So my name is Penny Chisholm and I am a professor in the Department of
Civil and Environmental Engineering with a joint appointment in biology
and I'm a microbial ecologist. I'm an ecologist by training and I
work on microbes in the oceans, photosynthetic plants in the oceans.
They're called phytoplankton. That's the research
that my lab does. Whenever I'm using examples in here,
a lot of times I'll be using examples of microbes and of plants
in the oceans. I know most people know a lot more
about trees than they know about things that photosynthesize in the
oceans. And I'll convince you that you should be as bonded to
phytoplankton as you are to trees in terms of recognizing your dependency
on them. So that's just a little bit about my background.
Now, let me find out a little bit about you. Is there anyone in here
who has taken the ecology class 7. 3 or 1.018 that I used to teach but
haven't taught for the last two years? No. Well,
that's good because there's some redundancy. And are there any
Terrascope students or alums in here? OK. Just curious.
So let me tell you just briefly about my hopes for our interactions
in this class, and I stress the term interaction.
I don't want to stand up here talking at you.
I really hope that you will ask questions and interact with me.
It makes it much more interesting for us up here if people are asking
questions and challenging. I expect you to challenge what I
say. The field of ecology is very broad.
It uses knowledge from physics, from chemistry,
from geology, all these diverse fields, and biology of course.
And so some of the things I'm going to talk about,
I know there are students here who know more about it than I do because
I think you all are majoring in very diverse majors at MIT.
So I expect that if I'm saying something wrong that you challenge
me because we're all in this together. The goal is to learn,
not necessarily to be right all the time. OK. So what I'm going to do
today is really to give you a broad overview of the field of ecology.
And the things that I'll talk about are things that we're going to go
into in depth in the next series of whatever lectures I have,
ten or eleven. So most of this you don't need to
take any detailed notes on or whatever. This is just to forecast
what's to come. At the end I'll tell you what the
important points are. So what is ecology? It's a very
broad field and it's basically the study of the interaction between
organisms and their environment. Or the study of what regulated the mediates the transformation of energy and mass on earth. So in this class I'm going to be talking about all levels of
distribution and abundance of organisms. And what we're going to do is talk about the subdivisions of ecology and look at the
characteristics of ecological systems, define some general characteristics and then we'll begin to talk about ways that life
organization of living systems from the biochemical level,
the gene level to the entire biosphere, no small challenge.
So the discipline of ecology, I'm going to move this down a little bit,
is only about a hundred years old. It's a relatively new discipline
but it's a strict natural science. Ecology is not environmentalism.
Ecology is not recycling. Ecology is a branch of the
biological sciences. And it uses the Scientific Method
which I know you're all familiar with.
But let's just review how ecologists
learn about how systems work. And it starts with observation.
Sometimes called descriptive. You go out and you describe what
you see in nature. So description. And from that you
form a hypothesis about the things that are structuring that system.
And then you actually do experiments. And this is something
that a lot of people don't realize. We can do experiments with
ecological systems, be they microsystems,
or some of the things we'll talk about in this class are experiments
with whole ecosystems. For example, they'll clear-cut an
entire forest watershed and compare then the behavior of the elements
cycling in that watershed with a control watershed where you don't
clear-cut the trees. So it's a basic experiment,
just like you would do in test tubes. And there are numerous other
examples about manipulating whole ecosystems, fertilizing
whole lakes. Obviously, it's more difficult
because you have to get permission to do these kinds of things and have
hundreds of replicates of these experiments, but it is a way of
understanding the systems. And then from the experimental
results you test the hypothesis and then you develop models of how the
system works. And then you revise your description based
on the models. And then you keep going through that
cycle. Ecology. At many universities you can get a
PhD in ecology. They have whole departments of
ecology. We at MIT don't. We have very few people even who
are ecologists, but it is a field and you can go
onto get a PhD. OK. So what are the subdisciplines of
ecology? And it has to do with the hierarchical organization of living
systems. Let me go onto my first slide here. And I have
a pointer. Oh, well. I have to stand back.
So when we think of living systems you can start with the atom.
