Prof: Today we're going
to talk about inter-specific
competition.
 
And you will remember that last
time I was talking about
intra-specific competition,
and we were looking a bit at
the impact of density on
population growth.
And I want you to remember that
if an individual is encountering
increased population density,
during its life,
as it grows from a zygote up
through an adult and reproduces,
the effect of increasing
density will be to decrease its
growth rate.
 
It will be smaller when it
matures.
It will have fewer babies.
 
Often the babies will be of
lower quality,
and the mortality rates in the
population will be higher.
So that's the general impact of
intra-specific competition.
However, animals don't live in
a world, and plants don't live
in a world, where they only
encounter other members of their
own species.
 
They're often living in a
complex ecological community.
And so what we want to do is
understand what happens to them
in a complex community.
 
And I'm going to do this by
showing you some classical
descriptive patterns;
then move through to some
experiments to demonstrate
competition;
give you an abstract conceptual
framework in which to think
about this;
and then return to the complex
reality of competition,
at the end.
Okay?
 
So, a reminder that it's often
useful, when you're thinking
about interactions between
species, to just use simple
logic.
 
You'll remember in an earlier
lecture I said,
"Don't be afraid of using
simple logic in constructing
your papers."
 
This is the kind of thing that
could lay down a nice simple
introductory logical framework
for something that you're
dealing with.
 
This particular one is about
biological interactions in
communities.
 
So if species' 1 effect on
species 2 is in this column,
and species' 2 effect on
species 1 is in this column,
and species 1 has a negative
effect on species 2,
and species 2 has a negative
effect on species 1,
then we've got competition.
 
If species 1 has a positive
effect on species 2,
and species 2 has a negative
effect on species 1,
then species 1 is food,
and species 2 is eating it
>
 
, and you've got predation,
parasitism or grazing.
If species 1 has a negative
effect on species 2,
and species 2 has no effect on
species 1,
we call that amensalism,
and you can contrast that with
commensalism,
where the relationship is --0.
So here, for example,
species 1 might be
benefiting--its effect on
species 2 is positive.
So species 2 is benefiting from
its presence,
but there's no effect the other
way around.
So basically what's going on
here is species 2 might be
living in species 1,
but not impacting it otherwise.
It might be very small or it
might not--it might just fit in,
in a very comfortable way.
 
So this kind of plus/minus
framework is a useful overall
way to see what we're
concentrating on today,
which is competition.
 
And here are some of the
natural history patterns that
suggested to ecologists that
inter-specific competition is
important in Nature.
 
This is one of Jared Diamond's
observations,
from New Guinea.
 
And this is a thrush.
 
There are a couple of species
of thrush there.
There are two species in the
same genus.
Usually when two species are in
the same genus,
they're ecologically pretty
similar, they tend to eat pretty
similar things.
 
And the interesting thing about
this pattern is that you're
going up a mountainside,
and this species is getting
more and more and more abundant,
and then suddenly it disappears.
And if you're coming down from
the top of the mountain,
this species is getting more
and more abundant,
and then suddenly it
disappears, right here at about
5500 feet elevation;
just about one mile high.
So it looks like the two
species are butting into each
other and they're excluding each
other right at the point where
they're each doing really great.
 
So that's a suggestive pattern.
 
It doesn't demonstrate that the
reason for the border here is
inter-specific competition.
 
Anybody have an idea what else
might do that?
They each carry a disease that
knocks out the other one.
Okay?
 
That'll do it.
 
It could be that this one
actually can resist a predator
that eats its eggs,
or something like that,
and this one can't resist that
predator,
and vice-versa.
 
So it could be apparent
competition.
Okay?
 
Without an experiment,
you don't actually know what is
making that sharp line.
 
But it's suggestive,
it looks like it could be
competition.
 
Then Robert MacArthur did a lot
of work on warblers for his
Ph.D.
 
thesis.
 
And the thing that you want to
concentrate on in this picture
is the different parts of the
spruce tree that the different
warblers use.
 
So the Cape May--this is a
diagram of half a spruce tree,
cut in half -- using the middle
and lower inner part.
The Blackburnian warbler is
using the outer middle third
about.
 
The Bay-breasted warbler is
using about the upper outer
third.
 
The black-throated green
warbler is using some of the
inner part here,
and the Myrtle warbler is down
inside the tree,
about here, and then down close
to the ground.
 
