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All righty.

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So we've been talking about,
the last time we talked about the

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different ways that this
extraordinary process called

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evolution has resulted in different
ways that organisms exploit the

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carbon and energy sources that are
available on earth.

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And as we talked about that we find
that whatever is thermodynamically

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possible in terms of redox gradients
in the environment you will find an

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organism that has evolved to exploit
it.  And one thing I haven't

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mentioned to you guys yet --
Maybe I've mentioned it but I

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haven't driven it home is that in
the microbial world right now 99.

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% of the microbes that life on earth
have never been cultivated.

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So it's a vastly unknown universe,
the details.  So the metabolic

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pathways that I'm describing here
are just the tip of the iceberg in

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terms of what's possible.
So last time we were talking at the

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biochemical scale.
This time we're now going to scale

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up and think of these biochemical
processes on a global level.

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And we're going to talk about
primary productivity which is really

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just another word for the collective
photosynthesis of all of the

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photosynthesizers on earth.
So who are the dominant primary

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producers?  Well,
we've already talked about this.

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And just to get ourselves oriented
let's think of in water the primary

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photosynthetic organisms,
the primary producers are

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phytoplankton.
And let's just look at one

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phytoplankton cell,
OK?  This is a single cell so this

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would be maybe ten microns in size.
And inside that cell we have a

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chloroplast, and you've learned all
this in other lectures and including

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my lectures, which takes up CO2 and
evolves oxygen and makes sugars

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which go to, what's the organelle
where respiration takes place?

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Mitochondria.  Which goes to the
mitochondrion and is respired.

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CO2 is evolved and oxygen is taken
up.  So this little single cell gets

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through life through photosynthesis
primarily, but it also takes the

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products of photosynthesis and burns
them inside the cell.

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So this is what we call respiration.
And it's important to remember that

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even photosynthetic organisms
respire, OK?  But then there's this

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whole other group of organisms that
last time we called heterotrophs

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that can only respire.
They rely on the photosynthetic

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product from photosynthetic
organisms.  So that's in water.

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And then on land the main primary
producers, productivity are plants.

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And we'll just symbolize these as a
tree.  And we all know that the tree

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takes up CO2, evolves oxygen.
If you blow this up, you just take

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a leaf, OK?  That's a blowup.
And then you blowup again and take

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a cell from the leaf.
This cell is identical in function

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to the single microscopic
phytoplankton cell, OK?

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So the process in both of these
ecosystems, this could be grass,

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it could be moss, it could be
anything else,

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the process is the same in terms of
what's involved in productivity.

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OK.  Now, you have a handout for
this lecture that has a lot of the

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definitions that I'm going to now
very briefly put on the board.

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Oh, I should tell you also that in
the last ten minutes of the lecture

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I'm going to show you another
incredible clip from the Blue Planet

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series that I showed you last time
so you have something to look

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forward to.  So let's
define some terms.

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Biomass we're going to call B.

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And as I said this is all in the
handout.

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So I'm just going to give you the
short version here.

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And the units of this can be of
course anything,

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but normally it's something like
grams carbon per meter squared.

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And what does that mean?  Well,
it means if you have, in a

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terrestrial ecosystem,
OK, where you have trees,

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it would mean that you would
calculate the biomass.

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You'd take a square meter and you'd
integrate up and collect all of the

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biomass in that surface area,
OK?  So that's what that means.

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So grams carbon in that square
meter.  For an aquatic ecosystem

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it's the opposite.
You take a square meter of the

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surface water,
let's put some waves on here,

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and you integrate all the way down.
How far?  When will you not have

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anymore primary,
oh, no, biomass?  When will you no

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longer have any more photosynthetic
biomass in the oceans or in a lake?

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Pycnocline.  Oh,

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that's a good answer.
Not necessarily, though.

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But it often corresponds to that.
The pycnocline is a density gradient

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in aquatic ecosystems.
But what is absolutely essential

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for photosynthesis?
Light.  Light.  So you go down

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until there's no light.
Absolutely.  There won't be any

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photosynthesis where there's no
light.  This is the one thing we

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know for sure,
OK?  And that's usually in the

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oceans around 200 meters.
And in lakes it depends on how rich

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they are.  In the Charles River it's
about one meter because

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it's such a mess.
In fact, well,

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I shouldn't say a mess,
but it's very productive.

