This course surveys the diversity, structure and functioning of California’s ecosystems through time and the ways they have influenced and responded to human activities and stewardship. Topics include ecosystem drivers such as climate, soils, and land use history; human and ecological prehistory of the state; comparative marine, freshwater, and terrestrial ecosystem dynamics; and managed ecosystems such as range, fisheries and agriculture in California. The course also emphasizes important skills to understand as a scientist or consumer of scientific information, including data collection, natural history, and writing.
Deserts
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Del curso dictado por University of California, Santa Cruz
Ecosystems of California
15 calificaciones
Conoce a los instructores
Erika ZavaletaProfessor
Ecology and Evolutionary Biology
[MUSIC]
I am Ericka Zavaleta, and this is ecosystems of California.
I'm standing at the southern end of the White Mountains
at a little over 10,000 feet, or 3,100 meters, and
I'm standing also at the northern edge of the Mojave Desert.
California's deserts make up almost 40% of it's total land area.
And of that 40%, more than three quarters are the Mojave Desert.
The Mojave Desert's divided up into different subregions.
And the northern part of the Mojave Desert,
which includes most of Death Valley and
the surrounding areas, is the most diverse part of California's deserts.
Part of that diversity has to do with the incredible range of
climates encompassed in the northern Mojave.
So today we're going to take a look at that diversity.
Starting here at the very, very top of the Mojave and going down to it's bottom,
at 282 feet below sea level, or 86 meters.
Which is the lowest point in North America in Badwater Basin in Death Valley.
As we travel that transect,
what we'll see is that we're moving not only from colder climates.
But also from wetter areas,
that are able to capture some of the rain as it comes over the Sierras.
Into drier areas, and so up here at 3,100 meters
annual rainfall totals about 13 inches or 32 centimeters per year.
And by the time we get to Badwater Basin,
many years there's no recorded rainfall at all.
We're now at about 7,300 feet or 2,200 meters, and
have entered a single-leaf pinyon pine, Utah juniper woodland.
So different species of pinyon pine and junipers occur together in woodlands
like these throughout the region on the arid side of California.
And their band extends down to about 1,200 meters or 4,500 feet in some areas.
It's warmer here, but we still have these relatively poorly developed soils.
You can see how rocky these soils are, they're relatively nutrient poor,
there's not much of an under story here.
That's especially true here in the White Mountains because we're on these
sedimentary substrates like dolomite.
At this elevation, we've come down far enough that we started to pick up
the diverse herpetofauna, the reptiles of the desert.
And so, in the pinyon juniper woodland zone,
we're starting to see a higher biomass of reptiles, in particular.
And also higher biomass of other vertebrate animals and invertebrates.
We've descended to about 1,600 meters, little over 5,000 feet, and
we have lost the tree vegetation.
So we're now in classic desert scrub.
The desert scrub in different parts of the Mojave Desert depends on what kind of
geomorphic setting you're in.
Here, we're on a hill slope, which is a pretty typical thing in the Mojave,
because of its topography.
And we have a variety of things like blackbush, ephedra and other species that
are tolerant of dry conditions, but not necessarily of salient conditions.
If you get down into the flats often, then you're in an environments like playas.
Where there's a lot of evaporative water loss, and accumulation of salts.
And so the vegetation there, we'll see is going to be a little bit more on the salt
tolerant end of the spectrum, with species like shadscale and maybe pickle weed.
The two other geomorphic settings we're going to take a closer look at later today
are alluvial fans and dunes.
Alluvial fans occur on hill slopes, and dunes are sort of a special
wind driven feature of the deserts of California with their own flora.
Desert soils, in general, develop slowly, they're thin, they're low in nutrients.
And then you also get these unusual features of deserts' soils that have to do
with the interacting forces of wind which plays a big role in these systems.
And other kinds of erosion.
You can see the surface here covered almost entirely with rocks.
This is called desert pavement.
And it can develop to such a strong degree that you don't get plants anymore because
nothing is able to penetrate the nearly solid surface of stones
pushed together side by side.
Desert pavement is formed by a combination of erosion
of silt and other soil particles that are overlying subsurface rocks.
