- Excluding herbivores from restoration areas may lead to an increase in both vegetation abundance and plant diversity, according to a new analysis.
- The global-scale analysis, which reviewed hundreds of studies, found that herbivores tend to be more common in areas undergoing restoration and can slow down vegetation recovery.
- While native herbivores play a crucial role in healthy ecosystems, researchers argue it may be beneficial to keep them from entering heavily degraded areas in the early stages of restoration.
- The impact of herbivores on restoration varies, and project managers should consider timing and local conditions when deciding whether to exclude, tolerate, or introduce herbivores.
When restoring or replanting vegetation in degraded ecosystems, it may be tempting to let nature run its course, and allow native herbivores to claim their fair share. Yet a new analysis in Science reveals that plant eaters at restoration sites don’t just reduce vegetation abundance — they may depress plant diversity as well.
Scientists evaluated hundreds of existing studies, comparing how restoration fared when herbivores were allowed in or kept out. The analysis was inspired by their own experience.
“Here in Shanghai, in the Yangtze River estuary, the government is doing a saltmarsh restoration project,” says lead author Qiang He of Fudan University in Shanghai. “They’ve planted a lot of Scirpus here, a genus of club rushes from the sedge family, but we’ve found that most of them are eaten by herbivorous crabs. So, we thought: why not do a global-scale analysis to find out how herbivores are generally impacting restoration projects?”
Co-author Brian Silliman, of Duke University, had a very similar experience in the southwestern U.S., where a snail called the marsh periwinkle (Littoraria irrorata) tends to gobble up newly planted saltmarsh cordgrass (Sporobolus alterniflorus).
The study reviewed more than 450 scientific articles involving nearly 1,900 field experiments across 64 countries.
Tender baby plants
Perhaps unsurprisingly, herbivores of all kinds (native, exotic or domesticated) strongly reduced the abundance of vegetation — on average by about a third in relatively undegraded areas, and by half in areas under restoration. Plant eaters, especially invertebrates like insects, snails and crabs, also tended to be more common in recovering ecosystems.
Compared to undisturbed areas, the biomass of herbivores per square meter was twice as high in naturally regenerating vegetation, and three times as high when restoration involved planting.
“Herbivores like to eat baby plants because they are often unprotected,” Silliman says. Young stems and leaves are tender, juicy and nutritious, as most plants invest in growth first and defense later. In addition, degraded areas often have a higher contingent of herbivores that aren’t very picky eaters, and fewer of the specialists that depend on particular plants.
Ecologist Jens-Christian Svenning of Denmark’s Aarhus University, who also studies herbivore impacts but was not involved in the study, agrees herbivores often reduce vegetation abundance. “This is expected and will often be positive for overall biodiversity,” he adds, as it may create opportunities for more vulnerable plants that would otherwise be overshadowed by faster-growing ones. Yet, in contrast to what happens in healthy ecosystems, herbivores at many restoration sites actually reduced species diversity, which may imperil their resilience and conservation value.
“I found it surprising that this study shows herbivores had a negative impact on restoration areas,” says Yuri Souza of the University of Miami, who was not involved in the new analysis, though it did review a study of his on herbivore impacts in Brazil’s Atlantic Forest. “This is a crucial takeaway, and particularly significant in tropical regions and biodiversity hotspots in countries where resources and funding [for restoration] are scarce.”
Vulnerable tropics
These impacts tended to be even more pronounced in the tropics, the study found. Among the hardest-hit areas was a restoration site in Bolivia where white-lipped peccaries (Tayassu pecari) ate more than 98% of the seeds of the chonta negra (Astrocaryum gratum), a spiny palm tree. In the Caribbean, a study found that recovering green sea turtle populations (Chelonia mydas) at Buck Island Reef reduced the abundance of seagrass in shallow areas by more than 80%.
In addition, a number of restoration studies from Ethiopia found that excluding livestock and other large herbivores improved both species richness and diversity in naturally regenerating forests and grasslands.
