Energy Flow in Ecosystems


Hi. It’s Mr. Andersen and this is environmental
science video 8. It is on energy flow in ecosystems. In the last video we talked about the importance
of producers producing food that is consumed by consumers. But we did not talk about where
that energy comes from. What is the major energy source on our planet? It is going to
be the sun. And this model also does not show that we are losing energy to heat at each
step along the way. And so a better model is an ecological pyramid that looks like this.
And so the producers on our planet produce food, make energy usable. Where did that energy
originally come from? It came from either the sun or chemicals. And so all plants do
photosynthesis and so they are taking energy from the sun and putting it into the energy
of the bonds of the food. Now some chemosynthetic bacteria can do the same thing with chemicals,
like hydrogen sulfide. They are making that energy usable. Now once they have made that
energy usable they can respire it. And so can all of the consumers that sit above or
below them on this trophic level. Now we measure this amount of energy that is converted through
productivity in one of two ways. Either gross primary productivity or net primary productivity.
Gross is the overall amount of energy converted and net is just how much the plant gets after
it uses some of the energy for respiration. Now each of the levels within this food chain
are going to be a trophic level. And we are losing energy along the way and so a good
way to measure this is using an ecological pyramid. We can measure the efficiency, in
other words how much energy makes it to the next level. We can measure the energy at each
level or we could measure the biomass, how much living material do we have. And so energetics
is the study of how energy gets from something like the sun into organisms. What is the most
important first step is going to be photosynthesis. In photosynthesis we take carbon dioxide in
the air, water, and energy and sunlight and we convert that into oxygen and glucose. If
you have not memorized the equation for photosynthesis now is the time to do it. You should know
what is going into the reaction and what is coming out of the reaction. And why that is
important is we can simply turn the arrow around and now we have the equation for cellular
respiration. So that is what you are doing. You are taking in the oxygen that is produced
by plants, taking in the sugar and you are converting that into carbon dioxide and water.
The nice thing is that this can be recycled again back into plants. And so this is really
how we take energy, put it in plants, store it in the food so we can utilize it as well.
Now one major misconception is that plants are doing respiration as well. They are making
the sugar for themselves to release that energy. Now something very similar to photosynthesis
is called chemosynthesis. And so if we look down deep in the oceans at these hydrothermal
vents we are producing not only heat but we are producing a chemical called hydrogen sulfide.
Also methane can be used this was. And so certain chemosynthetic bacteria, look how
similar this is to photosynthesis, can use the energy in the bonds of the hydrogen sulfide
to make glucose. They release water and then sulfur. And so the equation looks very similar
to photosynthesis. Now what happens is things living around the chemosynthetic bacteria
can take in that glucose and they can use oxygen to do cellular respiration. And so
we have a totally different system. It is built on the energy inside the chemicals.
So no matter where the energy comes from we can measure the amount that gets into the
producers using productivity. Now the bad news is that hardly any of that energy actually
gets into the plant. Ninety-nine percent is going to move through it, bounce off of it.
The plant does not get it. Only one percent actually goes into the producer and we call
that the gross primary productivity. It is the amount of energy that the plant actually
gets. Now what is the plant going to do? It has to survive. And so it is doing respiration.
That is where most of the energy goes. And a small percent of it goes to what is called
the net primary productivity. That is the amount the plant gets if we subtract the amount
that it used for respiration. So the bad news, not much energy goes into the producers. What
is the good news? There is so much energy contained within the sun. If we look at the
productivity on our planet we could compare different terrestrial and aquatic biomes all
the way from the tropical rainforest which has high productivity. We are measuring that
as the amount of material per meter squared per year all the way down to something like
a desert. It is not very productive at all. What is interesting is cultivated land actually
does not produce that much. We could compare that to aquatic systems like coral reefs which
are incredibly productive. We could even look at how it changes over time. So this is net
primary productivity. So this is terrestrial, on land. And watch what happens as it changes
over an eleven year period of time. You could see that is just moving back and forth. It
is moving from the southern hemisphere to the northern hemisphere. You can see there
is no production in areas where we have massive deserts. But what is causing that change?
It is simply going to be the seasons. During the summer we are going to have way more production
where there is way more sunlight. And so an accurate model of measuring how energy is
used is an ecological pyramid like this. And so what we are really looking at here is the
net primary productivity, the amount at this level. And so if we start here with the producers,
let’s say that that small percent is actually one hundred percent, what percent goes to
the next level? Well we are losing energy at each level. And that is because the organism
has to survive. It does respiration so we are losing heat at each level. And so in general,
of the one hundred percent that the producers get only ten percent goes to the next level.
What percent of this goes to the next level? Ten percent of that. So now we are down to
one percent. And what about the next level? Ten percent of that. And so we are losing
a huge amount of energy at each step along the way. And that is going to be why we have
way less tertiary consumers then we are going to have producers in an area. A good way to
study this is using an energy pyramid. And let’s look at an actual energy pyramid from
Silver Springs Florida. What we are looking here is the amount of energy. They are measuring
it in kilocals per meter squared per year. And so it is 20,000 kilocals. Now where is
the energy found? Just looking at this picture it is going to be for the most part in these
trees. What amount makes it to the next level to the consumers? Well when they studied it,
it was this amount. So what is the ecological efficiency? What amount made it to the next
level? Well you could just take this and divide it by that. And we could find that sixteen
percent moved to the next level. We could look at the secondary consumers. You can see
it is around ten percent there. And we could look at the tertiary consumers and you can
see that it is around five percent. And so ecological efficiency is going to be somewhere
between five and twenty percent depending on how efficient that ecosystem is. Now what
are we really missing on this energy diagram are the decomposers. They are going to make
use of a lot of the energy as well. Another way to measure it is biomass. Just how much
material is made. What is different here that we do not measure it over a given period of
time. We measure what is called standing crop. It is the amount that is there at one point
in time. But you can see the same thing occurs if we are looking at a Wisconsin lake or a
field or a coral reef the amount of biomass that we have at the producer level is going
to be way more than what we have at the levels above it. And so did you learn the following.
Can you fill in all the blanks? Pause the video. If not I would say the energy comes
from chemicals through chemosynthesis to producers. We could measure productivity as gross or
net primary productivity. How do we utilize that energy? This would be respiration all
the way down to heat. We then have the trophic levels. Those are going to be the feeding
levels. Ecological pyramids measure efficiency. And we can also measure the biomass. And I
hope that was helpful.

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