Add this to the challenges facing efforts to re-green the world: Out of control herbivores.
Often overlooked during habitat restoration projects, these creatures can put a major dent in revegetation work, according to new research.
“In the early stages (of a restoration project) these plants are like lollipops, irresistible little treats for grazers,” said Brian Silliman, a Duke University ecologist involved in the study.
Grazers can bring myriad benefits to landscapes. Bison can improve plant diversity in Midwest prairies by cropping dominant grasses. Caribou and muskox help preserve tundra plants when they eat encroaching shrubs. Elephants boost carbon storage in African forests by munching on less carbon-rich trees.
But there’s another side to the coin. Their appetite for greenery, particularly when inflicted on already damaged ecosystems or left untamed by predators, can wreak havoc. One classic example is the harm to streams in Yellowstone National Park when elk populations, left unchecked when wolves were wiped out in the early 20th century, demolished streamside bushes and trees.
Now, as people work to revive ravaged ecosystems through initiatives like the United Nation’s Decade on Ecosystem Restoration, Silliman joined with scientists from Asia, Europe, Latin America and New Zealand to conduct a globe-wide examination of the effects herbivores might have on such work.
The researchers gathered data from nearly 2,600 experiments detailed in more than 600 papers that observed interactions between herbivores and vegetation in different settings: relatively untouched ecosystems, places where damaged areas were left to regrow on their own, and ones where people tried to give recovery a boost with plantings.
The survey revealed that the presence of herbivores at a restoration project was tied to a 50% reduction in the number of plants growing there and a 15% drop in the number of plant species in places such as forests and grasslands, the scientists reported in early November in Science.
The problem showed up regardless of the ecosystem – in aquatic areas and on land, in the tropics and in temperate regions. The effect was more pronounced in hotter, drier places where plants confront a particularly challenging growing environment.
The findings don’t necessarily contradict research extolling the ecological benefits of grazers. While the presence of grazers reduced total plant numbers by 32% on average in studies of relatively pristine landscapes, plants diversity increased by 14% in those places.
The difference could be in part because vegetation at degraded sites was generally less productive than at healthy ones, making it easier for animals to overgraze them, the scientists found.
The research also revealed that measures to control herbivores can reduce the damage a lot. “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 Silliman.
In wild places like Yellowstone, that could mean the return of creatures such as wolves to reduce grazing pressure. There, shrubbery in streams bounced back in many places after wolves were let loose in the park in the 1990s.
Where that’s not realistic, people doing restoration might need to resort to other ways to keep animals from chomping down on new plants, such as fencing. In just one example, earlier this year I visited the Klamath River in northern California, where the world’s largest dam removal is underway. Scientists there are leading an ambitious initiative to revegetate drained reservoirs covering nearly 1,000 hectares of land. Conservation groups are helping by paying for kilometers of fencing around the barren ground to keep out cattle and feral horses that might otherwise feast on the hundreds of thousands of new seedlings.
Joshua Chenoweth, a restoration ecologist with northern California’s Yurok Tribe, which is leading the revegetation work there, said he considered the fencing critical to the success of what some have described as among the world’s biggest single habitat restoration projects. “It will be a game changer,” he told me.
He, et. al. “Herbivory limits success of vegetation restoration globally.” Science. Nov. 2, 2023.