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In fact, legend has it, and I don't
know if this is true,

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that it's actually safe to swim in
the Charles in terms of the water

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quality, but the reason you're not
supposed to swim is because the

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visibility is so bad that if
anything happened they'd never find

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you.  But I don't know if that is
true.  That could be

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an urban legend.
OK, so we're going to define gross

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primary productivity.
GPP, gross primary productivity is

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the rate of photosynthesis --

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-- and grams carbon per meter
squared per year.

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And of course the units here are not,
the absolute units,

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this could be per day and this could
be per square millimeter if you

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wanted, but the units are amount per
unit area per unit time,

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OK?  And then we're going to define
the respiration rate,

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the respiration of the autotrophs.
So this would be the respiration

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rate of the photosynthetic organisms,
which is why we call them R sub A.

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And that's going to have the same
units, OK?  So it's grams carbon per

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meter squared per unit respired.
And net primary productivity is

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then gross primary productivity
minus, so net primary productivity

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of an ecosystem is the amount of
carbon, CO2 that's drawn into the

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system through photosynthesis minus
the amount that was respired by the

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plants, by the organisms that did
the photosynthesis.

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In a sense, you can think of it as
the amount of carbon that actually

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goes into a plant growing,
OK, that goes into the biomass of a

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tree or that goes into the division
of a single celled phytoplankton

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into two phytoplankton.
And then a lot of that is lost

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through respiration.  OK.

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We can also define mean
resonance time --

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-- as MRT, which is the biomass

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divided by the net primary
productivity.

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So what are the units of the mean
resonance time?  It should

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be obvious but --

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Years, right, or time.
Mean resonance time is time.

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And that's really, if you want to
think about it,

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if you think of yourself as a carbon
atom that's drawn into a tree

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through photosynthesis,
it's the average amount of time you

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will spend, your one
atom in that tree.

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OK?  It's the average resonance,
the mean resonance time.  Well,

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actually, that's wrong,
right?  It's not the amount of time

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that you, that one atom will spend,
but it's on average the amount of

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time that atoms will spend in the
tree.  OK.  And then the

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fractional turnover --

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-- is equal to one over the mean
resonance time.

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And it has the units of obviously
years to the minus one.

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And it's the fraction, if you think
of a tree again,

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it's the fraction of the carbon in
that tree that is renewed by new

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carbon every year.
Now, these two concepts will become

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very important again when we start
to talk about global biogeochemical

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cycles.
And we'll talk about the resonance

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time, the various elements in
various components in the earth

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system.  OK.  So now let's go on and
look at, first of all,

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I should have shown this slide last
time.  I'm trying to not walk around

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too much because this fellow is
filming me or filming

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these classes.
But we should briefly take a look at

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the absorption spectrum of all of
the plants in the biosphere.

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Could we turn the lights down a
little?  Or I guess I'm

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in charge of that.

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There.  A little bit better maybe.
This is the absorption spectrum of

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the pigment chlorophyll,
chlorophyll A that is the pigment

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that all green plants have and they
use to absorb sunlight.

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And you'll notice that over the
course of evolution all of these

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white bands are accessory pigments.
That different organisms have

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evolved to also capture light.
And they pass on that energy to the

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chlorophyll molecule.
And the point is that if you look

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across all of the visible light and
even beyond there are pigments that

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have evolved to capture that solar
energy collectively in the ecosystem.

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OK, so let's look at,
now we're going to look at world net

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primary productivity.
So essentially photosynthesis on a

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global scale.  And I'm going to tell
you right up front.

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These numbers are extremely
approximated.  And I've taken these

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numbers from various textbooks.
Your textbook doesn't even have a

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table like this in it.
In fact, your textbook is very weak

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in this section of biology.
But there are always tradeoffs in

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choosing textbooks.
So I've taken this from textbooks

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and I've rounded off these numbers.
And so I just want you to go

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through and understand the structure
of the table.

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The idea is not to memorize
particular numbers but understand

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and have a feeling for the relative
amounts of productivity in different

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ecosystems.  So,
first of all, here's our units of

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grams per year,
world net primary productivity.

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These are grams of carbon, OK?  And
if we first look at the total amount

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on land, in this particular table is
177 versus the marine total is 54.

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But the new estimates,
I've taken this out of sort of more

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primarily literature than textbooks,
really show these numbers to be more

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like this.  That shows you how
variable this is.