And together they come together and make up molecules,
come together and make up the cell. And then the cells come together
and make up tissues and organs, and then eventually you have an
organism. Although, you can have unicellular organisms.
Here's an organism. And then a group of organisms
belonging to the same species is called a population in ecology.
And there's a whole field called population ecology.
We'll have a few lectures on that where you study what regulates the
growth and the life cycles of a particular species of organism.
And then a collection of populations is called a community.
And there's a whole field called community ecology.
And then a collection of communities.
This is a coral reef. Can you guys see these slides in
the back or is there too much light? Is it OK? OK. Good. So this
would be a coral reef ecosystem. And then all of the ecosystems
collectively on earth constitute the biosphere. So ecologists study
these systems at different levels of organization depending on what
questions they're interested in. But of course this is a gross
oversimplification of what we know about living systems.
For example, an organism. I mean is an organism really an
organism? Think of yourself, for example. Are you only one
species?
You're a human, yes. But there are parts of you
that aren't human, right? What's in your gut?
E. coli. There are more bacteria in your gut than there are human
cells in your body. And without those bacteria you'd be
in [trouble?]. Also your skin is an ecosystem.
It's teaming with mites and little creatures that if you looked under
the microscope you'd be appalled. All of your pours have tiny little
ecosystems in them. And most of these are doing their
job. You're their habitat. And they're helping you be a living
being. So you yourself are an ecosystem. So it's really difficult
to talk about these in a very clean way. I mean you cannot really study
an organism all by itself because each organism is in itself an
ecosystem or a community. Maybe not an ecosystem but a
community of organisms. But this is the way we have come to
divide up the living world at different levels of organization.
And we will talk about all of these in this class.
So ecology really has two broad branches. And that's how my series
of lectures are divided up. One is called biogeochemistry.
And in this branch is the study of
how organisms mediate the transformation of energy and matter
in the biosphere. And this is essentially talking
about the metabolism of ecosystems. In some ways we will talk about,
you've been learning all about the biochemistry of cells and how cells
work and how they process energy, etc. The sum of all of that
biochemistry in cells results in these biogeochemical cycles,
the interactions between these organisms and their environment.
When we talk about the metabolism of
the biosphere you almost can think of yourself as being inside of a
cell looking at the biochemistry of the whole cell from the inside.
It's all a matter of scaling. And we'll see that.
You've learned about photosynthesis and respiration, right?
Have they? Yes. Well, you'll see that those
processes that you've learned about that are subcellular collectively
also have a metabolism for the biosphere. OK.
The second, and this will be my second set of lectures,
is population and community ecology.
And here we talk about organisms. Not just the biochemistry, their
collective biochemistry, but this is the study of the
processes that regulate the distribution and abundance
of organisms. What determines the rates of
population growth of a particular population? What determines the
distribution of different species over the landscape in a particular
habitat? This part of ecology we often talk about as being the
function of ecosystems, and this the structure.
And the really important thing to remember is that these two are very
much obviously dependent on one another. If you change the
structure of an ecosystem, if you cut down all the trees you
will change the function of the ecosystem, there will be no
photosynthesis. OK? So this is a really important
point that these two are related. And we'll talk about that a lot in
more detail when we go forward here.
If we can turn the lights off I'll show you one of my favorite,
favorite, favorite slides, if it works. Ah, yes.
This is to give you a feeling for the earth as a living organism,
in a sense. This is the biosphere. This is obviously a NASA image
showing the green is the plants on earth and this is the dessert.
The green in the ocean is the phytoplankton.
And so where there's lots of green there's more phytoplankton.
Where there's red there's even more. And now we're zooming into the
Equatorial Pacific. Here's a big bloom of phytoplankton.
And this is about three seasons in the life of the earth. OK?
But it shows you how dynamic the surface film of the earth is that we
consider the biosphere. And all of this life and all of its
cycling and dynamics influences the composition of our atmosphere and of
our oceans, all of the nonliving components of the earth.
And really at the biosphere level what ecology is,
is understanding the interaction between the living processes and the
nonliving processes in the earth and how they coevolved to shape
this living planet. If you were somebody out on mars and
you saw that, if you could see that from mars, but you probably could if
you were living on mars because you'd be a different species and you
could do all kinds of things, but you would really get a feeling
for this planet is alive, just seeing those dynamics.
