand the potential consequences of broadscale species loss.”

Atwood is lead author of a paper in the journal Science Advances that classified more than 24,000 vertebrate species into groups of carnivores, omnivores and herbivores and compared their numbers of threatened and extinct species from the late-Pleistocene to the present.

Once the researchers discovered that herbivorous mammals, birds and reptiles were at highest risk, they further explored this pattern across different geographic regions and habitat types, including savannahs, wetlands, deserts, caves, oceans, forests, mountains and shrublands.

As a group, herbivores consistently showed the highest extinction risk, amounting to around 300 more species listed as threatened than would be expected.

This was highest for large mammals such as elephants and large birds, whose disappearance could have sweeping consequences for nature and human wellbeing, says Atwood, with important ecosystem functions that may not be compensated by their smaller cousins.

“Prehistoric extinctions of megaherbivores (plant-eaters weighing more than 1000 kilograms) in the late-Quaternary … drastically changed the trajectory of life on Earth by altering its vegetation, its fire regimes, and global greenhouse gas dynamics,” she says.

Overall, herbivorous species had increased risk of being wiped out in 100% of aquatic and 57% of terrestrial environments. On land, forest-dwelling herbivores were worst off, while herbivorous reptiles, such as turtles, drove much of the increased risk in marine habitats where more than half of these species were classed as threatened.

Omnivorous reptiles, that eat both plants and meat, aren’t doing well either, with increased extinction risk in 80% of oceans and 46% of land regions.

It’s not “clear sailing” for predators though, says Atwood; they face high risk of extinction in 23% of land and 40% of marine habitats.

In particular, scavengers that eat dead flesh, such as vultures, and animals that feed primarily on fish, such as seabirds, face a heightened risk – and we’ve already seen significant declines of many large carnivores such as tigers, dholes, and several leopard species.

Until now, this entire group of meat-eating animals was thought to be most vulnerable, and for good reasons.

A major factor is thought to be depletion of the prey that they depend on for survival. Conflict with humans as their natural ranges decline is also key, attributed to an 80% drop in predators through hunting and trapping.

But assumptions about their elevated threat have been based on single species research, highlighting the need for a systematic analysis of at-risk species across different groups and habitats.

The researchers also explored human influences and found there wasn’t one single driver; they varied across species.

Herbivorous reptiles, for instance, were disproportionately impacted by invasive species such as rats, insects and plants. Small herbivorous bird populations were most affected by invasive species, pollution and habitat damage.

Understanding different risks has broader implications for ecosystems.

In India, for instance, declines in vultures have allowed zoonotic diseases like rabies and anthrax to spread. Large frugivores (fruit-eating animals) in tropical forests are altering tree communities which could in turn reduce forest carbon storage.

Although there is much more to understand, the researchers point out that herbivores have been linked to other diverse ecosystem functions such as the evolution of plants and predators, ecosystem resilience, nutrient recycling and plant regeneration.

Further human drivers of extinction risk include climate change and activities such as hunting and poaching. Their variable and heightened impacts on herbivores have yet to be unravelled, the authors write, a “new challenge for conservation biology”.