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It changes every decade.
Showing that the amount of

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photosynthesis in the oceans is
roughly on par with the amount on

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land.  And to remind you of our
units here, weight-wise this is 50

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to 70 billion Volkswagen's
worth of carbon.

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I mean that's a lot of carbon every
year that is going into these

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ecosystems.  OK,
so let's look and dissect the table

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a little bit.  Looking at tropical
forests like the rainforest in the

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Amazon that are some of the most
productive ecosystems in the world.

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You can see that over here their net
primary productivity per meter

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squared is 2,000.
And then you look down here at the

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open ocean which on a per meter
squared basis,

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a very unproductive ecosystem,
there are tiny little phytoplankton,

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is 100.  But looking further, before
I get to the but which

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is the punch line.
That's the trouble with animation.

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The biomass in the tropical forest
is enormous, obviously trees,

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whereas the biomass in the open
ocean is very tiny.

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But if we look at the percent of
the surface of the earth that is

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covered by these two different
ecosystems the open ocean is huge

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compared to the tropical forest.
So on a global scale, because of

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the aerial coverage here these two
ecosystems contribute equally, OK?

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So it's a combination of net primary
productivity and the coverage of the

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global ocean.  OK.
Let's go back to that for a minute.

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So let's --

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-- talk about turnover times or mean
resonance times.

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Just eyeballing it,
can you give me an estimate in terms

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of days, months,
years, decades, centuries,

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order of magnitude for the mean
resonance time of carbon in all the

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phytoplankton in a
marine ecosystem?

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Is it days, months, years,
centuries, decades?

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Well, don't guess.

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Well, you can guess but minutes is
wrong.  So having guessed minutes is

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wrong now you use your analytical
brain and you look at this,

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mean resonance time, which is
biomass divided by net primary

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productivity.  Here's the biomass.
Round that off.

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And here's the net primary
productivity.

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And the units here is years.

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Like a month did I hear?  I didn't
hear.  Right.  It's about one-tenth

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of year which is about a month.
About a month.

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Does everybody follow that?
It's just to round this off,

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five over 50 is 0.1, biomass over
MPP.  How about for the terrestrial

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ecosystem, what's the average amount
of time a carbon atom spends

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in the average tree?

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Years, right?  Decades.
Many years.  Maybe decades to

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centuries.  This is a way we think
about these things.

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We don't have an exact number.
But you want to get an order of

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magnitude feeling.
So carbon is turning over very,

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very fast in the marine ecosystem
but very slowly in the terrestrial

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ecosystem.  And the simple way to
think about that is that

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phytoplankton don't have trunks,
but there's a more complicated way

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to think about it.
OK, so now, all of this primary

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productivity that we've made,
all of this photosynthesis, as we

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talked about, is the base of the
food webs in all ecosystems.

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And so we're going to start to
dissect this.  This is a marine food

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web showing phytoplankton that are
eaten by zooplankton.

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We used to talk about food chains,
but we well now that it's not a

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chain.  It's really a very complex
web and very hard to put organisms

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and assign them to particular
sections.  Phytoplankton are eaten

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by zooplankton.
Zooplankton are eaten by worms.

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00:20:32,000 --> 00:20:37,000
You've got blue crabs, barnacles,
and the top predators shore birds

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00:20:37,000 --> 00:20:43,000
and sea bass.  Now,
an important part of these food webs

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00:20:43,000 --> 00:20:48,000
is also all of the carbon,
the primary productivity that is not

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00:20:48,000 --> 00:20:53,000
eaten while it's alive.
So some things just die,

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00:20:53,000 --> 00:20:58,000
right?  You have dead carbon lying
around.  And that dead carbon falls

228
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into what's called a different food
web, the detritus food web.

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And we're going to talk about that.
And it comes from all of these

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different components in the food web.
So now we're going to more

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00:21:13,000 --> 00:21:17,000
analytically look at the flow of
carbon.  And when we talk about

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carbon we are also talking about
energy, right?

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00:21:22,000 --> 00:21:26,000
Carbon and energy are the same
thing.  I mean they're

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00:21:26,000 --> 00:21:31,000
interconvertable.
So we're going to look at the flow

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00:21:31,000 --> 00:21:37,000
of carbon from the phytoplankton to
the zooplankton through that trophic

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level.  And to do this we have to
talk about some definitions.

