Biological soil crusts, which some researchers describe as “the living skin of the Earth”, are a diverse community of organisms – microbes, fungi, plants and lichens – that play a crucial role in the management of entire ecosystems. And they are in trouble.
From deserts to polar regions, dryland ecosystems are home to about a third of the human population and make up about 40% of Earth’s land surface – and 30% of that land is covered in biocrusts. They may not look like much, but if you zoom in closely, you’ll begin to see an intricate web of life, vital to the health of its surroundings.
“I encourage [people] to get on all fours and really check them out,” said Sasha Reed, biogeochemist at the United States Geological Survey.
They help plants get needed nutrients like nitrogen and they stabilize loose soil, protecting it from erosion. This means that the loss of this upper layer – often due to human activity, as well as climate change – has far-reaching consequences.
Temperatures in arid regions have already risen and droughts have become more severe. Climatologists predict that temperatures will continue to warm in drylands at a faster rate than in other ecosystems, and that the frequency and severity of droughts will increase more rapidly.
The effects are already visible in the southwest, with Lake Mead at record highs and the entire region in a mega-drought worse than any seen in the past 1,000 years.
A 2018 study estimated that 25-40% of the Earth’s existing biocrust could disappear within 65 years.
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“[Biocrusts] are only active under certain conditions,” said Anita Antoninka, a soil ecologist at the Center for Adaptable Western Landscapes at Northern Arizona University. “[They] grow slowly and once lost, [are] forever lost.”
Young biocrusts are almost indistinguishable from their surroundings, but mature crusts may appear dark and knotty or brightly colored. Some can be incredibly old and live to be thousands of years old.
There are different types of biocrust, generally categorized by the shape of the crust and the mixture of relationship-forming organisms within the community. Lichens often form a structure for crusts and are classified by their shape, such as smooth, rough, pinaculate, or rolling. Rougher types of crust, those with more lichen elements, slow erosion while helping retain water and nutrients for other larger plants to grow.
Biocrust communities – like “tiny coral reefs in the desert,” according to soil ecologist Matt Bowker – are also very fragile. They can be destroyed by livestock, heavy machinery, or even just people walking on them. This led to “don’t burst the crust” campaigns throughout the American Southwest, often led by the National Park Service.
Although drought-resistant, biocrusts are still “quite sensitive to climate change,” according to Reed. Experiments have revealed that temperature changes of around 4 degrees Celsius can seriously hamper their growth and recovery rate.
Too much water can also lead to the death of biocrust communities. “Biocrusts are special in that they can completely dry out,” said Jayne Belnap, research ecologist for the US Geological Survey. It takes a lot of photosynthetic energy for these organisms to rehydrate, and more frequent rain events with an increase in temperature will cause energy to be lost through the repetitive process of desiccation and rehydration. The more often they have to perform this process, the more energy they risk losing.
How biocrusts benefit the environment
Biocrusts protect the earth from erosion, but if the crust is lost, more dust is thrown into the atmosphere.
Dust has been the third deadliest weather hazard in Arizona in the past 50 years. as the biocrust degrades and exposes more soil to erosion, the problem will only get worse. Antoninka and Reed agree that cases of Valley fever, a fungal disease often carried in dust, could increase as the biocrust disappears.
Biocrust is also an important factor in the water cycle, especially along the Colorado River. Without the crust to hold it in place, dust is picked up by the wind and deposited on the snow, causing the snowpack to absorb more sunlight and melt too quickly. As a result, mountain plants become active earlier in the season, absorbing water before it reaches the rivers. This makes the seasonal cycles of rivers more erratic, leading to increased pressure on water supplies for communities and agriculture.
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This is all a “result of the crust bursting”, according to Belnap.
Reed also suspects that drylands play an underappreciated role in the global carbon cycle. Carbon sequestration is part of the cycle that pulls some of this greenhouse gas out of the atmosphere and stores or sequesters it in places that may contain carbon, such as the oceans, tall trees in the rainforest, or even in cacti and herbs. Despite all the ecosystems capable of absorbing carbon, about 45% of the carbon dioxide produced by humans remains in the atmosphere.
Forests are major centers of carbon sequestration, where large trees are able to absorb and use carbon as they grow. But an estimated 30% of the world’s carbon resides in and beneath dryland ecosystems, contained in biocrust, cacti and other dryland flora.
Restore the crust
Efforts are underway to restore the biocrust in places where it has been lost. One of the biggest hurdles will be tracking large swathes of biological soil crust simultaneously to measure where and how much of it is disappearing. Scientists are turning to NASA for help. The objective of working with NASA is to access different technologies, such as satellites, planes or even drones, to better map and measure changes in the global surface of biocrusts in the world.
“We want to start building a network,” Reed said.
Separately, Bowker, who works with Antoninka at Northern Arizona University, is developing a global research network called CrustNET. This would involve scientists around the world collaborating and bringing their research together in large datasets to better understand the crusts globally.
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One of the primary goals of biocrust research is to restore landscapes that have already lost most of their native biocrust populations. Ecologists are experimenting with techniques to transport or graft healthy biocrust onto damaged landscapes the same way a doctor grafts healthy skin onto wounds on a body.
“We borrow a lot from plant restoration,” Antoninka said of the grafting techniques.
As the biocrust gets lost in nature, scientists have been able to reproduce and grow these biological communities in laboratories. Under ideal conditions, the biocrust can grow quite quickly.
“We were able to inflate it and grow it very quickly,” Reed said, “[but] we have made life too easy. Once the lab-grown crust was introduced to outdoor conditions, it could not survive.
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Reed and Antoninka are experimenting with ways to make the crust tougher by hardening it to the conditions it would experience in the wild. They also work to soften potential transplant areas with a regular watering system until the scab establishes.
Just as our own skin helps regulate our bodies, biocrust plays a key role in our world. With rising global temperatures and increasing drought, Earth’s skin is in more trouble than ever, but scientists are scrambling to salvage what they can and urge the public to appreciate what’s left.
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