And also by the wind deposition
of silt that over time can lift up rocks to the surface.
A lot of wind transported particles in the desert are really fine.
And so
another feature of some desert soils that you can get, especially in older soils,
is over time a deposition of a fine layer of silt over the top of the existing soil.
That silt is so fine that moisture from the soil, as it's rising,
sometimes will leave air bubbles trapped underneath that fine layer of silt.
You can develop something called a vesicular horizon.
It's like a layer of air that then makes it very difficult for
water to penetrate down beneath it into the soil.
And so, vesicular horizons can combine with desert pavement to create conditions
that are really impossible for plants to get a foothold in.
For plants in the desert to reproduce, and
effectively outcast can be challenging because they're few and far between.
And the vectors that could move their pollen for them may also be few and
far between.
One of the iconic species or genera of Californian deserts are the yuccas.
And they include a variety of species ranging from small ones to these big
tree like Joshua trees.
And the yuccas are a classic example
of an evolved mutualism to overcome that challenge in the desert.
So, what the different yuccas do, and of 27 species,
at least 20 of them are known to have single moth associates,
is that they form a mutualism with a particular moth species.
And this even extends down within the species level.
The different varieties of Joshua tree each form a one to one mutualism
with a particular moth.
And the moth is moving pollen from the flowers of the yuccas.
And then is in turn getting services from the yucca,
because it lays its eggs in that flower.
And the eggs of the moth develop into larvae, which can feed on some of
the seeds in the developing fruit, and survive to reproduce as moths.
So, the reason that there are these close one to one associations
from the plant's point of view, is that that way they can ensure pollen transfer
to another individual that they can successfully reproduce with.
From a moth's point of view each yucca's flower
is a slightly different size and shape.
And the moths have ovipositors, these long blade like,
ovipositors through which they lay their eggs.
So, if you match the right moth to the right yucca, then the ovipositor
will put the eggs at just the right distance from the developing seeds.
Water cycling in deserts is really epispodic.
Long periods of drought will be punctuated by rain events that,
because there's so little vegetation, cause big run off and flash floods.
I'm walking next to a normally dry water course that right now has a little bit of
water in it because it rained just a little bit.
Water's so important to animals in the desert.
When it does happen, a rain event can just trigger a flood of germination by plants,
and also laying out by things like desert toads.
You can see here that a lot of mammals and
birds are taking advantage of the presence of this ephemeral water.
Another place you can see, at a large scale,
the flashiness of desert hydrology is in the alluvial
fans that come out of mountain ranges throughout California's desert.
So behind me, there's a valley in the mountains.
And out of that valley is a broad fan of dark green vegetation.
So that alluvial fan is covered with creosote because when extreme events
happen, and water comes flowing out of that valley, it carries with it rock and
soil, and silt particles.
Depositing them with the large pieces at the top, the larger pieces coming out
first and settling, and then as you get further down progressively finer material.
So that layer of alluvium, this fresh soil that's available for colonization.
It can be more soil than in these rocky more desert pavement areas,
in the lowlands, and on hill slopes.
And it can also, because it has those coarser pieces in it,
it can allow water in future events to infiltrate the ground faster.
So you get this inverse texture effect in deserts sometimes.
Where it's the coarser soils with the rocks mixed in,
that actually can have water infiltrate during a rain event, and
more effectively reach the rooting zone of plants.
And in the finer material, which in a more wet system we might think of as the better
soil, water may not actually infiltrate very far before it evaporates again.
Creosotes another important species in the Mojave Desert.
It occurs over large areas of the Mojave, in almost monodominant stands,
in the canopy, with subshrubs underneath it.
And one of the things that creosote does to cope with the desert environment
is that it can reproduce vegetatively.
So we know that bristlecone pines are the oldest individual organisms on Earth.
But clonal species like creosote, that are able to send out runners, and
then sprout from branch to upsurf from roots.
And produce new individuals,
that are clones of the parent, can persist even longer than that.
And so there are rings of creosote that are found throughout the Mojave Desert.
