02 December 2011 by Julienne du Toit

The Secret World of Rock-Biters

Pull off the road and go and examine a piece of weathered rock. It would be unusual not to find a patch or six of lichen on it.

I always just thought lichen was a kind of oddment of the plant kingdom, and so it is. But talk to an expert on the subject, and you’re sucked into a whole soap opera of warring rock-biters.

Dr Gideon Groenewald, based in Clarens, Eastern Free State once took us to the Golden Gate Highlands National Park, a world of spectacular sandstone formations and mountains.

There is a certain type of lichen here that occurs nowhere else in the world. It doesn’t look like much. In the best examples, you see a lighter section on the rock, usually in a circular formation, like a hoof mark, with black speckles in the middle. If you look at them through a microscope, they look like miniature trees, and if you scratch them, you can see the green.

This lichen lives only here because of the potassium feldspar in the sandstone. The lichen sends its hyphae (roots) five millimetres deep into the rock, precisely penetrating the potassium crystal in the feldspar. It depletes it to such an extent that crystal collapses, and the structure of the sandstone starts to decompose. After a while it is not much harder than wet beachsand. Meanwhile, the lack of potassium pushes silica and aluminium in the rock to the surface. The silica makes the surface hard so it forms a brittle, glassy crust. The aluminium runs off into the nearest earth.

Now it really gets interesting.

The aluminium gets absorbed by a plant of the onion family, called alium. The grey and mountain rhebuck come and eat these specific plants first thing in the morning, for breakfast, says Gideon. This makes their blood unpalatable, and chases away ticks and other bloodsuckers. Is this not an incredible example of the interlinks between species in an ecosystem?

But back to the lichen. The potassium-eating lichen eventually eats itself out of food, and dies. It can’t survive where there is only hard silica. But there is another lichen that can. This is an epilithic lichen - it lives on the surface of the rock as opposed to penetrating it. But it chows up the silica rock and leaves hollowed out square netting on the rocks.

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Because of this undermining and inestimably slow digestion of delicious rocks, the Clarens sandstone is dangerous to rock climbers. Don’t even try to come near it with a piton.

Then the rock gets attacked still more. The lichen hollows out little potholes, or birdbaths in the rock. Blue-green algae sits on the edge of the water, and stains the rock black. It also eats up all the calcium carbonate, and makes the hole even deeper. The black streaks on the huge creamy pale buttresses in the park are from this blue-green alga.

Lichens are merciless. And they fight each other, emitting antibiotics to stop attack. That’s why they’re so valuable to science.  The potassium eating lichen eats rock at a very specific rate - 9 mm every 100 years. In other words, 9 metres every million years.

Respect.

Category: Wildlife

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