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And again this is in your handout
so I'm not going to write all this

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00:21:48,000 --> 00:21:54,000
on the board but we'll just walk
through this.  The flow of carbon or

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00:21:54,000 --> 00:22:00,000
energy through a trophic level,
which is one of these links, OK?

240
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This is one trophic level.
This is the next trophic level.

241
00:22:04,000 --> 00:22:09,000
Or you can also think of this as an
organism.  This type of analysis

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00:22:09,000 --> 00:22:14,000
applies to both.
And we start out with the

243
00:22:14,000 --> 00:22:19,000
productivity at trophic level and
minus one, OK?

244
00:22:19,000 --> 00:22:24,000
And so the first would be primary
productivity coming into the system.

245
00:22:24,000 --> 00:22:29,000
Some of that productivity, some of
that carbon is not ingested by the

246
00:22:29,000 --> 00:22:34,000
next trophic level.
That's lost as dead organic matter,

247
00:22:34,000 --> 00:22:40,000
detritus, whatever we want to call
it.  So that's D sub n the portion

248
00:22:40,000 --> 00:22:45,000
not consumed.  Then some of it is
ingested by the organism,

249
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I sub n.  And then some fraction of
that is assimilated by the organism.

250
00:22:51,000 --> 00:22:57,000
That means it's taken into its
biochemistry and goes toward

251
00:22:57,000 --> 00:23:02,000
building biomass.
And some of it is lost as fecal

252
00:23:02,000 --> 00:23:07,000
matter produced,
that's F sub n.  And urine would

253
00:23:07,000 --> 00:23:12,000
also be a part of this,
waste products.  And then some of it

254
00:23:12,000 --> 00:23:17,000
is then, the rest of it is then
available, oh,

255
00:23:17,000 --> 00:23:22,000
some of it, this is important,
is lost as respiration.  And then

256
00:23:22,000 --> 00:23:27,000
the rest is available as
productivity for the next

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00:23:27,000 --> 00:23:32,000
trophic level.  OK.
So different types of organisms.

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First, before we get to that, using
this analysis we can start to define

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00:23:38,000 --> 00:23:44,000
efficiencies of energy conversion
through this system.

260
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And this is because different types
of organisms assimilate carbon with

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00:23:49,000 --> 00:23:55,000
different efficiencies.
And that is important in the flow

262
00:23:55,000 --> 00:24:01,000
of carbon through different
types of ecosystems.

263
00:24:01,000 --> 00:24:05,000
So let's look at the first
efficiency that would be I,

264
00:24:05,000 --> 00:24:10,000
ingested, reflecting the amount
that's ingested relative to the

265
00:24:10,000 --> 00:24:14,000
amount that's available.
And this is called the exploitation

266
00:24:14,000 --> 00:24:19,000
efficiency, OK?
I sub n divided by P sub n minus

267
00:24:19,000 --> 00:24:24,000
one.  The next one similarly would
be A sub n, the amount that's

268
00:24:24,000 --> 00:24:29,000
assimilated relative to the amount
that's ingested.

269
00:24:29,000 --> 00:24:34,000
And that is the assimilation
efficiency.  And finally the amount

270
00:24:34,000 --> 00:24:40,000
that goes to the next trophic level
divided by the amount that's

271
00:24:40,000 --> 00:24:45,000
assimilated, which is the production
efficiency, the amount that actually

272
00:24:45,000 --> 00:24:51,000
goes to productivity that is
assimilated.  And these all

273
00:24:51,000 --> 00:24:57,000
multiplied together give you the
ecological efficiency.

274
00:24:57,000 --> 00:25:03,000
And that is sometimes called the
trophic transfer efficiency.

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00:25:03,000 --> 00:25:09,000
That's the amount of carbon that is
basically lost as you go through one

276
00:25:09,000 --> 00:25:15,000
trophic level.
And usually, and we'll talk about

277
00:25:15,000 --> 00:25:21,000
this in a minute,
this is 10% to 20% actually makes it

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00:25:21,000 --> 00:25:27,000
through the system,
and the rest is lost to respiration

279
00:25:27,000 --> 00:25:34,000
or detritus or fecal matter.
OK, so let's talk about now how

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different organisms vary in terms of
efficiencies.