In some places, however, the opposite was true: excluding herbivores from naturally regenerating tropical forest in Papua New Guinea or from a degraded limestone forest on the island of Guam resulted in forest dominated by only a few tree species, some of them exotic.
So, how should project managers decide whether to exclude, tolerate or introduce herbivores?
Predators, please
Timing is of the essence, Silliman says.
“In places where the vegetation has been strongly degraded, where many young plants are trying to establish themselves or are being planted, it’s probably a good idea to exclude herbivores for a number of years until the plants have settled in,” he says.
Later on, of course, native herbivores should be allowed back in, so they can resume their role of spreading seeds and creating the kind of benign disturbance that benefits biodiversity and growth.
Though they may indeed pose problems for tree-planting efforts early on, herbivores are key to a healthy forest, says Souza of the University of Miami. “Peccaries, for example, are crucial to shape soil structure and ensure nutrient and water availability for plants.”
Restoring an ecosystem will eventually require a healthy food web, he adds, where herbivores play their role and are controlled by predators.
“Excluding herbivores may be necessary at the onset to protect plants and make the best use of precious restoration funds. But the key message of our paper is not that we need to exclude herbivores everywhere,” study lead author He says. “The crucial point is that we need to strive to restore the entire ecosystem, the entire food chain, including the predators that can keep herbivores in check.”
Banner image: A feral population of Philippine sambar deer (Cervus mariannus) introduced to the island of Guam in the late 1600s contributed to reduced tree diversity, benefiting of the exotic white leadtree. Image by Shukran888 via Wikimedia Commons (CC BY-SA 4.0).
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Citations:
Xu, C., Silliman, B. R., Chen, J., Li, X., Thomsen, M. S., Zhang, Q., . . . He, Q. (2023). Herbivory limits success of vegetation restoration globally. Science, 382(6670), 589–594. doi:10.1126/science.add2814
Souza, Y., Villar, N., Zipparro, V., Nazareth, S., & Galetti, M. (2022). Large mammalian herbivores modulate plant growth form diversity in a tropical rainforest. Journal of Ecology, 110(4), 845-859. doi:10.1111/1365-2745.13846
Aliaga-Rossel, E., & Fragoso, J. M. (2014). Defaunation affects Astrocaryum gratum (Arecales: Arecaceae) seed survivorship in a sub-montane tropical forest. Revista De Biologia Tropical, 63(1), 57. doi:10.15517/rbt.v63i1.13042
Gulick, A. G., Johnson, R. A., Pollock, C. G., Hillis‐Starr, Z., Bolten, A. B., & Bjorndal, K. A. (2021). Recovery of a cultivation grazer: A mechanism for compensatory growth of Thalassia testudinum in a Caribbean seagrass meadow grazed by green turtles. Journal of Ecology, 109(8), 3031-3045. doi:10.1111/1365-2745.13718
Gebregerges, T., Tessema, Z. K., & Birhane, E. (2017). Effect of exclosure ages on woody plant structure, diversity and regeneration potential in the western Tigray region of Ethiopia. Journal of Forestry Research, 29(3), 697-707. doi:10.1007/s11676-017-0512-6
Mekuria, W., & Yami, M. (2013). Changes in woody species composition following establishing exclosures on grazing lands in the lowlands of Northern Ethiopia. African Journal of Environmental Science and Technology, 7(1), 30-40. Retrieved from https://www.ajol.info/index.php/ajest/article/view/88664
Szefer, P., Molem, K., Sau, A., & Novotný, V. (2020). Impact of pathogenic fungi, herbivores and predators on secondary succession of tropical rainforest vegetation. Journal of Ecology, 108(5), 1978-1988. doi:10.1111/1365-2745.13374
Nafus, M. G., Savidge, J. A., Yackel Adams, A. A., Christy, M. T., & Reed, R. N. (2017). Passive restoration following ungulate removal in a highly disturbed tropical wet forest devoid of native seed dispersers. Restoration Ecology, 26(2), 331-337. doi:10.1111/rec.12559