Herbivore Diversity Helps Maintain Arctic Tundra Diversity

Herbivore Diversity Helps Maintain Arctic Tundra Diversity

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Musk oxen, their brown fur coats reaching nearly to the ground, have been dubbed “shaggy survivors of the Ice Age.” Now, researchers have shown that the funny-looking herbivores help stave off declines in Arctic tundra diversity brought on by climate change. These results, based on data from a long-term environmental monitoring project in Greenland, suggest that efforts to reintroduce large herbivores into Arctic landscapes—that is, so-called rewilding—might help ensure that a wide range of plants, fungi, and lichens continue to thrive there as the planet warms, the team suggests.

For more than a decade, Eric Post, a climate change ecologist at the University of California, Davis, has been visiting Greenland with the goal of answering one of the biggest questions in ecology: What are the drivers responsible for shaping communities of plants, fungi, and lichens? Post and his colleagues are particularly keen to answer that question in the context of the Arctic, a region currently warming several times faster than the rest of the planet. Ambient temperature is a clear-cut contender when it comes to affecting tundra vegetation, said Post, but biotic interactions like herbivory could also play a significant role.

From 2003 through 2017, Post and his collaborators collected environmental data from a site near the town of Kangerlussuaq in southwestern Greenland. Rolling hills, lakes, tundra meadows, and thickets of birch and willow characterize the site, which is also home to populations of musk oxen and caribou. The presence of those animals is key, said Post, because a major research goal was to study their impacts on the landscape. “That’s what led us to work in the site in first place.”

Grazed and Ungrazed, Warmed and Unwarmed

Both musk oxen and caribou are herbivores, and they consume large amounts of vegetation such as grasses, leafy plants, sedges, lichen, and moss. It makes sense that the animals, by way of their prodigious appetites, would affect an environment, said Post. To investigate that idea, it was necessary to establish places that would remain ungrazed by musk oxen and caribou. To that end, Post and his colleagues erected three circular exclosures, each roughly 15 meters in diameter. The structures’ wire mesh fencing effectively prevented musk oxen, caribou, and other local fauna (like Arctic hares) from entering. The team also defined corresponding areas that were not fenced off. Finally, Post and his collaborators installed passive warming chambers—basically, miniature open-topped greenhouses—on some grazed and ungrazed areas so they could investigate the effects of a warming climate.

“There’s been substantial warming happening while we’ve been doing this experiment.”

“We can compare grazed and ungrazed, warmed and unwarmed, and all of the interactions among them,” said Post.

Every July, the team traveled to the Kangerlussuaq region to determine the diversity of the tundra. To consistently quantify tundra diversity year after year, Post and his colleagues carried a square piece of clear plastic, half a meter on a side, to 50 preset locations across their research site. The team then tabulated all of the vegetation directly beneath specific points on the plastic surface. “That gives us a continuous record of the abundance and diversity of plants, fungi, and lichens in all of these plots every year,” said Post.

The researchers found that tundra biodiversity decreased across all of their sites over time. That wasn’t entirely a surprise, said Post, given that the area has warmed by nearly 1°C on average during the 15-year study period. “There’s been substantial warming happening while we’ve been doing this experiment,” he said. Losses in biodiversity have been linked to a slew of human-driven processes, including changes in the uses of terrestrial and marine landscapes and climate change, according to a 2019 report.

The Benefits of Herbivores

Post and his colleagues showed that the losses at their site weren’t uniform, however: Biodiversity declined more gradually in the grazed plots compared with the ungrazed plots. In other words, the presence of large herbivores like musk oxen and caribou seemed to forestall biodiversity losses.

The difference in biodiversity was significant, said Post. “It declines about twice as fast without large herbivores.” (Herbivores have also been shown to confer other environmental benefits, such as helping to cycle an ecosystem’s carbon into more permanent reservoirs.)

But the changes in biodiversity that Post and his colleagues recorded weren’t just due to shifts in the absolute number of musk oxen and caribou over time, the researchers noted. Instead, the diversity of the herbivores appeared to matter the most. The team quantified herbivore diversity using an index known as true Simpson’s diversity. “It takes into account both the number of taxa and their relative proportions,” said Post.

Musk oxen and caribou help maintain Arctic tundra diversity by eating shrubs like willow and birch that often shade out a wide range of lower-lying vegetation, Post and his colleagues propose. And having both animals around is important, said Post, because they have very different impacts on vegetation. “They have different preferences, both for what they will eat and when.” (Caribou tend to migrate through the Kangerlussuaq region from May through July, whereas musk oxen reside there year-round.)

Bring Back the Beasts

An important takeaway from this work is that boosting herbivore diversity in the Arctic might help delay anticipated losses in tundra biodiversity, Post and his colleagues suggest. That effort could take the form of rewilding, an idea in conservation biology that refers to deliberately reintroducing species into a landscape, the team proposed.

“When we’re thinking of adapting to climate change, there are contexts in which animals can help.”

Rewilding the Arctic might strike many people as incongruous, said Yadvinder Malhi, an ecosystem ecologist at the University of Oxford in the United Kingdom not involved in the research. “We tend to think of the Arctic as a wild space.” But in fact, high-latitude regions used to be home to a much wider range of animals—horses, mammoths, and antelope all roamed the Arctic as recently as the Pleistocene. Today, many of those animals are gone, said Malhi. “A lot of the populations of animals are depleted.”

It makes sense to think about rewilding as a way to stave off biodiversity losses, he added. “When we’re thinking of adapting to climate change, there are contexts in which animals can help.”

—Katherine Kornei (@KatherineKornei), Contributing Writer

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Citation: Kornei, K. (2023), Herbivore diversity helps maintain Arctic tundra diversity, Eos, 104, https://doi.org/10.1029/2023EO230304. Published on 11 August 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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