So it looks like the warblers
of North America have taken the
spruce tree and they've divided
it up so that they don't run
into each other too much,
and they're each using a
different part.
 
That's an interesting
observation, because if you
think about it,
what it means is that they're
guaranteeing that they're going
to run into more intra-specific
competition.
 
They're avoiding inter,
but they're running into more
intra-specific competition.
 
So that if you say, "Hey,
this has been caused by
competition,"
you're making an implicit
assumption about how strong that
those relationships are.
This one is saying
inter-specific has probably been
more important than
intra-specific competition.
Okay?
 
So again a very suggestive
pattern, but not confirmed by
experiment.
 
Now, those sorts of patterns
led Evelyn Hutchinson to suggest
this kind of view of why we find
so many species on the planet.
He says basically over a long
period of time evolution will
fill the world up with species,
and they will get better and
better at exploiting the
resources they encounter,
and competition will limit the
number of species you can pack
onto the planet.
 
At about the time that
Hutchinson was thinking about
this,
various people,
including Garrett Hardin,
were enunciating what's called
the competitive exclusion
principle;
that two species with very
similar resource needs in
physiology can't co-exist in the
same place, at equilibrium.
So the assumptions behind that
are that we're looking at an
equilibrium that's established
after a long time;
that the competitive exclusion
principle applies.
That means two species cannot
occupy exactly the same niche--
which is what it looks like is
going on with the warblers and
the thrushes in New Guinea--
and that diversity is
determined primarily by
competition rather than by
predation or by disease or
something like that.
So let's see to what extent
this kind of thinking has
occasionally been confirmed in
Nature.
And one of the first field
experiments, showing that
competition was really quite
important, between two species,
was done by Joe Connell.
 
Joe Connell is much revered
Emeritus Professor at the
University of California at
Santa Barbara now.
He did his Ph.D.
 
thesis at the University of
Edinburgh in Scotland,
and he worked on barnacles that
were living in the rocky
intertidal in Scotland.
 
And in particular he contrasted
Balanus balanoides and
Chthamalus stellatus;
and Balanus and is big and
Chthamalus is small.
 
And the pattern that he
found--and he confirmed this by
doing manipulation experiments
in the field--
is that the larvae of Balanus,
which is big,
settle out over a big tidal
range.
This is mean higher water;
this is mean lower spring water;
this is mean lower normal tide
range.
Okay?
 
So once a month the tides get
this high, and at that point the
larvae of Balanus can settle out
over the whole tidal range.
However, Balanus is sensitive
to drying out,
and so as these larvae grow up,
many of them die because
they're getting desiccated,
and so the upper range of
Balanus drops.
 
However, it does just fine over
the whole tidal range below
that.
 
And down here,
the problem that Balanus is
encountering is that primarily
there are other Balanus around
crowding them out.
 
And, by the way,
when two barnacles grow right
up next to each other,
one of them can actually grow
under the other and pry it off;
so it'll fall off.
So you might think that
barnacles look like extremely
boring,
slow moving rocks,
but in fact they have a little
bit of direct competitive
activity for space.
 
Chthamalus is a little guy,
and what Chthamalus does
basically is it gets a refuge up
in the dry part of the upper
intertidal.
 
Its larvae can actually settle
pretty high up,
and it can survive up here,
where Balanus cannot.
So its problem is primarily
getting scrunched by Balanus at
lower tidal ranges.
 
And by doing cage experiments
and removal experiments,
where he actually took one type
or the other off the rock,
and came back--and he actually
mapped them out,
so he followed each
individual--Connell could show
that this was what was going on.
 
Another famous set of early
competition experiments were
done by Gause.
 
Gause was a Russian ecologist,
who became a Russian
epidemiologist,
and he did some early work,
back in the 1920s,
using Paramecia.
And he used three species:
Aurelia, Caudatum and Bursaria.
And what he showed basically is
that if you grow them alone,
this is their density,
that you can measure,
per milliliter;
and if you grow them together,
that Aurelia will exclude
Caudatum completely;
and that if you compete
Caudatum and Bursaria together,
they can actually coexist.
 
So from these early experiments
it did look--
by the way, in this
circumstance they are both
persisting at much lower
population densities than when
they're alone.
 
