DURHAM, N.C. -- Re-establishing plantings of trees, grasses and other vegetation is essential for restoring degraded ecosystems, but a new survey of almost 2,600 restoration projects from nearly every type of ecosystem on Earth finds that most projects fail to recognize and control one of the new plants’ chief threats: hungry critters that eat plants.  

“While most of the projects took steps to exclude competing plant species, only 10% took steps to control or temporarily exclude herbivores, despite the fact that in the early stages these plants are like lollipops — irresistible little treats for grazers,” said Brian Silliman, Rachel Carson Distinguished Professor of Marine Conservation Biology at Duke University’s Nicholas School of the Environment.

By not protecting plants in their early states, conservationists are missing out on great opportunities to significantly speed restoration, improve its outcomes, and lower its costs, he said.

Plant-eating snails

Plant-eating snails overwhelm both naturally occurring and artificially planted marsh plants as this ecosystem tries to regrow after drought and grazing. Credit: Brian Silliman

“Our analysis shows that installing barriers to keep herbivores at bay until plantings become more established and less vulnerable, or introducing predators to keep herbivore populations in check, can increase plant re-growth by roughly 100-400% on average,” said Silliman, who helped conceptualize the study and was one of its coauthors.

Those gains are equal to or greater than the gains realized by excluding competing plant species, the new survey shows.

“This begs the question: Why aren’t we doing it more?” he asks.

The new survey was conducted with input from an international team of researchers affiliated with 20 universities and institutions. They published their peer-reviewed findings Nov. 3 in Science.

Qiang He, professor of coastal ecology at Fudan University and a former postdoctoral research associate of Silliman’s at Duke, co-led the study with Changlin Xu, a member of He’s Coastal Ecology Lab at Fudan.

The survey’s findings have far-reaching implications for efforts to restore vegetation at a time of climate change, He said.

“Herbivores’ effects were particularly pronounced in regions with higher temperatures and lower precipitation,” He noted. 

All of which leads to one inescapable conclusion, Silliman said.

“If we want more plants, we have to let more predators in or restore their populations,” Silliman said. “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.”

“Conventional restoration is slowing our losses, but it’s not expanding vegetation in many places, and climate change could make that even more difficult,” he said.

“Harnessing the effects of predators to keep herbivores in check at restored sites is a relatively untapped approach that could help us boost plant diversity and restore ecosystems that are vital to human and environmental health, in less time and at lower costs,” Silliman said. “It’s like learning a new gardening trick that doubles your yield.”

Once a planting is established, the herbivores are essential too, he added. “Plants just need a small break from being eaten to get restarted making ecosystems. Once they establish, herbivores are key to maintaining plant ecosystem diversity and function.”

Researchers from Northeastern University, Northern Illinois University, the Smithsonian Environmental Research Center, the University of Maryland, the University of Florida, and Sonoma State University coauthored the study.

Coauthors also came from the University of Canterbury (N.Z.), Aarhus University, Pusan National University, the Instituto de Investigaciones Marinas y Costeras, East China Normal University, Peking University, Nanjing University, the Royal Netherlands Institute for Sea Research, the University of Groningen, the Chinese Academy of Sciences, Lanzhou University, and Yunnan University. The team was led by Qiang He, professor of coastal ecology at Fudan University and a former postdoctoral research associate of Silliman’s at Duke.   

Primary funding was provided by the National Key Basic Research Program (#2022YFC2601100 and 2022YFC3105402), the National Natural Science Foundation of China (#31988102 and 32271601) and the Global-ERCaN project of the Chinese Academy of Sciences (#177GJH2022020BS).

CITATION: “Herbivory Limits Success of Vegetation Restoration Globally,” Changlin Xu, Brian R. Silliman, Jianshe Chen, Xincheng Li, Mads S. Thomsen, Qun Zhang, Juhyung Lee, Jonathan S. Lefcheck, Pedro Daleo, Brent B. Hughes, Holly P. Jones, Rong Wang, Shaopeng Wang, Carter S. Smith, Xinqiang Xi, Andrew H. Altieri, Johan van de Koppel, Todd M. Palmer, Lingli Liu, Jihua Wu, Bo Li and Qiang He; Science, Nov. 3, 2023.  DOI: 10.1126/science.add2814