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00:25:42,000 --> 00:25:50,000
We have, in terms of the

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00:25:50,000 --> 00:25:58,000
exploitation efficiency,
if you're talking about, for example,

283
00:25:58,000 --> 00:26:06,000
tree insects.
So insects feeding on trees is about

284
00:26:06,000 --> 00:26:15,000
1% to 10%.  They don't take that
much of the tree.

285
00:26:15,000 --> 00:26:24,000
If it's grass to animals it's more
like 20%.  And if it's phytoplankton

286
00:26:24,000 --> 00:26:33,000
to zooplankton it's more
like 20% to 40%.

287
00:26:33,000 --> 00:26:37,000
In other words,
zooplankton harvests much of the

288
00:26:37,000 --> 00:26:41,000
primary productivity,
no, almost half of the productivity

289
00:26:41,000 --> 00:26:45,000
that the phytoplankton have made.
OK.  What about assimilation

290
00:26:45,000 --> 00:26:50,000
efficiency?  How does this vary
between organisms?

291
00:26:50,000 --> 00:26:54,000
Well, this one, you can think about
it as if you eat food that is,

292
00:26:54,000 --> 00:26:58,000
I was going to say not you, but it
is true, we are all animals so it

293
00:26:58,000 --> 00:27:04,000
applies to us, too.
If you eat food that is similar in

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00:27:04,000 --> 00:27:10,000
composition to your own bodily
composition you're more efficient at

295
00:27:10,000 --> 00:27:16,000
assimilating it because you don't
have to break it down as much and

296
00:27:16,000 --> 00:27:22,000
reorganize it.
So herbivores,

297
00:27:22,000 --> 00:27:28,000
organisms that eat plant matter are
20% to 50% efficient in

298
00:27:28,000 --> 00:27:34,000
their assimilation.
But carnivores is more like 80%.

299
00:27:34,000 --> 00:27:41,000
Because you are meat and if you eat
meat you don't have as much waste

300
00:27:41,000 --> 00:27:49,000
than if you eat a lot of fiber.
So there are big differences there.

301
00:27:49,000 --> 00:27:56,000
And then in terms of, that is not a
value judgment on whether you should

302
00:27:56,000 --> 00:28:03,000
eat meat or not.
I just want to make that very clear.

303
00:28:03,000 --> 00:28:09,000
What?  OK.  But later on we'll talk
about the difference between eating

304
00:28:09,000 --> 00:28:15,000
meat globally and being vegetarians
in terms of utilizing primary

305
00:28:15,000 --> 00:28:21,000
productivity on the earth.
But in terms of production

306
00:28:21,000 --> 00:28:27,000
efficiency you have warm-blooded
organisms having a 2% production

307
00:28:27,000 --> 00:28:35,000
efficiency.
Whereas cold-blooded have a 40%

308
00:28:35,000 --> 00:28:45,000
production efficiency.
Why would that be?

309
00:28:45,000 --> 00:28:52,000
Yes.  If you're warm-blooded,

310
00:28:52,000 --> 00:28:57,000
does anybody know the technical term
for that?  It is if you

311
00:28:57,000 --> 00:29:02,000
are homeotherm.
Or what did you say?

312
00:29:02,000 --> 00:29:08,000
Endothermic.  I'm not sure.
I think that's chemistry.  It

313
00:29:08,000 --> 00:29:13,000
sounded good, though.
So there are homeotherms and

314
00:29:13,000 --> 00:29:19,000
heterotherms.  It doesn't matter.
The point is that these have to

315
00:29:19,000 --> 00:29:24,000
maintain their body temperature like
we do.  That takes a lot of energy.

316
00:29:24,000 --> 00:29:30,000
Whereas, these go with the flow so
to speak, that technical term.

317
00:29:30,000 --> 00:29:36,000
But that doesn't take as much energy.
If it's cold they just let their

318
00:29:36,000 --> 00:29:42,000
body get cold.
They don't burn,

319
00:29:42,000 --> 00:29:48,000
burn, burn to keep the temperature
constant.  OK,

320
00:29:48,000 --> 00:29:54,000
so that's how organisms differ.
Now let's move on to the next

321
00:29:54,000 --> 00:30:00,000
chapter in which now we are
going to look at --

322
00:30:00,000 --> 00:30:04,000
We were looking at the flow through
one trophic level.