You can see that from the
y-axis.
This goes up to 75;
that goes up to 200.
So they are depressing each
other's densities.
You can tell that they're
competing because of that
observation.
 
But they manage to coexist,
they don't go extinct.
So it looked here like there
were two possible options.
One wins and the other goes
extinct, or they co-exist.
Now over the last fifty or
sixty years there have been a
lot of competition experiments
done,
and after Connell,
and then later Bob Payne,
who was working more on
predators, did these removal and
caging experiments,
people had a frenzy of
experimentation out in Nature,
where they would remove one
species and then see what
happened to another.
Nelson Hairston did it with
salamanders,
down in North Carolina,
on a huge scale in the
Appalachian Mountains,
and managed to demonstrate that
if you pull one salamander out,
the density of the other one
increases,
and they grow faster and they
have more babies.
 
So you can--by removing one,
you can demonstrate that there
is competition going on.
 
In all those many experiments,
one of the important take-home
points is that the results are
usually highly asymmetric.
That means that the removal of
one of the competing species has
a much bigger impact on the
other;
species 1 having a big impact
on species 2,
and removing species 2 often
doesn't have such a big impact
on species 1.
 
So asymmetric competition
appears to be fairly common.
And it can get so asymmetric
that it may be amensalism rather
than competition.
 
So in some cases you get
removing one has no impact on
the other, whereas you do it the
other way around and there's a
big impact.
 
Okay?
 
So there's a continuum of these
kinds of cases.
And often what's going on is
that competition for one
resource is reducing the ability
of a species to compete for
another.
 
So, for example,
if plants are competing by
shading each other out,
then the plant that's getting
shaded is having a harder time
building roots,
which is giving it difficulty
getting the water.
So you can see there can be a
cascade of effects that
inter-specific competition might
trigger.
And just as a side note,
plants have developed an early
warning system to indicate
whether they're coming into
competition.
 
They can't tell them whether
they're dealing with another
species or not,
but if they're starting to get
shaded by another species,
or by another plant,
then the ratio of near-red to
far-red light,
that's coming into their
chloroplasts,
is getting shifted by being
shaded,
and they actually produce a
hormone,
that gives a signal to the
plant, oh,
I'm getting shaded in that
direction,
and they will grow a branch out
in the other direction;
so they'll grow away from shade
and toward light,
and they actually have an early
warning system that's detecting
competitions.
 
Annie Schmitt,
up at Brown University,
has done interesting work on
that.
Now how to conceptualize all of
this?
Well if we take those
observations we had,
from the last lecture on
density dependence,
this guy Verhulst,
who was a Belgian demographer,
developed a simple modification
of the exponential equation that
we were looking at.
 
And the key idea there is that
as density goes up,
the per capita rate of increase
is going to decline.
So it's going to decline
linearly until it reaches 0 at
K.
 
And if you look at this,
right here--
so if density goes up,
and when N = K,
you have 1 minus 1,
which is 0, and that means that
the rate of change of population
per unit time equals something
times 0.
 
Okay?
 
So it levels out;
and that's what's going on
right here.
 
This is increasing,
but the rate at which it's
increasing is being affected by
the density.
So density is on this axis,
K is right here.
And as you get that N closer
and closer to K,
this part of it's getting
closer and closer to 0.
So the multiplication rate is
dropping and it smoothes right
out.
 
In fact, in this simple model,
the maximum rate of increase is
here, when N is equal to K/2.
 
These are the two guys that
extended that into multi-species
competition: Alfred Lotka and
Vito Volterra.
And Lotka was a demographer at
Johns Hopkins,
and worked oh between about
1915 and about 1935 mostly.
And Vito Volterra was a really
eminent Italian mathematician,
who had a son-in-law who was
engaged in fisheries management
in the Mediterranean,
and the son-in-law would
occasionally have dinner with
the father-in-law,
and would bring to the
father-in-law certain conceptual
problems dealing with the
fisheries,
like, "Well let's suppose
that we have two fish species
that are competing with each
other,
but we can conceive of the
fishing fleet being a predator
acting on them;
how should we predict the
dynamics?"
 
And Volterra,
who really was a pretty
profound mathematician,
found these problems amusing,
and he tended to write down the
answers on napkins at dinner,
and hand them to his son.
 
