Most Plant Rewilding Efforts Fail To Control Herbivores

Conservationist efforts around the world are under attack from ravenous herbivores, according to a new meta-analysis of over 600 studies published in Science.

According to the analysis, herbivorous creatures living near rewilding sites reduced vegetation abundance by 89%, on average.

To help conserve natural ecosystems and nurture young plants, the study’s authors recommend that conservationists take extra care to deter local herbivores, whether through fencing or introducing natural predators.

Protecting young plants from hungry mouths

Re-establishing trees, grasses and other vegetation in areas with degraded ecosystems can help boost biodiversity, something the United Nations describes as the “strongest natural defense against climate change.”

But these fledgling plants can be delicate when they’re first planted, and delicious.

In many conservation cases, according to the new meta-analysis, young seedlings are bitten, chewed and swallowed by plant-eating animals before they’ve had a chance to sink their roots. Plants in warmer, dryer regions are especially prone to being consumed in their early years.

So, to protect their plants, conservationists need to get tough with their local herbivores.

“Our analysis of the surveyed projects shows that introducing predators to keep herbivore populations in check or installing barriers to keep them at bay until plantings become more established and less vulnerable, can increase plant re-growth by 89% on average,” said Brian Silliman, a professor of marine conservation biology at Duke University’s Nicholas School of the Environment and co-author of the meta-analysis.

“If we want more plants, we have to let more predators in or restore their populations,” Silliman added. “Indeed, the decline of large predators, like wolves, lions and sharks, that normally keep herbivore populations in check, is likely an important indirect cause of high grazing pressures.”

According to Silliman’s meta-analysis, excluding herbivores at restoration sites increased vegetation abundance by an average of 93% and 158% at natural regeneration and planted restoration sites, respectively, and introducing predators increased abundance by 138% and 372% at natural regeneration and planted restoration sites, respectively.

Only a minority of conservationists, however, are using such tactics, says Silliman.

“While most of the projects took steps to exclude competing plant species, only 10% took steps to control or temporarily exclude herbivores,” he said, “despite the fact that in the early stages these plants are like lollipops – irresistible little treats for grazers.”

Beyond these early stages, herbivores are, of course, welcome members of the ecosystems, says Silliman. They just need to be kept away in the beginnings of such regeneration projects.

“Plants just need a small break from being eaten to get restarted making ecosystems,” Silliman said. “Once they establish, herbivores are key to maintaining plant ecosystem diversity and function.”

Reference: Changlin Xu et al. Herbivory limits success of vegetation restoration globally. Sci. 2023. (382)589-594. doi: 10.1126/science.add2814

This article is a rework of a press release issued by Duke University. Material has been edited for length and content.