323
00:30:04,000 --> 00:30:09,000
Now we're going to connect a whole
bunch of trophic levels.

324
00:30:09,000 --> 00:30:13,000
We're going to look at the flow of
energy through this component of

325
00:30:13,000 --> 00:30:18,000
this food web and do a more thorough
analysis.  OK,

326
00:30:18,000 --> 00:30:23,000
so this gets kind of messy but let's
start here.  I better use the

327
00:30:23,000 --> 00:30:27,000
powerful one.  OK,
so each one of these is what we call

328
00:30:27,000 --> 00:30:32,000
a trophic level.
And these are the primary producers,

329
00:30:32,000 --> 00:30:37,000
the photosynthetic organisms.  Here
is our gross primary production

330
00:30:37,000 --> 00:30:42,000
absorbing sunlight.
Some of that is lost to heat.

331
00:30:42,000 --> 00:30:46,000
Some of that is lost to respiration.
Here is our little R sub A,

332
00:30:46,000 --> 00:30:51,000
remember?  Right here.  And some of
it, the net primary productivity is

333
00:30:51,000 --> 00:30:56,000
available for ingestion at the next
trophic level which are

334
00:30:56,000 --> 00:31:01,000
the herbivores.
So all we're doing here is ganging

335
00:31:01,000 --> 00:31:06,000
up a whole bunch of those individual
analyses.  And then the next trophic

336
00:31:06,000 --> 00:31:11,000
levels are the carnivores and then
the second carnivores.

337
00:31:11,000 --> 00:31:15,000
And the number of links you have
here is something that is obviously

338
00:31:15,000 --> 00:31:20,000
determined by the efficiency of
transfer from one to another and the

339
00:31:20,000 --> 00:31:25,000
total amount of energy that comes
into the system.

340
00:31:25,000 --> 00:31:30,000
So here in our ecosystem we have
carbon being lost at each

341
00:31:30,000 --> 00:31:35,000
step to detritus.
And you have feces at each link.

342
00:31:35,000 --> 00:31:39,000
This all goes down and becomes part
of the detritus food web.

343
00:31:39,000 --> 00:31:44,000
And in that you have two forms of
carbon.  You have particulate

344
00:31:44,000 --> 00:31:48,000
organic carbon which is pieces of
dead carbon floating around,

345
00:31:48,000 --> 00:31:53,000
dead leaves, dead phytoplankton,
whatever.  And then you also have

346
00:31:53,000 --> 00:31:57,000
dissolved organic carbon.
When these plants die and the

347
00:31:57,000 --> 00:32:02,000
phytoplankton die they burst open.
And the glucose and amino acids and

348
00:32:02,000 --> 00:32:06,000
all of that dissolve into the water
in the system.

349
00:32:06,000 --> 00:32:10,000
And that becomes dissolved organic
carbon which is available for this

350
00:32:10,000 --> 00:32:14,000
microbial food web,
an entirely different food web

351
00:32:14,000 --> 00:32:18,000
that's coupled to the system.
So you have the detritivore.  This

352
00:32:18,000 --> 00:32:22,000
the grazing food web here.
The waste from that goes to the

353
00:32:22,000 --> 00:32:26,000
detritivores and the microbes.
And then whatever is left over

354
00:32:26,000 --> 00:32:30,000
after that is called refractory
carbon.

355
00:32:30,000 --> 00:32:36,000
That means none of the creatures are
able to break it open and really get

356
00:32:36,000 --> 00:32:42,000
energy out of it.
And in the big picture what is this?

357
00:32:42,000 --> 00:32:48,000
We talked about it actually last
time.  Fossil fuel,

358
00:32:48,000 --> 00:32:54,000
exactly.  This is the carbon that
actually accumulates over time that

359
00:32:54,000 --> 00:33:00,000
when ecosystems are finished
processing all of the

360
00:33:00,000 --> 00:33:07,000
primary production.
OK, so I'm trying to time this right

361
00:33:07,000 --> 00:33:15,000
so that we make sure we have time
for the movie.

362
00:33:15,000 --> 00:33:24,000
So let's look at a couple of ways
we can think about this.

363
00:33:24,000 --> 00:33:31,000
Let's just compare.
If we look at the open ocean

364
00:33:31,000 --> 00:33:36,000
ecosystem versus the
tropical forest.

