I Didn't Know That!: Biological Soil Crusts

You’re exploring in the high desert of the Colorado Plateau, and you see a cool rock formation off the trail. You want a better look, so you detour from the trail and walk over to the formation. You’ve heard people say to stay on the trail, but what does it matter in the desert? It’s just dirt... right?

Did You Know?

It’s alive! Although it may just look like dirt, biological soil crusts are full of living organisms like lichens, mosses, algae, micro fungi, and bacteria. In the high deserts of the Colorado Plateau (which includes parts of Utah, Arizona, Colorado, and New Mexico), biological soils may represent 70 to 80 percent of the living ground cover! 

A mature biological soil crust stabilizes the soil at Colorado National Monument.

NPS Photo

What are biological soil crusts?

Biological soil crusts are found throughout the world and play important roles in their ecosystems. They are sometimes called biocrusts, or, our favorite, cryptobiotic soils— where “crypto” means hidden, while “biota” means life. They are just what their name suggests, hidden life—a living soil that creates a crust over the landscape. These crusts are made up of micro-fungi, soil lichens, mosses, green algae, bacteria, and cyanobacteria. You can find these intricate living communities in the top few millimeters of the soil.

Old-growth soil crust at Arches National Park, draped in lichens.

NPS / Neal Herbert

In dry regions, these living soil crusts are dominated by cyanobacteria, some of the oldest known life forms on the planet. Cyanobacteria is dormant when dry. When wet, cyanobacteria become active, moving through the soil and leaving a trail of sticky material behind. This sticks to surfaces such as rock or soil particles, forming an intricate web of fibers throughout the soil. In this way, loose soil particles are joined together, and an otherwise unstable surface becomes very resistant to both wind and water erosion.

How do I know if it’s biological soil crust?

Where are biological soil crusts found?

Biological soil crusts are found in dry and semi-dry environments throughout the world. In the United States you can find these crusts in the high deserts of the Colorado Plateau, as well as deserts like the Mojave, Sonoran, and Chihuahuan Deserts. They are also found in semi-dry environments in Alaska. You may see biological soil crusts in parks like:

• Arches National Park, Utah

• Grand Canyon National Park, Arizona

• Saguaro National Park, Arizona

• Joshua Tree National Park, California

• White Sands National Park, New Mexico

• Dinosaur National Monument, Colorado/Utah

• Katmai National Park, Alaska
• and more!

Pink flowers grow out of biological soil crust near Novarupta in Katmai National Park.

Photo courtesy of A. Ramos

What do biological soil crusts look like?

Young crusts can be hard to identify. They are flat and brown and look like bare earth. As biological soil crusts develop it becomes easier to identify. A mature crust is bumpier and darker in color. If you take a closer look, you can see ridges and spires—like miniature sandcastles. Often these “sandcastles” are sprinkled with lighter lichen or moss. It’s a diverse miniature world, worth taking a closer look at!

Left image
A young biological soil crust
Credit: NPS / Neal Herbert

Right image
A mature biological soil crust
Credit: NPS Photo

There’s more than meets the eye. Take a closer look in the photo gallery below.

Why are biological soil crusts important?

Biological soil crusts are incredibly important to healthy desert ecosystems. In sandy places, these crusts literally shape their landscapes—they hold the place in place, so to speak. The crusts hold the soil and sand particles in place, so it doesn’t wash away in the rain or blow away in the wind. They also absorb and hold water—a precious resource in dry environments. Additionally, the cyanobacteria and lichens found in biological soil crusts are nitrogen fixers. This means that they can covert nitrogen in the atmosphere into a form usable by plants.

A mature biological soil crust in Canyonlands National Park.

NPS / Amy Washuta

Learn more about biological soil crusts in this podcast featuring Dr. Sasha Reed, research ecologist with the U.S. Geological Survey.

• Tiny Forests in the Desert

  
  

  Like trees in a forest, biological soil crusts play a key role in the ecosystems in which they are found. Listen as Dr. Sasha Reed, research ecologist with the U.S. Geological Survey, explains why damage to soil crust communities can have greater consequences than you might think. Dr. Reed was awarded the Presidential Early Career Award for Scientists and Engineers in 2011.

  Credit / Author:

  Jennifer Jerrett

  Date created:

  10/14/2020

  Audio Transcript

  [chain saw]

  [Jennifer Jerrett]: You might not associate the sounds of clear cutting with the desert, but something similar is happening around the national parks in southern Utah, and it's right under foot.

  [footsteps]

  [Jerrett]: I'm in Arches National Park, and I'm on the hunt for biological soil crust. This stuff is alive. It's made up of lots of little organisms, and it's found all over the world, almost anywhere that the soil can see the sun.

  In deserts like this one, the soil sees a lot of sun, so I'm not going to have to go too far to find some really nice specimens, and in fact, here we are.

  So, from where I’m standing, it just kind of looks like a dark, well, crusty layer on the top of the soil, and I suppose that's not too surprising.

