Life and landforms

Life’s tough. I don’t mean winning the World Series or getting a PhD, tough though they are. I mean living organisms. They’ve evolved in a capricious world, neighbour competing against neighbour for that speck of sunlight or this drop of water. They’ve adapted to their environment, and in turn the environment, too, has evolved. Atmospheric chemistry is a prime example. What keeps our aerobic metabolism chugging along is the waste stream of other organisms. But cast your gaze down to Earth, to the hills and valleys. Is there a telltale sign of life in the landforms that adorn the Earth, or in the evolution of these landforms?


“No. Well, maybe. Well, give us some money and we’ll find out.” This is how Bill Dietrich and Taylor Perron of U.C. Berkeley answered the question in Nature. There are no characteristic landforms where you can say, “Life did that.” Hills are hills, and valleys are valleys. Some have suggested that rivers would not meander as they do if not for the riparian vegetation holding their banks in place, and yet we see meanders on Mars. Instead, life changes more subtle characteristics of landforms – like their steepness or smoothness. Hilltops are round, but even rounder with a mantle of vegetation.

Plants and animals can alter the way landforms develop by changing the rules of how rock and regolith move about the place. Worms churn up the soil (hat-tip to Chuck Darwin); prone grass protects soil from being eroded; tree roots anchor soil into bedrock, while at the same time wedging the bedrock apart. These effects are most readily apparent when they're removed. If a wildfire burns a grassy slope, erosion picks up. In fact it may have been erosion following the wholesale collapse of terrestrial ecosystems in the end-Permian that led to the parallel mass extinction in marine ecosystems. Massive soil erosion brings with it nutrients and, if great enough, disruption of aquatic ecosystems through eutrophication.

Humans are also part of this “life”. We’ve dug one serious trench from the Atlantic to Pacific, and have built great pyramids (though probably not in Europe). We plow up fields and cut down trees, causing erosion and landslides. But we also replant trees to stop erosion, as China is doing on a grand scale (with dire effects for Japan’s chopsticks imports).

Life leaves its signature on the Earth’s landforms, written in the landforms’ longevity as much as their profile. Discovering this can be a real pleasure for geomorphologists, tough though it may be. I know. This is what I’ve been doing for my PhD, with only a few weeks left. Wish me luck.