Hello students.  This is plants and animals of Southern California.  And today we're at the edge of the desert.  And I wanted to point out an aspect of the desert that you might think is kind of cryptic, that is not easily to notice.  And that is the cryptobiotic crust of the soil.  If we went to a place that was completely undisturbed by humans where there were just jack rabbits and coyotes and road runner then what we would find is we would find places in the soil, especially flat places, where it wasn't dusty like this.  Instead there was a thin layer where all the sand grains were held together.  And that they be held together by these organisms.  They would be held together by mosses, lichens and cyanobacteria.  And these mosses, lichens and cyanobacteria are organisms that are able to just go into suspended animation.  They go to sleep.  So all winter they're doing their thing.  They're photosynthesizing.  If cyanobacteria are probably fixing nitrogen.  They are growing and forming this crust.  And then what happens is March 20th it stops raining and March 25th they dry out.  And then the whole time, ever since March 25th they've just been dreaming of the rain.  That's what mosses do all summer long.  All summer long mosses dream of when it will rain again.  And they're completely dormant.  You know, like if you were to measure their metabolism it would be a tiny, tiny fraction of normal metabolism.  You know, one thousandth of normal metabolism.  Just the barest bones stuff that keeps them going.  And then when the rains come again, when the rains do come again then within hours they green up, the mosses and lichens and cyanobacteria all green up.  And they all start photosynthesizing again.  And they start doing their thing.  They'll repair themselves.  And the intracellular repair that happens, that'll happen within a few hours.  And then they'll start growing again.  And so as the rains come throughout the winter there will be cycles of wetting and drying where they go dormant and then they grow and then they go dormant and then they grow.  And as long as the periods of being wet are long enough to replenish themselves, they can dominate the landscape.  Now of course this is only dominating this landscape if you're the size of a mite because these are really tiny organisms.  But the cryptobiotic crust, it has a big influence on the ecosystem nevertheless even though it's just this tiny thin film of life that covers the sand in flat places in the desert.  Because first of all it's holding everything together.  And as soon as it gets broken up then what happens is the wind blows in the desert a lot of time and it just makes these big dust storms and kind of turns everything over.  Another thing that it's doing is it's forming a certain surface, right, and so then when vascular plants come, there's some vascular plants that are really good at breaking into that surface and then growing.  And then there are other vascular plants that are not very good at breaking through the cryptobiotic soil crust and those plants will be less abundant because of the crust.  And then if the crust is broken up then they'll become more abundant.  So the cryptobiotic soil crust is kind of determining what the vascular plant community will be in the long term.  Another thing that's happening is a lot of the nitrogen that's being added to the soil is being added by cryptobiotic crust.  It's like that and cat claw which is a legume.  And then the cryptobiotic soil crust they also, they kind of hold onto the water.  As the soil is drying out it dries out more slowly because of this cryptobiotic soil crust.  Some of them have like pectin in their, in their cell walls and they're able then to dry more slowly than just sand would or just dust would.  That's that way it was.  And a lot of places in the desert have been kind of disturbed by people and their motorized vehicles and other things.  And so the cryptobiotic soil crust in those areas, they get, you know, kind of plowed up.  And if it gets broken up too much it can't repair itself.  And so then you'll have areas that are kind of like dust bowls that start forming which is sad if you are somebody who loves the mosses of the desert.  But you might not have thought there were so many mosses of the desert but there's actually quite a few.  There's like 100 species of mosses in the Mohave Desert and a fair number of them are soil mosses.  And most of the time they're dormant.  And you don't see them very much because you'd have to be crawling around on your hands and knees and looking for mosses.  But they're there and they're, you know, part of the community doing their thing.  And that's just a little bit but that's really all I have to say about that.