365
00:33:36,000 --> 00:33:46,000
And the average,

366
00:33:46,000 --> 00:33:56,000
oh, I forgot one thing.
OK.  If we take now the gross

367
00:33:56,000 --> 00:34:03,000
primary productivity.
And I'm going to circle all of the

368
00:34:03,000 --> 00:34:09,000
respirations.  See the purple here?
So this is all the carbon that's

369
00:34:09,000 --> 00:34:15,000
being fixed.  This is that is lost
to respiration.

370
00:34:15,000 --> 00:34:21,000
And we can define a new parameter
which is net ecosystem production.

371
00:34:21,000 --> 00:34:27,000
And that is gross primary
production minus the respiration of

372
00:34:27,000 --> 00:34:33,000
all of the autotrophies,
R sub A, minus the respiration of

373
00:34:33,000 --> 00:34:39,000
all of the heterotrophic components
of the system.

374
00:34:39,000 --> 00:34:45,000
So that would be the herbivores,
carnivores, detritivores,

375
00:34:45,000 --> 00:34:51,000
microorganisms.
That's all the carbon that's

376
00:34:51,000 --> 00:34:58,000
respired and lost to CO2.
So you can add this to this.

377
00:34:58,000 --> 00:35:05,000
So we'd have net ecosystem
production.  Production equals GPP

378
00:35:05,000 --> 00:35:12,000
minus RA minus the sum of all the
heterotrophs.  Now,

379
00:35:12,000 --> 00:35:19,000
in very mature ecosystems this net
ecosystem production is essentially

380
00:35:19,000 --> 00:35:26,000
nothing.  All right?
Everything that's produced is

381
00:35:26,000 --> 00:35:32,000
consumed.
For example, in a tropical

382
00:35:32,000 --> 00:35:36,000
rainforest you don't have a huge
buildup of organic carbon in a

383
00:35:36,000 --> 00:35:41,000
tropical rainforest.
Everything that's produced is

384
00:35:41,000 --> 00:35:46,000
basically consumed and this is zero.
But in a young forest, say a

385
00:35:46,000 --> 00:35:50,000
plantation at the extreme where you
plant trees and they're increasing

386
00:35:50,000 --> 00:35:55,000
in biomass, then obviously this net
ecosystem production is a positive

387
00:35:55,000 --> 00:36:00,000
number.  What if net ecosystem
production is a negative number?

388
00:36:00,000 --> 00:36:09,000
It won't be there for long,

389
00:36:09,000 --> 00:36:15,000
right?  You've got to have things
photosynthesizing net at least

390
00:36:15,000 --> 00:36:21,000
enough to maintain the ecosystem.
I mean you can have it for a

391
00:36:21,000 --> 00:36:27,000
transient for not any steady state.
OK.  All right.  Talking about

392
00:36:27,000 --> 00:36:33,000
ecological efficiencies again.
If we talk about the average --

393
00:36:33,000 --> 00:36:43,000
The average ecological efficiency of

394
00:36:43,000 --> 00:36:51,000
the open ocean is about 25% and of
the tropical rain forest is about 5%.

395
00:36:51,000 --> 00:36:59,000
So the average number of trophic
levels, that's basically the number

396
00:36:59,000 --> 00:37:08,000
of links in the food web in these
two systems is about 7.1 and 3.2.

397
00:37:08,000 --> 00:37:14,000
In other words,
when you have more efficiency of

398
00:37:14,000 --> 00:37:21,000
transfer from one link to another
you can have more links obviously.

399
00:37:21,000 --> 00:37:28,000
So getting back to humans again,
and this gets back to in terms of

400
00:37:28,000 --> 00:37:35,000
what we might think about as a
global human society.

401
00:37:35,000 --> 00:37:49,000
If you go from wheat to man you lose
90% of the energy in that transfer.