And it is. And we don't know if we're the only ones but it's
certainly worth understanding how the whole thing works.
So that's the goal which is rather intimidating but very exciting.
OK. And very important concept in ecology is a concept of
emergent properties.
And the idea is that at each level of organization that we talked about
here, well, if I cannot go back, I cannot go back. Oh, there we go.
At each level of organization of the system that we talk about there
has properties that only exist at that level or organization.
So if you want to understand population ecology or community
ecology you cannot study cellular ecology and just scale up.
Now, this runs up against a lot of MIT reductionist philosophy,
right? I mean what we're all about here is taking things apart down to
their component parts and understanding them and then building
our knowledge from there. But this whole field of complexity
theory that has emerged with a force, say in the last ten years,
is showing us that that's too oversimplified and that systems,
at different levels of organizations, have their own properties.
So if you want to understand how the earth works you've got to understand
the earth. You cannot just understand how all the atoms are
working. They're coming together to make the organisms.
And this is fundamentally because of feedbacks in the system.
So let me give you an example. OK.
The brain, and this isn't ecology but it's an example you can relate
to, has ten to the twelfth cells with ten to the fourteenth
connections. And the emergent property is your behavior,
right? Those cells are connected in a certain way which results in the
way that you think and feel, your emotions, your behavior, etc.
And in brain research we now have learned that the way these cells are
connected is influenced by the learning environment for young
children. How the brain is stimulated influences how these are
connected. So there's a feedback between the environment experienced
by a child and how these connections are made.
Similarly in ecology systems and throughout evolution there's a
feedback. You have a bunch of cells doing a certain kind of metabolism
which changes the atmosphere of the planet. And then that atmosphere
selects for certain types of cells that weren't there before.
So there's a constant feedback between the emergent property
feeding back on the system level below and changing it.
And that's really the characteristics of a complex system.
And we'll talk about, in a minute, some major examples of that.
One that I always like to think about and throw out is that if you
knew everything, did any of you had an ant colony
when you were a kid? Ah, you're deprived.
Oh, yes. OK. You get the sand and the two pieces of glass and you put
some ants in and they make all these tunnels and everything.
So if you knew everything about ants and ant behavior and sand and
the mechanics of sand could you ever predict the pattern of those tunnels
that they're going to build? Actually, most of the classes I
talk to say yeah. If I knew all that I could predict
it. But think about it. OK. Because I don't think you
could. And this whole field of complexity theory is really
exploding now. In fact, Northwestern University,
I just noticed on the Web, has just formed an entire school of
complexity that has people from all different fields under one roof,
social sciences all the way to physics under one room studying
this phenomenon. And ecology is complexity at its
best. OK. So one other thing, before we move on, is that I said
ecology is not environmentalism, but it's a fundamental science. But
the knowledge that is gained from the study of ecological systems and
the principles of ecology, that knowledge is used in a field
that is growing now called applied ecology.
And that's where you use ecological knowledge to influence human
activities on earth. And, for example, you'd use
ecological knowledge to understand how much of the earth's forests can
we cut down before the system won't sustain us, that you won't have
enough photosynthesis to maintain the oxygen levels in the atmosphere
or draw down enough CO2 so that we don't all boil up.
We'll talk about that. If you want to protect a particular
species, how large of an area do you have to set aside as a reserve?
In order to know that you have to do fundamental studies of the
ecology of that species. Basically, how far can we,
as humans, push these systems from their ìnatural stateî until they
won't function for us anymore? Because we rely on natural systems
for food, fiber and clean water and the composition of our air.
But another point here, I just said how far from the natural state can
we push them, another big important point, as we talk about ecological
systems, is there is no original state of nature. There
is no baseline. Nature is ever-changing.
Nature is evolving. And we'll talk about how much the earth has evolved
since the formation of the earth. So it's important to keep in mind
that what humans are doing now, as we try to have conservation
programs and worry about global warming and the extinction of
species, is that we're trying to keep the earth the way we think we
want it right now. I mean the way we think it was
before the Industrial Revolution, before the massive flux of energy
into the system has allowed us to completely change the biosphere.
