Friday, May 3, 2013

The background is most important

Western Corsair, Rasahus biguttatus
The Western Corsair is an impressive Assassin Bug. He is fast, nicely marked, night-active, and a hunter of other insects. He runs them down, paralyzes them with his poisonous bite and sucks up their liquified tissues.

But there is something else in the picture that is at least as important as this predator for the ecosystem that we call desert: the cryptobiotic soil crust, those dark green specks he is walking on.

The spots are communities of cyanobacteria, green algae, lichens, mosses, liverworts, and other microorganisms that colonize the surface of bare soil.  "Cryptobiotic" means "hidden life."  This crusts in our backyard goes mostly unnoticed. and does not even look alive when it is very dry (as it usually is).  When it rains enough, though, the crust looks greener and more three-dimensional, like many little mounds of moss. There are more impressive and well studied examples in moister shaded areas in many national parks. But this unassuming variaty that covers the drier desert habitats may be of special importance and is probably the most fragile.

Haboob over Eloy AZ
 The crust is vital to the health of soils and ecosystems.
It holds the soil in place and protects underlying sediments from erosion. Lately we have seen our mountains disappear behind dust clouds that were blown in all the way from the Californian desert were agricultural use has laid the soil bare. Last year haboob clouds rose up horrifyingly from the Phoenix area. The thin living layer that covers undisturbed desert soil can prevent many of these events, but it is very fragile. Agriculture, development, motor vehicles, even grazing cattle will destroy it. Also, I am not sure how the current ongoing drought conditions will impact it. Once disturbed it may take decades for the fragile system to grow back to its former productivity, even under 'normal' weather conditions.

Where the layer is intact it can be the first start of organic soil development on bare inorganic sediments, absorbing water and, through photosynthesis and fixation of atmospheric nitrogen, enriching the surface with nutrients and organic matter. This in turn creates a favorable environment for seeds to germinate and for insects and other soil organisms to live. For more info and great photos go to this link.

Here in the arid Southwest we are learning the hard way that this fragile, living component  of our soil deserves diligent protection. As a kid in Germany I would not have thought that I would ever come to value this kind of ground cover so much. I remember that there was a particularly shady area in the yard behind my grandmother's house where we were not supposed to play because the ground was slimy and green and to our mothers 'full of germs' (these were the sixties). But I did see my first liverworts (probably Marchantia sp.) there and marveled at their shapeless not-quite-floral not-quite-fungal existence that sometimes did give hints of a higher organization. Even at four years old (the house was sold when I was five) it peaked my curiosity.   


  1. It is odd that the seeming harshness of the desert is counterbalanced by this extreme fragility. I often wonder if it is difficult for humans to develop a nurturing attitude to xeric environments because we intinctually perceive them as hostile. Or perhaps this is only because it is so evident when people have despoiled deserts due to the long time it takes for the traces to be erased...

  2. Your future was obviously decided at an early age!