402
00:37:49,000 --> 00:38:03,000
If you go from wheat to cows to man
you lose 90% here and

403
00:38:03,000 --> 00:38:12,000
you lose 90% here.
So obviously in terms of feeding the

404
00:38:12,000 --> 00:38:17,000
world it's much better to go
directly from wheat to humans than

405
00:38:17,000 --> 00:38:21,000
to go from wheat to cows to humans.
You all know this but it's

406
00:38:21,000 --> 00:38:26,000
important to remember that.
And unfortunately the trend in the

407
00:38:26,000 --> 00:38:31,000
world is to go more from here to
here instead of the

408
00:38:31,000 --> 00:38:36,000
other way around.
Just something to remember in terms

409
00:38:36,000 --> 00:38:41,000
of the application of this knowledge.
OK.  Finally very quickly I'm going

410
00:38:41,000 --> 00:38:46,000
to skip over this and go to another
way to look at what we've been

411
00:38:46,000 --> 00:38:51,000
talking about.
It's just another diagram of the

412
00:38:51,000 --> 00:38:56,000
same thing.  You have the
photosynthetic organisms that the

413
00:38:56,000 --> 00:39:02,000
entire world depends on,
this productivity for food and fiber.

414
00:39:02,000 --> 00:39:06,000
And that most of that is lost
through respiration and the rest

415
00:39:06,000 --> 00:39:10,000
goes to the other organisms
including detritivores.

416
00:39:10,000 --> 00:39:15,000
So a big question is how much of
this global primary productivity,

417
00:39:15,000 --> 00:39:19,000
this global photosynthesis have
humans taken over?

418
00:39:19,000 --> 00:39:23,000
It's really hard to answer that
question, OK, but a lot of

419
00:39:23,000 --> 00:39:28,000
ecologists have been working very
hard at understanding the fraction

420
00:39:28,000 --> 00:39:32,000
of global photosynthesis that has
been what's called co-opted

421
00:39:32,000 --> 00:39:38,000
by humans.
And the estimates range from 10% to

422
00:39:38,000 --> 00:39:44,000
55%.  The amount that we use
directly as food or fuel or fiber or

423
00:39:44,000 --> 00:39:50,000
timber is not that great,
but there's a lot of productivity

424
00:39:50,000 --> 00:39:56,000
that's diverted as crop waste,
burning, et cetera.  And land

425
00:39:56,000 --> 00:40:03,000
conversion obviously uses up a lot
of habitat and productivity.

426
00:40:03,000 --> 00:40:06,000
So the significant point here is
that as we co-opt this primary

427
00:40:06,000 --> 00:40:10,000
productivity we change dramatically
all of the food webs that rely on it,

428
00:40:10,000 --> 00:40:14,000
and that is what is part of the path
to extinction of a lot of species.

429
00:40:14,000 --> 00:40:18,000
And the big question is how much
can we co-opt?

430
00:40:18,000 --> 00:40:22,000
I mean we're on the road to taking
over the primary productivity of the

431
00:40:22,000 --> 00:40:26,000
earth, there's no question,
completely.  Unless we set up

432
00:40:26,000 --> 00:40:30,000
reserves that's it, that's
where we're marching.

433
00:40:30,000 --> 00:40:34,000
Now, when we're in charge of it,
is it going to function the way we

434
00:40:34,000 --> 00:40:38,000
need it to function to maintain our
atmosphere and to provide us with

435
00:40:38,000 --> 00:40:42,000
the food and fiber that we need?
That's still an open question.  OK,

436
00:40:42,000 --> 00:40:46,000
so now I'm going to show you this
really neat DVD because these

437
00:40:46,000 --> 00:40:50,000
pictures of food webs are deadly
dull and don't represent anything at

438
00:40:50,000 --> 00:40:54,000
all of what the reality is like.
So I'm going to show you three

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weeks in the life of
a real food web.

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00:40:58,000 --> 00:41:02,000
And I want you to think about two
things when you're watching it.

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Don't just think you're at home in
front of your TV watching a nature

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00:41:06,000 --> 00:41:10,000
show and going brain dead or
something.  Think about what you've

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00:41:10,000 --> 00:41:14,000
learned in this class.
So think about the gigatons of

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carbon that are flowing through this
system.  More importantly this

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00:41:18,000 --> 00:41:22,000
entire food web and everything
that's going on in it is

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00:41:22,000 --> 00:41:26,000
orchestrated by the information in
the genes of the organisms

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00:41:26,000 --> 00:41:30,000
in the food web.
That's the information content that

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00:41:30,000 --> 00:41:34,000
structures this whole thing.
And how it's all orchestrated and

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coordinated and happens the same way
more or less every year is

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absolutely mind blowing as far as
I'm concern and a major,

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major challenge for ecology and for
molecular ecology.  OK.