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Big herbivores could save tundra flora from growing heat and shrinking sea ice—if they can survive themselves

DAILY SCIENCE

Big herbivores could help save the tundra from rising heat and shrinking ice—if they can survive themselves

Scientists in Greenland found that tundra vegetation fares better when caribou and muskoxen are around to dine on encroaching, heat-loving shrubs
June 28, 2023

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In a world of breakneck environmental change, the Arctic is in a class of its own. Temperatures there are rising nearly four times faster than the global average. The ice that once reliably covered much of the Arctic Sea each winter is shrinking and could regularly vanish in the summers by the 2030s. Permafrost is turning to swamps. Forests are marching north across the warming tundra.

It turns out two animals could help put the brakes on at least some of the change. Caribou and muskoxen can slow the decline of tundra flora by dining on encroaching shrubs, new research has revealed.

The ecosystem-shaping power of large herbivores has been getting a lot of attention from ecologists in recent years. While the damage done by livestock on fragile ecosystems is well known, plants in many places have evolved in the company of grazing animals. Bison aid native plants in tall grass prairies in the Midwest by chomping back dominant grasses. Forest elephants in Africa promote the growth of trees that sequester more carbon by dining on their low-carbon counterparts. Now, herbivores in the far north are getting their due, thanks to a 15-year study of their effects on plants, fungi and lichens in southwest Greenland. 

Starting in 2002, scientists from the U.S., United Kingdom and Europe built 3 fenced enclosures on the tundra 20 kilometers outside the village of Kangerlussuaq, an area where both caribou and muskoxen live. Each year until 2017 they tracked the species and number of different plants, lichens and fungi they found from inside the enclosures and in nearby unfenced study plots. 

The results revealed that grazing had a major effect on the diversity of such organisms. While the constellation of vegetation species shrank both inside and outside the fenced areas as temperatures climbed, their diversity fell by almost half as much in places where the animals could graze, the researchers reported last week in Science.

 

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“The biggest takeaway from this project is that herbivores helped reduce the loss of tundra diversity associated with rising temperatures and diminishing sea ice,” University of California, Santa Barbara postdoctoral ecologist Christian John wrote in an email response to questions.

The primary reason is the grazers’ appetite for dwarf birch and gray willow. The two shrubs thrive in hotter temperatures, overshadowing the cold-adapted, tundra-dwelling small plants, lichens and fungi. 

In an earlier paper from the same project, scientists reported that of 14 plant species, 7 species (including 5 rare ones) became less common inside fenced areas, while the birch and willow thrived.

The combination of muskoxen and caribou—a mixture not found in much of the Arctic—also proved important at this site. Caribou numbers in the area fell during the course of the study, from several hundred to just over 100, while muskoxen more than doubled from 20 to 50. While the overall number of herbivores fell, the increased mixture of the two species had a positive effect. 

“These results suggest that the diversity of large herbivores—and not just the number of herbivores—is really important,” John wrote. 

The two animals have different food preferences and different eating styles, meaning their impacts on tundra vegetation varies. Caribou are selective browsers and migrate from one place to another. Muskoxen, by contrast, “are almost like lawn mowers,” according to John, ripping entire plants from the ground and feeding in the same area year-around.

While it’s hard to draw Arctic-wide generalizations from findings in a single location, the authors note that research there and similar findings elsewhere point to the potential benefits of preserving or even increasing Arctic grazing herds.

But such an effort will confront another problem. It’s not just tundra vegetation that’s in trouble. Caribou numbers have been falling in recent years around the Arctic. It’s not clear why. But one potential factor is wintertime rains – expected to increase in parts of the Arctic in a warming climate. The rains can form sheets of ice over snow that caribou can’t penetrate to reach food.

“If caribou or muskoxen eventually go locally extinct from parts of the Arctic, or even fall to severely low abundance, what we’ll likely see in response to warming is a tundra increasingly dominated by a few common species, like shrubs,” warned Eric Post, a University of California, Davis ecologist who led the project.

Post, et. al. “Large herbivore diversity slows sea ice–associated decline in arctic tundra diversity.” Science. June 22, 2023

Photo: ©Eric Post, UC, Davis

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