So they were more or less
throwaway lines for him.
Now the way that they--these
two guys, who came up with the
same way of conceptualizing
these problems--the way they
conceptualized it was this.
 
This is the essence of it right
here.
They've both decided that what
they would try to do is
basically use the single species
framework and just convert the
density of the other species
into an equivalent number of
this species.
 
Okay?
 
And that's what these alpha
competition coefficients are
going to do.
 
And so they wrote down
differential equations that have
a term up here,
that includes the
inter-specific effects.
 
And if you look at the logistic
equation,
which had that nice smooth
approach to a saturation point,
you'll notice that there is a
chunk of these equations that
looks just like the chunk of the
logistic equation,
except they've stuck in this
little term here.
So what they're basically
saying here is this.
I'll read it out to you in
English.
"The rate of change of
species 1 is equal to the
intrinsic rate of increase of
species 1,
times the number of species 1,
which are present,
times a factor that you're
using to account for
density."
 
And the way you account for
density is take the carrying
capacity of species 1,
and you ask,
"How far away from that
carrying capacity are we?"
Well we have to subtract the
number of species 1 which are
there.
 
Okay?
 
So we might be a long away from
carrying capacity,
because we have some of species
1;
and we convert species 2 into a
number that's equivalent to a
number of species 1.
 
And we do the same thing over
here.
So this is very similar to K -
N/K.
And you'll see that basically
when this number here,
N1 alpha 1,2 N2 = K1,
we have K1 - K1,
which is 0/K1.
 
So as this gets larger and
larger, the rate of increase is
going to smoothly go to 0.
 
So they did posit what is
probably the simplest way to
make that one species situation
into a two species situation,
and they did it by assuming
that you could just convert one
species into the other,
in terms of its impact on
inter-specific competition.
 
Now, let me go back there for a
minute.
These differential equations
are not easy to solve;
and in fact it's a system of
differential equations that are
linked to each other.
 
But there are some simple
tricks that you can use to get
some insight into the dynamic
behavior of a system like this,
whether you have a numerical
solution or not.
And so I'm now going to show
you that simple trick,
and we'll practice it a little
bit.
So here we have plots of N1
versus N2.
So what we have done is we've
created a phase space.
And it's important to realize
what I'm talking about now.
I am not talking about an axis
in which we are plotting the
population density of the
species against time.
I'm talking about axes where we
have the population density of
one species plotted against the
population density of the other
species.
 
And time has disappeared,
for the moment,
from our consideration,
when we look at it.
So this is the zero-growth
isocline, and basically what
this line here is expressing,
at what points is species 1
increasing?
 
And the answer is that if there
aren't any species 2 present--
so we're down at 0,
on the y-axis,
no species 2 present--
it reverts to the simple
logistic model.
 
And species 1 will increase up
to the point where it hits its
carrying capacity,
K.
If it goes over that carrying
capacity, it will decrease until
it hits the carrying capacity.
 
That's the assumption of the
model.
The species 2 isocline
intersects the y-axis--
not the x-axis but the
y-axis--at the carrying capacity
of species 2,
and if there are no species 1
present,
then we're just down to this
one axis,
and species 2 will increase
until it hits its carrying
capacity of species 1,
and species 2 will decrease if
it's over that carrying
capacity.
 
What the isocline does is it
gives you the effects of
converting the other species
into equivalent competition
units for this species.
 
So if you're looking here at
the species 1 situation,
and you start adding in some
species 2,
then it will hit its
zero-growth point,
its carrying capacity,
at a lower and lower density of
species 1,
because there's some species 2
there,
that are competing with it.
And this line tells you exactly
where that will happen.
Everywhere along this line
species 1 has a zero growth
rate.
 
Similarly, for species 2.
 
What this line is doing is
telling you what the impact of
competition, coming in from
species 1 is,
on species 2.
 
And as you add in species 1,
by increasing the numbers out
here,
it is reducing the density at
which species 2 reaches its
carrying capacity and its zero
growth rate;
which is why this line is
pointing down here,
this line is pointing up here.
I'm now going to show you the
four possible outcomes,
of putting these things
together, but before I do it,
I'd like you to take a moment
and turn to your colleague,
and explain to them what these
axes mean and why the arrows
point in those directions.
 
