Cryptobiotic Crusts: What's a Biological Soil Crust?

The black stuff between the rocks is cryptobiotic soil crust, composed of cyanobacteria, algae, and fungi interwoven together.  If you look closely green may be also seen on the crust.  These green areas are the beginnings of moss growing on the crust, indicating a healthy crust that hasn't been disturbed for many decades or longer.  White Tank Mountains, west of Phoenix in the Sonoran Desert.

Cryptobiotic is sort of an obscure weird word.  Cryptobiotic soil crust probably is even weirder, like some odd disgusting thing nobody really wants anything to do with.  The reality is, these crusts are very useful and important to all desert life.  The very word "crypto" means obscure or hidden, and many desert hikers may have walked past miles of this crust without every noticing it.  "Biotic" means living, which is also odd being most of these crusts simply look like... well... crusts.  If we were to zoom in on these crusts with a microscope however we would find a complex interweaving of cyanobacteria, algae, and fungi.  This seemingly lowly inconspicuous  crust is essential to the desert, and without it desert life and landscapes can become much different.

For the majority of the year these crusty organisms are all dried out and dormant, appearing almost completely lifeless.  Even in this crunchy dry state cryptobiotic soil crust function in extremely crucial ways by covering the soil, thus preventing it from blowing away and also helping to retain water in the soil.  Given even a little rain however these crunchy crusts soften and become biological powerhouses.  As rain drops fall, the crusts protect the soil again by holding it in place, preventing erosion and therefore preserving the soil and landscape.  Infiltration of these raindrops is also facilitated so more water goes into the soil.  Lastly, cyanobacteria remove nitrogen into the air and fixes it into the soil, increasing soil nutrients.  So basically, these crusts protect the soil, increase and preserve soil water content, and enrich the soil with nutrients.

Cryptobiotic soil crust with pink lichens forming on the surface in the Sonoran Desert.

So why are all these function important?  Well, preserving the soil, increasing soil water content, and increasing soil nutrients all allow for more plants to be established.  Being deserts are in short supply of both water and nutrients even modest increases of these can have significant effects on the food chain.  When more plants are established the more animals an area can support.  More and healthier plants also means a healthier more stable habitat.  This is known as a bottom-up effect in ecosystems or food chains.  The bottom-up effect is when things at the bottom of the food chain, such as cryptobiotic soil crusts, have strong effects on organisms higher up the food chain.

Found at: http://www.soilcrust.org

Being these crusts have such a strong bottom-up effect, their destruction can have huge effects on entire landscapes.  Foot traffic, trampling by grazing animals, and off-road vehicles all can quickly destroy crusts that took anywhere from years to century's to form.  Grazing and off-road vehicles are especially destructive being these activities cover so much area so quickly.  Once a cryptobiotic soil crust is destroyed the soil becomes prone to erosion and there are less nutrients and water in the soil, harming the entire food chain.  So the best way to preserve these crusts is to limit grazing and keep all foot and vehicle traffic on the trail or road.
After disturbance, it can take anywhere from years to centuries for a crust to form again.  Deserts with summer rains, such as the Sonoran, form crusts more quickly in a matter of years to decades.  Cooler deserts with winter precipitation, such as the Great Basin, can take centuries to form crusts.  If a crust is undisturbed for very long periods of time mosses, liverworts, lichens and even ferns will colonize on top of the cyanobateria, algae, and fungi.  Protecting soil crusts is essential to maintaining healthy desert ecosystem function.

For a lot more great information check out: http://www.soilcrust.org
This website even has a great free downloadable field guide for soil crusts.