Mosses That Move and the Rocks They Reveal

by Jennifer Frazer on February 7, 2011

Many of you may know about the Racetrack, the mud-cracked waste at the north end of Death Valley that is home to the mysterious sailing stones.

See, if you tilt the camera, the rocks slide downhill. Wait. . . let me check my notes . . . A photograph of the fascinating Racetrack Playa in Death Valley, California, USA. Creative Commons djfrantic. Click for link.

It turns out there’s a perfectly cromulent explanation for this effect; if you don’t know it and want to spoil the magic, see here.

Well, it turns out that this organism,

Sooooooo cute. I'm waiting for the plush version from Etsy. Grimmia trichophylla. Creative Commons johndal.

or one of its close relatives, I should say, can apparently do the same, or something like it. Don’t believe me? There were three photographs and a fascinating account of the phenomenon over at Botany Photo of the Day last week — the most important bits are paragraphs two and three. Don’t miss it! Once you’ve read it, come back here for a little commentary . . .

OK, finished?

Now, my friends, you can see why geologists hate “vegetation”. For in addition to your garden-variety and annoyingly rock-obscuring trees, shrubs, flower, and soils, they must also contend with the biofilm of lichens — little fungus-alga co-ops — and naked algae that encase every rock in sight after a few decades. That means that nearly every rock face you look at is not its true color; it’s the color of the encrusting life. The day the light bulb blinked on and I thought, “That cliff isn’t gray-green. The rock is pink and the stuff living on it is gray,” was one of revelation for me.

This further explains why geologists flock to newly blasted road cuts like flies to honey, and further why they carry around rock hammers* for splitting rocks to see what they truly look like. It also explains why I get nervous around them when they get that glimmer in their eyes suggesting that if they could napalm the countryside in their research area, they would.

When I reach the summit of mountains in Colorado, I’m astounded by the variety of lichens, moss, and algae I find there. Mountaintops are lichen biodiversity hot spots, splashed with green, yellow, black, gray, and orange.

A fiesta of lichens (can I coin that term for a lichen herd?) at Rocky Mountain National Park. Creative Commons shrocket.

Thrillingly orange lichens are particularly common up there, since they thrive in places birds poop (and thus fertilize), and birds seem to like perching on rocks near the summits of mountains where they, like us, have a clear view of the countryside. And yet, almost no one looks down or looks carefully, and with enough foot traffic, the encrusting life dies and peels off.

Apparently, mosses can also induce lichen holocausts as they slide down rock faces. How they do this, I know not, although the lack of light may play a role. But notice there aren’t simply dead lichens in their wake; the rock is scrubbed clean. Which leads to another interesting hypothesis: the moss excretes acids or some other chemicals that allow them to digest the biofilm on the rock surface and absorb the resulting nutrients. Would that make these mosses . . . herbivores?

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*Also because physical and chemical processes called “weathering” alter the surfaces of rocks. Also, by coincidence, they note that in the first two photos of the galloping moss, they are galloping upon the fossils of 1.9 billion year old cyanobacterial mats — thin films of blue-green bacteria that slowly build up characteristic striated rocks called stromatolites. In ancient times, these were prolific and their fossils are common, but today, stromatolites, crowded out by us pesky multicellular organisms, are found in only a few places on Earth, most prominently in the Hamelin Pool in western Australia’s Shark Bay. Interesting biological coincidence!

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{ 2 comments… read them below or add one }

kati February 7, 2011 at 9:39 pm

crazy! i like the term “lichen herd”. these roaming mosses digesting lichen might qualify as a herd, too :) :)

Jennifer Frazer February 8, 2011 at 1:21 pm

Definitely. : )

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