And that's a really important thing because the earth ìhas a mindî of
its own. And we know that, it doesn't have a mind, it has a
behavior of its own. And we know that there were massive
climate changes in the past and there will be massive climate
changes in the future. And asteroid might hit us.
And who knows all of these things? It's going to change. So our
challenge is understanding the degree to which we want to try to
control that natural change and try to not impose rapid change in the
rates of things that are already going on. So it's a very tricky
problem, and you guys are the ones that are going to inherit
it big time. You're going to be in charge of this
biosphere and you're going to have to figure out how to manage it,
because, like it or not, we are influencing it to such a degree that
we have to manage it. OK. So another important concept
is that, and I've already alluded to this, the organism environment
interaction is two-way.
I mean we're all used to thinking about the ìsurvival of the fittestî,
right? That you have this environment out there and you have
organisms, and the organism that's more fit, has most adapted to that
environment will survive and reproduce and that's how
evolution works. Well, it's more than that.
The important thing is that not only do organisms adapt to the
environment but they actually change the environment.
They co-evolve with the environment. And so life in the non-living
components of the earth are interacting very intimately and life
has fundamentally shaped the nature of the non-living part
of our planet. So an example of this that I draw
from your textbook, just a very simple example,
has to do with the influence of microbes on the succession of plants
in a northern ecosystem. Succession is --
If you start with blank soil and you just let it sit,
how will communities of organisms come in and colonize?
You'll start out with very small plants and then larger plants and
then bushes and then trees over time. So sometimes ecologists study this
by clear-cutting an area and then following it over time.
That takes years and years and years, much longer than the average
scientist's lifespan. So another way to do this is
substitute space for time. And that's what they've done here.
This is a glacier that is receding in this direction.
And so where it initially retreated soil was exposed and succession
started. And then it retreats more. So this area is growing while this
one is just getting exposed. So eventually, along this gradient,
you have the youngest community up here and the oldest community
down here. And so they were able to look at the
effect of that succession of different species on the composition
of the soil. And there's an important successional stage here.
So here's the nitrogen content in the soil as a function of simulated
time here and showing that it increases steadily as succession
goes on up to a peak and then levels off.
And that increase is largely due to the invasion of plants that can fix
nitrogen. Alder trees are a specific species of trees that have
in their roots symbiotic nitrogen-fixing bacteria.
Have you talked about this at all, nitrogen-fixation? This is what
Professor Walker actually works on his research in his laboratory.
He's a world expert on the mechanism of this.
But these nitrogen-fixing microbes are the only forms of life really
that can take nitrogen gas from the atmosphere, N2,
and using this nitrogenase enzyme convert it to pneumonia which is
what they need to make proteins, etc. and only through these microbes can we get that into the soil so that
So there's this huge reservoir of nitrogen gas in the atmosphere,
other organisms can use it. So here's a process at the
molecular level where the biochemistry that you're learning in
the rest of this class scaled up actually completely influences the
species that inhabit this ecosystem by increasing the nitrogen
in the soil. And just to show you how important
these microbes are globally, this is the global nitrogen cycle.
And don't worry about the details because we're going to talk about
this in future lectures in big detail. So this is just to make the
simple point that here's this biological nitrogen fixation on a
global scale. We have to start talking about big units here,
OK? This is gigatons of nitrogen. So I've converted it.
A gigaton of nitrogen is equal in weight to a billion Volkswagens.
So you have to think on an annual scale 140 billion Volkswagens worth
of nitrogen is dragged out of the atmosphere and put into living
organisms by the biochemical processes that you're learning about
at the subcellular level. So massive quantities of stuff is
flying around here that you cannot appreciate until you see it.
Now, if there was no life on earth, that nitrogen cycle wouldn't be a
cycle. None of these processes would work without the microbes.
And we're going to talk about them in much more detail later.
OK. The other really important cycle on earth is the carbon cycle.