  But if I get closer, I start to see tiny sand castles, and some of them look like they're covered in soot. And if I get really close—now I'm actually looking through a magnifying lens—I see a garden full of mosses, and multicolored lichens and cyanobacteria. (You can look that one up later).

  It really is a forest in miniature. There are other similarities between trees and crust too. Trees photosynthesize. So does crust. In a forest, leaves fall and decompose, boosting soil fertility.

  Here in the desert, crust organisms help soil fertility too. And just like trees in a forest, crusts prevent erosion.

  [Sasha Reed]: One of the things that makes them very special in these sandy places...

  [Jerrett]: That's Sasha Reed, scientist with the U.S. Geological Survey, in Moab, Utah.

  [Reed]: ...is that they exude this glue. they ooze out these polysaccharides, these sticky sugars, and that sticky sugar will hold the soil particles, the sand grains in place, and it's incredibly strong. It holds the place in place, is what we like to say.

  The crusts are very resilient to a lot of things like high-speed winds and crazy amounts of water. They've really evolved a way to be able to resist that kind of disturbance, but they're incredibly sensitive to disturbance that compresses them.

  Stepping on the crusts, riding your bike on the crusts, driving your car on the crusts, putting cattle on the crusts, those things will absolutely bust the crust.

  [Jerrett]: When crusts are disturbed like this, they can't "hold the place in place" any more, and because of blowing sand and dust, those initial disturbances can spread.

  [Reed]: If that role of holding the soil in place is taken away, that sand is going to blow, and the thing about dust-- it goes next door, it covers the crust next door, maybe healthy crust, and because that crust needs to see the sun to survive, to photosynthesize, it will kill that crust as well.

  [Jerrett]: So now we’re moving from something localized to something bigger, much bigger.

  [Reed]: There's some really interesting work that's happened here with dust lately, to show that dust that's produced from disturbance blows onto the snowpack of the Rocky Mountains.

  It changes the color of the snow, and that changes the energy balance, making the snow melt faster. And that's predicted to result in a five-percent decline in the Colorado River flow.

  [Jerrett]: Not so great for a place that's already pretty dry.

  [Reed]: So if you have more dust moving around the Earth, it's not just changing the place where the dust came from, it's changing the place where the dust goes to.

  [Jerrett]: Who knew organisms so tiny could be so important?

  [Reed]: And that when they go away, we just see things blow in ways that are apocalyptic. I mean, it's just a really big deal. It's a huge role that they play at the global scale.

  [Jerrett]: And while we're thinking big, thinking global, let's go back to that idea of deforestation.

  [chain saw]

  [Jerrett]: Clear cutting in the rain forest has dramatic effects. It changes local hydrology, species diversity, and soil fertility.

  [Reed]: Biological soil crusts and dryland ecosystems can be the same way, it's just really small. You've got to get on your hands and knees to be able to see the forest.

  [tree falls]

  [Reed]: Everything is connected. If you're cutting down the forests of crusts, if you destroy these areas, you change everything.

  [Jerrett]: So what do we do, stay home? Never get out of the car? Or maybe we should only read about the parks online. I mean, really, is there any hope?

  [Reed]: There's so much hope. Absolutely. I mean, these are survivors. These organisms just need us to give them the chance to either stay healthy or to return to health.

  [Jerrett]: If we just watch our step. Because if we pay attention, we stand a much better chance of not missing the forest for the trees. At Arches National Park, I'm Jennifer Jerrett.

What you can do

Biological soil crusts are very fragile. Unfortunately, we can destroy soil crust and often do so without meaning to. Vehicle tracks, bicycles, and footprints are extremely harmful—even a single footprint can kill the soil crust immediately below. Impacted areas may take decades to recover and some may never fully recover. Help protect these crucial desert organisms and don’t bust the crust.

Damaged biological soil crusts can take decades to recover!

NPS Photo

Don't Bust the Crust!

• Stay on existing trails or roads

• When you must go off trail, walk in wash bottoms or on slickrock

• Camp on slickrock or in previously disturbed areas

• Go out of your way to avoid biological soil crust

• Pay attention to signs

• Leave no trace of your visit

Pass It On!

Did you learn something new? Pass it on! Protecting our ecosystems is a job for us all, but there’s no way for everyone to be an expert in everything. That’s why sharing knowledge is so important!

Download or screenshot this card to share with a friend or help you remember not to bust the crust when you visit places where biological soil crusts are found. Thank you for helping protect our fragile desert ecosystems!

Check out other I Didn’t Know That! topics.

Text description of infographic

I Didn't Know That! Biological Soil Crusts

• It's alive! Althought it may look like dirt, biological soil crusts are full of living organisms. In the high deserts of the Colorado Plateau, biological soils may represent 70-80% of living ground cover.
• Biological soils are essential to desert ecosystems! They stabilize soil, reducing erosion from wind and rain. They also absorb water and add nitrogen to the soil.
• Don't Bust the Crust!
  • Stay on existing trails & roads
  • If you must go off trail, stay in wash bottoms or on rock.
  • Camp on rock or in previously disturbed areas. 
  • Leave no trace of your visit!