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Balls of Moss Travel in Herds Atop Some Glaciers and We Don’t Know Why

Glaciers may look like icy wastelands, but they actually support thriving, diverse ecosystems. The majority of species present in glacial ecosystems are microbial, but insects, birds, and even mammals also inhabit these vast, cold expanses. One such lifeform that has captured the attention of glaciologists worldwide is the glacier moss ball — a literal, free-rolling ball of moss growing around a rock or pebble core. The balls themselves provide key habitat for invertebrates and other life, acting as little green islands amidst a sea of white. Last month, researchers exploring the Alaskan Root Glacier published an article in Polar Biology on this understudied biological rarity.

Moss balls on the Root Glacier
Moss balls on the Root Glacier in Alaska. Source: Tim Bartholomaus

In 1950, an Icelandic glaciologist was crossing Hrútárjökull glacier, when he discovered stones covered with moss. “I have nowhere and never seen this kind of vegetation before,” he wrote in The Journal of Glaciology, and even the locals were largely unaware of their presence. He called them “jökla-mýs,” which translates to “glacier mice,” and wrote, “you will have noticed, Sir, that rolling stones can gather moss.”

These slow-moving green balls of vegetation—tumbling no more than inches a day—are a rare biological phenomenon. They have only been described on a few glaciers in Iceland, Alaska, South America, and Norway’s Svalbard archipelago. But while glaciologists have known about their existence for decades, no studies have analyzed their movements or their persistence across years — until now.

In 2006, glaciologist Tim Bartholomaus came across glacier mice on the Root Glacier in the Wrangell Mountains of Alaska. “What the heck is this!” he recalled thinking while in an interview with the National Public Radio. For four years, his team conducted research on the movement and longevity of these Root Glacier moss balls. Last month, Bartholomaus and coauthors Sophie Gilbert and Scott Hotaling published their findings in Polar Biology.

Many different species of moss have been found in glacier mice around the world, suggesting they are not dependent upon the presence of a certain type of moss. Rather, they form when their environment provides suitable growth conditions, including the availability of moss spores, nutrients, and the presence of a rock or stone substrate.

Hiking up to the flat, debris-covered Root Glacier in Alaska’s Wrangell Mountains. Source: Alan Grinberg/Flickr

Scott Hotaling, a postdoctoral researcher at Washington State University, explained that some amount of debris or dust is important as it provides the nutrients needed for moss ball development and growth. The researchers found the moss balls just downslope from a cone of dirty snow, which Bartholomaus believes may have been left over from an ancient volcanic eruption in the area. “So, that kind of dirt substrate and the dirtiness of the snow, paired with there being a lot of light in the summer, paired with there being forests and moss nearby on the margins of the glacier, is probably why the moss balls are able to form here,” Hotaling said.

For the researchers, the most interesting, and simultaneously baffling attribute of these glacier mice is their implicit, slow movement. Moss balls shield and insulate the ice underneath them from sunlight, reducing the rate of melt in these localized spots. As the rest of the ice melts around them, the moss ball is left elevated on a pedestal of ice. Scientists previously observed them teetering on the edges of their pedestals, and hypothesized that eventually, the moss balls would fall from their pedestals and roll into a new position to start the process over again.

However, the exact reason the moss ball falls the direction it does remains an enigma. “We had originally assumed the reason to be very simple, or at least totally random to the point where finding a pattern was futile,” Hotaling said. From 2009-2012, Bartholomaus and his team tracked the position of 30 moss balls on the Root Glacier by wrapping color-coded beaded wires around the masses and tracking how far and fast they moved in relation to a grounded reference stake, throughout the summer season.

closeup of a moss ball
Moss ball tracking. Source: Tim Bartholomaus

Glacier mice are present and mobile during the summer season when the ice is melting. During the winter season, for eight or nine months out of the year, the moss balls are covered by snow. The researchers found that the moss endures these winter seasons, and once the moss balls reach a mature size, they can persist for at least six years, and probably much longer.

Moreover, the researchers had expected the movements of the moss balls to be largely idiosyncratic, but for reasons still unknown, the entire colony of moss balls moves in unison—in the same directions and at the same speeds—in a coordinated, herd-like fashion. They change direction a few times during the summer season, moving together as an entity, like a school of fish or a flock of geese.

The researchers considered a number of potential influencers but were surprised because their movement does not follow the overwhelming direction of sunlight, the downward slope of the glacier, or the direction of the prevailing winds.

“We really thought the direction of sunlight would be it,” said Hotaling. But after analysis, it turned out that incoming solar radiation remained relatively equal throughout the day. And the Root Glacier is actually quite flat, so the moss balls don’t just fall down the slope of the glacier, either.

Many people liken glacier mice to tumbleweeds, dried grassy plants that detach from their stems and roll across the desert with the force of the wind. “Moss balls are way more dense than tumbleweeds,” said Hotaling. “They’re little, fairly well-packed chunks of moss and dirt and rocks and water—they’re very damp. So that’s why, perhaps somewhat unsurprising, the wind does not drive their distributions.”

moss balls in Iceland
Glacier mice on the Vatnajökull glacier in Skaftafell National Park, Iceland. Source: Carsten ten Brink/Flickr

In the end, the explanation for why these glacier mice move synchronously remains a mystery. “It is likely attributable to something very complex like thermal physics and some combination of where the wind is and how the ice melts, or maybe one side of the moss ball becomes more dried out from the wind than the other, and so it falls a certain way.” But there is still much to learn. “And that’s the really fun part,” Hotaling said. “It’s important to have questions, and to recognize that we don’t have all the answers.”

Heidi Steltzer, a professor of biology and environment and sustainability at Fort Lewis College, Colorado, agreed, saying that “Science is the question-asking, it’s the constantly being curious — saying, huh, what’s going on with these moss balls? It’s not just: ‘here’s the answer,’ it’s ‘here are the next ten questions.’”

A 2012 study from the Falljökull glacier in Iceland found that each glacier moss ball, itself, supports a diverse community of invertebrates, including the tardigrade (water bear, or moss piglet), an invertebrate that can survive the harshest of environments, including the deepest ocean trenches and the vacuum of space. Other invertebrates that inhabit glacier moss balls include springtails and nematodes.

This invertebrate colonization is crucial because, aside from a few species (like the ice worm) most invertebrates typically remain on the edges of glaciers, probably because sufficient habitat is lacking on the glacier surface. The moss balls, then, represent key habitat for glacier-associated invertebrates — and potentially other types of fauna — that may use the moss ball islands as a refuge.

Tardigrade. Source: Dino Hristopoulos/Creative Commons

“I kind of started to think about these moss balls as an archipelago, or a cluster of islands, so rather than being isolated habitat, they are maybe a little bit more like the Galapagos Islands. And like the Galapagos, you could start to do island biogeographic studies,” said Steltzer. Applying basic ecological theory could help understand the distribution of individual species on moss balls, and how their size and proximity to one another could benefit overall glacial biodiversity.

So, “there’s this broader point about these environments — that they are far from being ecological dead zones. And though it’s mostly microbial diversity, glaciers are also relevant to vertebrates,” said Hotaling. “The moss balls are just another piece in that puzzle, another example of how biologically interesting glaciers can be.”

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