And so I've put this up here to very simply --
Now, you've talked about these reactions in the biochemical sense
in great detail. This is what I call my
impressionistic biochemistry, all right? The net reaction of
photosynthesis is to take CO2 gas from the atmosphere using water,
using solar energy and convert it to organic carbon.
Think of this as glucose. Multiply that by six and you've got
glucose organic carbon and oxygen is evolved.
So this is what we call the ìmass from gas reactionî on a global scale.
This is the foundation of all life on earth. Without it there would be
no life. Life was created from this gigatons of CO2 in the atmosphere
and the solar energy. And then the reverse,
of course, is all of the animals and bacteria, all of the things that
cannot photosynthesis use this organic carbon and use oxygen and
burn it in respiration, that you've learned about,
to make chemical energy [as heat? and cycling the gas back.
Now, in order for this to run, I love that part. That took me
about ten minutes to figure out. This is that all the other elements
on earth need to cycle through. And it's microbes that do this
cycling. And we're going to talk about that
in the next lecture. Because the system would run down
if the elements on earth weren't cycled by microbes through various
redox states. So this is just a more detailed scaled down version of
what you've already learned in class, photosynthesis,
the Z scheme, making glucose from the Calvin Cycle.
And then that glucose goes into the mitochondrion.
And you have respiration and CO2 evolved. So this is the micro scale
version of what I just showed you and this is the macro scale version
of what I just showed you. This is the global carbon cycle.
And you have a hundred gigatons of carbon coming into the system
through photosynthesis and going out through respiration.
So that's the collective photosynthesis and respiration
metabolism of the earth. What I'm trying to have you get a
feeling for here is that this whole phenomenon of scaling,
that these processes occur at multiple scales from the subcellular
to the biosphere level. And this is the flat earth version
of the globe. In the northern hemisphere you can see winter.
There's winter, no green. There's summer,
everything green. Winter, no green. Summer, everything green. And now
wait until you see this. This is cool. Because that pulsing
you can actually see. This is the CO2 concentration in
the atmosphere over a period of three years or so showing that when
respiration is greater than photosynthesis in the northern
hemisphere in the winter, CO2 in the atmosphere goes up.
When photosynthesis greater respiration, CO2 goes down.
So you can see the signature of the earth breathing in the CO2
concentration in the atmosphere. Also you can see that this is
increasing. And does anybody know why that is?
Hello. Did somebody say something? Global warming, yeah. Greenhouse
effect, yeah. But what's causing that? There you go,
fossil fuel. Burning fossil fuel is causing an increasing trend in the
CO2 concentration which is causing the greenhouse effect which is
causing global warming. And we'll talk about that.
This is an emergent property of the system, the CO2 concentration.
Now, in the last slide I showed you these arrows were the same width.
But life on earth was not always that way. Respiration didn't always
balance photosynthesis. And this gets to my point about
there's no original state of nature. In the early earth photosynthesis
way outpaced the consumption of organic matter. So what
would that result in? If this arrow is much more rapid
than this arrow? Oxygen in the atmosphere increased,
exactly. Now, if oxygen in the atmosphere increases,
what happens? What else happens in order for that to happen?
This increased, too, right? And, in fact, that is the
deposition of fossil fuel. This is billions of years.
Now we're going back to the formation of the earth 4.
billion years ago, origin of life. Then we have the beginning of
oxygenic photosynthesis here. And it's the phytoplankton in the
oceans that started this. And so oxygen started to accumulate
in the oceans but not in the atmosphere at first.
So you have all this photosynthesis in the oceans which were a very
reduced environment. So the minute the oxygen was evolved
in photosynthesis it reacted with iron, all these reduced compounds,
particularly iron, and made iron oxides. So it never made it to the
atmosphere at first. So you had the deposition of these
banded iron formations in the ancient marine environment.
Then eventually all of that got oxidized and it escaped into the
atmosphere and you started to have a buildup of oxygen in
the atmosphere. And at the same time that was
building up there was burial of this organic carbon because it wasn't be
respired by the heterotrophic organisms. And that's the fossil
fuel that was built up over all of these billions of years.
