GEOGRAPHY 368
Human Impact on Biological
Soil Crusts
Desert Southwest, USA
Simenson, David Lloyd
10/2/2012
Abstract
Tourists visiting national parks in the southwest region of the United States will find an
increasing number of signs warning about stepping off the path. Rightfully so: each step causes
damage to that environment which takes decades, even centuries to repair. Not well known by
visiting tourists, biological soil crusts are an important life source in arid regions. This paper will
provide a brief analysis of the major functions of these organisms, and focus on how our
interactions with them will shape the landscapes of tomorrow.
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Introduction
While visiting the southwest you may look out into a sun beaten, hot desert landscape and
notice nothing but a mirage against the horizon. Focus out and you may be delighted to find the
seemingly unlivable desert is teeming with life. The desert’s floors of the United States are home
to a biotic blanket of organisms often referred to as biological soil crusts. Associated national
parks are decorated with signs warning you of these crusts and effects of stepping off the trails. It
is important to respect the limitations of the trails. Although not immediately obvious, these
organisms have many duties pertaining to the wellbeing and succession of the desert ecosystem.
Since this layers discovery in the 1950s researchers have used words such as cryptobiotic,
cryptogamic, microfloral, and organogenic, to describe it. Whichever term referred to, it is
describing a mutual interaction between cyanobacteria, green alga, microfungi, lichens and
mosses (Belnap 2003). Together they create a delicate layer on top of the desert soils. Though
research into their biology is relatively recent, its organisms (cyanobacteria), have been found in
rocks 1.2 billion years old (Horodyski and Knauth 1994).
Just as algae may be found all throughout history, biologic soil crust can be found all
over the world. They prefer dry climates and are especially abundant in hot, arid regions, such as
Death Valley. Varied with the given dryness in a climate, cryptogamic soils are forced to grow in
an extremely short growing season. Not only does this make growth difficult, but the dormant
stage they assume without moisture also leaves them in a very fragile state.
The physical structure they adopt is influenced greatly by the climate, disturbance
regimes and ratio of organism. Generally, younger or recently disturbed crusts will blend in with
soil color as cyanobacteria start to grow. Assuming the climate is moist enough for such, mosses
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and lichens will additionally grow and start
to give the crust color and texture as
depicted in Figure 1. If a climate receives
little to no rain, lichen and mosses will
grow slowly or may not develop at all. This
leaves crusts in dryer climates smoother
and less visible to human eyes.
Figure 1: Dark lumpy looking soils indicate moss and lichen
development. Light colors indicate here indicate disturbance
As alluded to earlier, the soils are
extremely fragile. This is due to their thin and fibrous structure, as well as moisture deficiency in
their climates (Belnap 2003). Mosses and lichens develop to have 75% of their photosynthetic
biomass on the top .3mm of crust (Belnap and Gillette 2005). Though optimizing light input, the
feature makes them susceptible to process harming inflictions.
Primary Functions of Cryptogamic Soils
Soil crusts play an enormous role in arid ecosystems. Infrequent rainfall and intense solar
radiation can rob unprotected soils of nutrients and water, which is in short supply to begin with.
Wind generated off the Southwestern desert plains in United States further increase the erosion
damage and lead to the degradation of the ecosystem. In three primary ways, biotic soil crusts
help to prevent desertification and the destruction of land.
The most prominent way in which cryptosoils benefit desert landscapes, is by protecting
against erosion. Organisms such as the cyanobacteria and micro fungi found in biological soil
crusts secrete mucilage around their cells (USGS 2001). When moist, the mucilage will move
down into the underlying soils and form a bond, securing the organism to the soils and loose soil
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particles together. Biological soil crusts also keep structure on soils through its other biotic
components. Lichens and mosses tend to have shallow root systems that form a web and anchor
into the layers of soil below (USGS 2001). The longer these systems go undisturbed and the
maturity they are allowed to reach, determine how strong their connections can be, thereby
increasing their ability to perform described functions.
As well as increasing soil stability, these webs do a lot in terms of soil health. The
photosynthetic nature of the mosses and lichens help to contribute carbon and phosphogen to the
soil below. Lichens and moss also have the abilities to “fix” atmospheric nitrogen levels by
absorbing it from the air and making it readily available via soil for other plants. (Rosentreter et
al, 2007). Though they can only metabolize and release the nutrients when moist, the xerophytic
developments of the crusts allow them to maintain function down to levels of 5% moisture
(Belnap 2003).
