Signs of life? Organisms' effects on terrain aren't all that easy to perceive

Science News,  August 4, 2007  by Sid Perkins

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Imagine our planet unmarred by humans: no buildings, no highways, no farms, no dams, no open-pit mines. Now, imagine the world suddenly swept clean of all life whatsoever: no plants, no animals, not a single microbe. Would the newly vacant landscape retain unmistakable evidence that life had ever existed?

And would that topography be noticeably different if life had never evolved at all? For instance, would Earth's mountains be craggier, or the courses of its rivers less sinuous, if life had always been absent?

These are the sorts of questions pondered by geomorphologists, the scientists who study the landscape and the processes that sculpt it. For these scientists, contemplating life's effect on topography isn't just an academic exercise. Learning how to identify the topographical signs of life on Earth could enable researchers to recognize the same signals on other worlds. It may also help scientists better understand how to restore ecosystems that have been disrupted by natural disasters or human activity.

At scales smaller than a meter, topographical signs of life such as gopher holes and anthills are abundant. However, if viewed at a larger-scale--as seen from space, for example--few if any natural landforms on Earth bear the unmistakable mark of life.

"Life's effects on topography are subtle," says J. Taylor Perron, a geomorphologist at Harvard University. They don't reveal themselves in dramatic signatures or single features. Instead, signs of life show up only in the large-scale patterns and general proportions of landscapes, he and his University of California, Berkeley colleague William E. Dietrich proposed in the Jan. 26, 2006 Nature. Now, a flurry of new research is bolstering that contention.

BREAKDOWN Without a doubt, life has a powerful effect on the landscape. The chemicals that microbes produce as they colonize rock surfaces pulverize minerals in the rock (SN: 11/15/03, p. 315). Plant roots squeeze into cracks smaller than a human hair and then exert pressures sufficient to split pristine bedrock as they grow. Overtime, as the detritus from such erosion blows or washes downhill, sharp, rocky ridges become smooth, rounded hills.

Yet rounded hills aren't necessarily a sign of life, says Perron. Wind, naturally acidic rain, and physical processes such as freeze-thaw cycles also break down rock. Field geologists can find rounded hills even in the Atacama Desert of South America, a wasteland so arid in some sites that even microbes can't grow, says Perron. There, soil forms as particles of salt waft in from the nearby Pacific and chemically attack rocks. Images beamed to Earth from rovers on Mars also show rounded hills. The landscape on the Red Planet--which has never seen life larger than a microbe, if that--is probably sculpted by freeze-thaw cycles, says Perron.

Even though vegetation can break apart rock, scientists have considered it an erosion suppressor. Foliage prevents precipitation from striking the ground beneath a plant as forcefully as it would if the soil were unprotected, and a plant's root system holds the soil in place. Severe erosion has been regarded as a sign of a foliage-free landscape.

However, this traditional view ignores the influence that plants have on the landscape at large scales, new research suggests. By keeping erosion in check at some spots, plants can dramatically boost erosion at bare locales nearby, says Stijn Temmerman, a geomorphologist at the University of Antwerp in Belgium. He and his colleagues analyzed how erosion processes have evolved on a tidal flat in southwestern portions of the Netherlands.

In 1989, this broad stretch of shoreline had only small, isolated patches of Spartina anglica, a salt-tolerant plant known as common cordgrass. As is the ease on most barren beaches, water running off the land during ebb tides carved small channels into the sand. Incoming waves largely erased the channels during the next high tide. So, the channels often formed and re-formed at various places along the shore, says Temmerman.

Over time, individual tussocks of grass expanded and coalesced into large, irregular patches of vegetation. Erosion within those patches of grass was minimal--which wasn't surprising, says Temmerman, because the root systems of the plants stabilized the sand. What was surprising, he and his colleagues note in the July Geology, was the extent to which erosion accelerated in areas of the tidal flat that remained bare.

The patches of vegetation increasingly funneled the water flow to plantfree areas of the tidal flat and boosted erosion there. By 1996, some of the channels were permanent features that measured as much as 10 meters wide and 1.5 m deep, says Temmerman.

After studying a computer simulation of channel patterns generated by tidal flow across a partially vegetated landscape, the team suggests that the more densely the vegetation grows, the larger the number of erosion channels that will develop in any particular area and the more erosion there will be overall. "That's counterintuitive, because dense vegetation is traditionally considered to decrease overall erosion," says Temmerman.