A friend and I have backpacked together for — never mind for how long — and we’re just back from our annual trip, this time to the high deserts of the Colorado Plateau where you have to be careful where you step lest you harm its “living dirt.” (At least that’s my name for it. Scientists who just read that sentence are groaning because that dirt has a fancy scientific name, “cryptobiotic soil.” Which sounds to me like a spy’s secret code. It also goes by another name, “biological crust.” I don’t care. I like “living dirt.” After all, the “crust” consists of soil plus living organisms and their by-products.)
The life in the dirt is made possible by what used to be known as blue-green algae. It too has a fancy name: cyanobacteria. (Fearless, I call that “little blue bugs.”) But it is entitled to its fancy name; without it, the birds wouldn’t be here nor would we.
It’s really, really old. In fact, it may be the oldest living substance on dry land. Fossils of it have been found, 2.8 billion years old, but it must be a billion years older than that.
Earth’s earliest solid land was lifeless. Single cell cyanobacteria were the first living colonizers of that land. Washing up on sandy beaches, it grabbed onto individual grains of sand and started wandering around, leaving behind a trail of mucilaginous glue which held the grains of sand together. Forming and then stabilizing soil, it formed thick mats all over the early earth.
In those days the air here was mostly carbon dioxide with hardly any oxygen. The cyanobacteria took care of that , converting much of the carbon dioxide into oxygen. Without it, earth would be as lifeless as the moon. By using water as an electron donor in photosynthesis, it made — and makes — oxygen. Combine little blue bugs with water, then add three billion years and you get sentient, oxygen-breathing life. You get crows, ravens, even humans.
When it is wet, it is mobile. It forms filaments which move away from the main biomass, bud, and make new colonies. All along the way it leaves behind a trail of sticky fibers which hold soil together, slowing wind and water erosion. When dry, it stays put, holding the soil and waiting for the next rain.
In the meantime, it is grabbing nitrogen out of the air and moving it into the dirt so plants can use it. (Most plants can’t use free floating molecules of nitrogen. They need the gas fixed in other forms. Without cyanobacteria fixing nitrogen, the world would have no plants.) The dirt becomes an ideal environment for lichens, mosses, green algae and other fungi which in turn harbor the larger plants of the desert which provide food and shelter for the birds and other sentient beings who live there or who visit.
Eventually, after several decades of growth, the living dirt forms erosion-resistant little hillocks of soil as you see in the photos.
Mature soil crust appears blackish. It is fragile. Compressing it kills it. Even the weight of a single human foot crushes it. Then, in its unceasing effort to turn the desert into lifeless, naked sand, the wind blows the top part away. It takes a century, or longer, to repair the damage from a single foot print.
So imagine what 150 years and millions of free-ranging cattle have done to it. Over-grazed desert land is easy to spot. Look for loose sand and flat-bladed scrub cactus. (I am sure that cactus also has a scientific name but I remain ignorant of it. I don’t care. Scrub cactus it is and scrub cactus it shall remain.) In such land you have to look hard to find any of the black-crusted soil that gives life to the desert.
So the next time you go birding on the Colorado Plateau step carefully. We’ve done a lot of damage there over the last 150 years and it will take a long time to repair. Stay on the roads or the trails or the slickrock or walk the washes. It’s the least we can do.
Mostly the facts in this post came from http://www.soilcrust.org/
The others I made up. Except for the part about bacteria and the first two billion years of the planet’s history. See Olivia Judson for that. For the resiliency of life, see The Wild Resiliency Blog.