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I Didn't Know That!: Biological Soil Crusts

title banner with text "I Didn't Know That! Biological Soil Crusts" and image of a biological soil crust

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?

black and knobby biological soil crust forms on the ground with a desert bush
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.

an old growth cryptobiotic soil with a minature world of lichens mosses and soils
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 amid a black and bumpy biological soil cruswt with mountains in the background
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!

a hard to spot thin crust along a sandy surface a hard to spot thin crust along a sandy surface

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 with bumpy dark ridges
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.

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.

tire tracks a clearly visible, crushing biologiocal soil crust
Damaged biological soil crusts can take decades to recover!

NPS Photo

Don't Bust the Crust!

Don't bust the crust logo with image of a boot stepping on cartoon soil crust with text "Stay on trails. Protect fragile biological soil crusts."
  • 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!

an infographic describing biological soil crusts - full alt text available below image

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

Arches National Park, Bighorn Canyon National Recreation Area, Canyonlands National Park, Capitol Reef National Park, Casa Grande Ruins National Monument, Colorado National Monument, Death Valley National Park, Dinosaur National Monument, El Morro National Monument, Glen Canyon National Recreation Area, Grand Canyon National Park, Hovenweep National Monument, John Day Fossil Beds National Monument, Joshua Tree National Park, Katmai National Park & Preserve, Mesa Verde National Park, Natural Bridges National Monument, Petrified Forest National Park, Saguaro National Park, White Sands National Park, Zion National Park more »

Last updated: December 4, 2023