Well, not billions. Yeah, billions of years. That we have now
burned over a period of a hundred years releasing all that CO2
into the atmosphere. So we're burning ancient
photosynthate over a tiny little period in the earth's history and
throwing it into the atmosphere. And the earth is saying I don't
know how to handle this. So there's a big question of what
the earth's system is going to do with all of that CO2 that we're
putting up there. And this is all going to happen in
like the next 50 years. And, as I always say in this class,
I'll be dead but you won't when it really hits the fan unless we do
something. So here are the banded iron formations.
We can see the legacy of that history of the earth in today's rock
formations, banded iron formations of marine origin and terrestrial
origin here. And we can see in the earth's composition of the earth's
atmosphere the signature of the evolution of life.
The composition of the earth's atmosphere is highly improbable.
Thermodynamically improbable if you didn't have the influence of
converting this solar energy into living biomass.
And so the CO2 concentration is much lower than these planets.
And the nitrogen concentration higher and the temperature a nice
balmy 16 degrees. OK. So this is just to remind you
that these processes operate at all scales. Biosphere.
This is a little ecosphere I'm going to bring next time showing you
a sealed ecosystem in which all of these properties go on.
And finally. So I've been talking about levels of organization from
the molecular level up to the biosphere level.
And there's an exciting new thing happening in my field that is so
exciting I have to tell you about it. And I'll probably be telling you
about it more and more as I go along here. And that is a whole new field
is emerging called molecular ecology where we're viewing the biosphere as
a network of genes. It's not that you either study
cellular molecular biology or biochemistry or you study the
biosphere but you try to think of the biosphere as a network of genes.
And in oceanography we're actually able to start to do this because the
oceans are dominated by microorganisms,
although in this picture I don't have that properly represented,
but we're starting to think about a sea of organisms as being simply a
network of genes. Most of the genetic information in
the oceans is in microorganisms. And I like to think of the oceans
as dissolved information essentially. When you look out and you see that
blue water, there is so much DNA in there and so many genes doing so
many different things it's just phenomenal. And there are a billion
microbes per liter in sea water. 99.9% have never been cultivated.
We know nothing about them. There is as much information in that liter
as in the entire human genome, and most of it is of unknown
function in this biosphere. And so just within the last year,
this is Craig Venter who was one of the people who sequences the human
genome. And now that that's over with, he needs a new challenge.
So he's taking on the ocean genome meaning sequencing all of
the DNA in the oceans. And he's taken his yacht,
because he's rich now, which is his research vessel.
And they're going around the oceans collecting samples,
filtering them onto filters and collecting all the microbes.
And then you take it and you grind it up, you extract the DNA
and you sequence it. And you just get a bunch of little
pieces that you identify as genes. You don't know what organisms they
belong to, but you've got genes. And this is his cruise trek which
you notice doesn't have many northern climbs here,
although they did do something up in Halifax. And it kind of resembles
the Challenger Expedition back in the 1800s which was one of the first
major oceanographic expeditions. So they published a paper in Science
last year. Just off Bermuda they found 1.2 million new genes that had
never been in the database. They had to create a new database
in order to even put these genes in it and 1800 new species.
And he is estimating the genetic inventory of the planet,
most of which are in these microbes, to be 20 to 30 billion genes. And
then we're going to have to figure out what they're all doing.
OK. So just the take-home message. I'm going to try to do this at the
end of each lecture so that you know what I think is important.
So we've talked about life at different scales.
Ecology is the study of life at all these different scales.
Emergent properties is a really important concept to understand.
There's nothing more important than understanding this organism
environment is a two-way street. There's really no such thing as a
free living organism either. They are all dependant on one
another. Even if you have a culture of say E. coli which you've supplied
glucose to so it can grow, where do you think that glucose came
from? It's the photosynthetic product of some plant somewhere.
So organisms are not really free-living. They are all dependent
on one another. Life has shaped the earth's features,
its atmosphere. The biosphere that I've shown you
and the geosphere, which is what we refer to the
nonliving part of the earth, have coevolved over evolutionary
time. And the genetic inventory of this is completely unknown.
And, of course, microbes rule the planet, don't forget that.
See you next time.