When rainfall occurs in arid regions, soils crusts regulate the amount of water the soils
below receive. This also is dependent on the morphology of the crust and dominant organism.
Roughened and pinnacled surfaces such as shown in Figure 2, slow water runoff and allow water
to infiltrate the crusts surfaces. Smoother crusts contribute using water stores that allow them to
take in high amounts of rainfall. They then release it at slower rate, benefiting surrounding
vascular plants (USGS 2001).
Figure 2: Different morphologies of BSCs are dependant on climate.
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Negative Human Impacts
Now with a brief understanding of major functions of cryptobiotic soils, we can see the
importance they have within an arid ecosystem. Unfortunately, their resilience against intense
solar radiation, severe drought, and extreme temperatures, means little when faced against
human disturbance.
Human impact is the number one cause of crust destruction (Belnap 2003). With the
growth of the U.S. economy and population, land managers look wide-eyed out to the untapped
desert. However, what could result from the continued intrusion onto arid landscapes?
Currently, the most common encroachment comes from the humble tourists who may
occasionally stray from the path. The impact of the common step was studied in 2006 with an
experiment conducted by J. Belnap and D. E. Gillette. They sought to find the friction threshold
velocities of biological crusts in different stages of development before and after disturbance.
Wind tunnels were set up on a selected research site 16km south of Moab, Utah. Soils of similar
sandy composition where split into classes depending on developmental stage as represented in
text above Figure 3. The wind tunnels would measure the wind speed generated before any carry
of sediment.
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Figure 3 Shows the classes and results. (a) indicates FTVs reached undisturbed. (b) indicates FTVs reached after walked over
with lugged boots. (c)
Results showed that an undisturbed Class 3 crust had 1283 times the resistance to wind
than that of bare sand, class 2 with 81 times resistance, and class 1 with 30 times resistance
respectively. This attested to the superiority of developed soils compared to undeveloped soils.
When a footstep with lugged hiking boots disturbed a well-developed Class 3 layer, following
results would show a reduction of friction threshold velocity by a staggering 73%. Low FTVs are
directly associated with sediment movement (Belnap and Gillette). A drop as such as that would
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allow average wind speeds of the area to pick up sediment off the disturbed soil and ultimately,
initiate erosion processes. This may come about from just a footstep.
According to USGS Research Ecologist and leading biological crust authority, Jayne
Belnap, the reduction of crust cover leads to reduced fertility of soil, reduced carbon/nitrogen fix,
and decreases in soil stability. The inability to stabilize loose grain soils could lead to accelerated
dust storm activity in the Southwest and go on to cripple the economies affected. Whether such
a severe consequence could result from a single step is unlikely. Unfortunately, increased
exploitation of the arid regions for commercial
and recreational reasons will only bring more
footsteps. Footsteps will give way to vehicle
trails and trails may lead to a landscape easily
destroyed, but not easily recovered.
Recovery
The recovery time for soil crusts vary
with region and what status defines recovery.
Figure 4: Depicts the variable affecting recovery of cryptobiotic
soils
Figure 5: Shows time estimated for BSCs to recover in dry regions.
Figure 4 shows the factors influencing
recovery. In returning to a productive state,
favorable conditions can lead to
redevelopment in 20 years (Belnap 2003). As
Figure 5 proves, the limited moisture in deserts
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like the Mojave, make lichen reformation impossible within the limits of our lifetime.
Soil Crust Engineering
Through decades of great research, land managers are becoming increasingly aware of
the time needed for recovery and now seek strategies to accelerate the recovery of soil crusts.
Recent research (Bowker and Belnap, 2005) identified repeated correlations between
micronutrient dense soils and moss and lichen abundance, suggesting micronutrient
supplementation may be an answer. However, researchers stress that more experiments need to
be designed. They hope to find nutrients advantageous to cyanobacteria, to help provide a more
complete soil crust remedy. Additionally, designed experiments need to find any negative effects
supplementation would have on desert ecosystems (Bowker and Belnap, 2005).
Conclusion
While researchers toil with the problem of trying to ensure biological soil crust recovery,
trail steppers similar to me should do our best to try not to disrupt anything further. Soil crusts
have much to offer us in return for just leaving it alone. It is a bargain that should ensure its place
holding down the desert floor. Land management specialists who are responsible for the safety of
the land should consider these studies, and consider the impact they have upon the landscapes of
tomorrow.
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References
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