And those rings can be as old as the Pleistocene, they can be 15,000 years old,
because they've formed from a central individual that's progressively reproduced
vegetatively in a ring that gets wider and wider over time.
During the Pleistocene the Mojave Desert was a much,
much wetter place than it is now.
And it contained many pluvial lakes or
lakes that formed during that period of wetter and cooler conditions.
Many of those old pluvial lake beds,
are now playas like the one you can see behind me over here.
There are both wet and dry playas.
Wet and dry playas can both be intermittently wet, but the difference
between them has to do with the depth of the ground water table beneath them.
And the shallower that ground water, actually, the saltier they typically are.
Because constant evaporation of water will keep bringing up salts that accumulate at
the surface.
So the playa is sand, it's very salty, and it can be unstable.
And that's why you don't see many plants growing on it.
We're here at the north end of Death Valley, 6 or 700 meters in elevation.
And even though, in some years, there's no measurable precipitation at all in
Death Valley, it's actually raining a little bit right now.
It's very unusual.
So it looks like I'm surrounded by snow but
this is salt on a playa in the bottom of Death Valley, and it's 104 degrees out.
Really, really unusual to have it be this cloudy and wet in August.
But because it's raining,
there is a little bit of water collecting in this playa.
This is a classic playa vegetation with just salt tolerant pickle weed,
and very little vegetation as you go out into the middle of it,
where the salt has accumulated more thickly.
Features created by the wind are called aeolian features, and
deserts are full of them.
The most obvious ones are sand dunes.
So we're at Mesquite Flat dunes in Death Valley.
And sand dunes are caused by wind deposition of fine material that's usually
carried from an upwind source like a playa or an old pluvial lake bed.
And there has to be some reason for the wind to let it's sand go,
as it reaches a particular place.
Often that's because,
turbulence created when wind is blowing down two different valleys intersect,
allows the winds to slow down and drop out their sand, silt that they're carrying.
Because sand dunes store moisture,
they're actually able to support a host of plant species.
And so around me you can see mesquite,
which is just one species that you find on dunes here in the Mojave Desert.
Mesquite's very deeply rooted.
And here in Death Valley it's also an important historical source of food for
the Shoshone people.
So Mesquite are a long lived woody perennial.
It's deeply rooted.
But there are other dune specialists.
Like there are dune primrose species with each dune system in the Mojave.
And so the Eureka Dunes, the Kelso Dunes, and these dunes, among others,
have their own primrose species.
We've reached Badwater Basin, the lowest point in the Western Hemisphere, and
one of the driest as well.
We're 282 feet, or 83 meters, below sea level.
And rainfall here averages a little under five centimeters a year.
Of course it's raining today, which is pretty unusual.
And the reason that this is the lowest point in North America on dry land has
partly to do with the climate.
Most places in the world are too wet for a point this low not to fill with water.
But here, rainfall is so low,
that even though we're that far below sea level this remains dry land.
The way that Badwater Basin was formed has to do with it's
place in relation to larger tectonic forces.
We're in part of the basin and range province, and
here the crust is stretching.
So as the crust stretches it's broken along fault lines.
And then some of those blocks that are formed are being uplifted.
So the blocks have tilted.
And so this is the Panamint Range on the west side of the Valley.
And this is the mountains here on the east.
But then what happened to create the basin is that this block in the middle, slipped,
like this.
And that dropped this point down as low as it is today.
And we're only 160 kilometers, or about 100 miles,
from the highest point in the Continental US, Mount Whitney, to our west.
And that's part of the same series of blocks that have been uplifted
with their edges high up above the valleys below.
So on the other side of the Panamint Range,
and on the other side of the Owens Valley beyond that, are the Sierras.
The highest of these block edge escarpments.
This is also one of the hottest places in the world.
The highest temperature recorded here is 134 degrees Fahrenheit was
measured in 1913, quite a while ago.
And the person who was here at the time measuring it,
described birds dropping out of the sky from the heat just as they flew.
Right now it's only about a 104 degrees out, unusually cool for
an August afternoon, because of the cloud cover and the rain.
[MUSIC]
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