And then I'd like to take any
questions about somebody that
doesn't understand this,
because when I show you the
next picture,
if you haven't got that clear
in your mind,
the next picture is just going
to be a little hard to digest.
 
So take a minute and try to
explain this to each other,
and then ask me questions if
you don't get it.
>
Prof: Okay.
 
We have the opportunity for an
immediate quiz on how well you
got it,
because when I show you the
four possible outcomes,
I can ask you to explain why
the vectors move in the
direction that they do.
These are the four possible
outcomes of competition between
two species.
 
The first one is that species 1
wins.
And that species 1 here is
always indicated in red,
and you can always tell which
isocline belongs to which
species by looking at where the
isocline is intersecting,
and how it's labeled.
 
Okay?
 
So we know that this isocline
belongs to species 1,
because the K1 is on the N1
axis.
And what you see here is that
species 1 is able to grow at
higher densities,
across the entire range of
densities of both species,
than is species 2.
In this intermediate area,
between the two lines,
species 2 is above its carrying
capacity and it's dying out.
So basically this is a case
where species 1 excludes species
2, completely;
it wins.
It was that first case we saw
in Gause's Paramecia
experiments.
 
The other case is where species
2 wins.
The black isocline belongs to
species 2.
We can tell that by remembering
to look for which axis has the
K2 on it.
 
Okay?
 
K2 is corresponding to N2 here,
and it is above the isocline
for species 1,
over the entire range of
possible combinations,
of mixtures of both species.
And what basically that means
is that species 2 can grow at
densities where species 1 is
declining, and it excludes
species 1.
 
The more complex,
and therefore more interesting
cases, are down here.
 
This case is an unstable
equilibrium.
Above the isoclines,
both species are declining.
Below the isoclines,
both species are increasing.
If they happen to be at this
point,
it's an unstable point,
because if they just get a
little bit off of it,
the system wanders off in this
direction and ends up going to
be only species 2.
And if it wanders a little way
off, down here,
it goes off in this direction
and ends up being species 1.
And over here you have the
interesting case that you have
co-existence,
and that's where intra-specific
competition is stronger than
inter-specific competition.
And above the isoclines they
both decrease,
and below the isoclines they
both increase,
and that means that the
critical thing to focus on is
what's going on in these
intermediate triangles here.
And if you look at where the
carrying capacities are for the
individual species,
you can see that carrying
capacity for 1,
in this case,
is below the intersection for
2, over here.
And that means that above this
point,
1 is decreasing and 2 is
increasing,
and that means that if you plot
the change in both of them,
you get a vector that points up
and to the left.
Similarly here,
the vectors are pointing down
to the right,
because 2 is decreasing,
but 1 is increasing;
1 is increasing in this
direction, and 2 is decreasing
in this direction,
and that yields a vector that
points like this,
which leads you to a stable
equilibrium point.
And if you look at the
relationship of where the
intercepts are on the axes,
basically you see that this is
a situation in which species 1
is being limited by its own
density,
below the point at which it is
being limited by the density of
species 2,
and species 2 is being limited
by its own density,
below the point at which it
would be limited by the density
of species 1.
 
And that tells us that this is
a situation in which
intra-specific competition is
stronger than inter-specific
competition.
 
Okay?
 
Now if you were faced with that
issue on an exam,
and I plotted N1,
N2, and I just put down two
lines like this,
and I drew K1 here,
and I drew K2 here,
which of the three cases do we
have?
 
Student:  Number two.
 
Prof: Which one?
 
Student:  Number two.
 
Prof: We have this one;
that's right.
And the thing that you
immediately want to focus on is
what is the relationship of the
intercept for itself to the
intercept for the other species?
 
And if the intercept for itself
is above that of the other
species, you have unstable;
and if it's below that of the
other species you have stable.
 
Now do any of you feel
confident that you know why this
looks like that?
 
If you were up here at the
board with me,
how would you convince me that
I should draw the arrow in that
direction?
 
Yes?
 
Student:  You could
break it down as a vector,
and you know that K2 has to be
going down.
Prof: Okay,
so you could just make a sketch
and say, "Oh,
I know K2's going like
that."
 
Student:  And then K1 is
going to the right.
Prof: Right,
because at this point it's
still below its carrying
capacity.
Student:  Right.
 
Prof: So you would go
like that, and that's how you
would get that vector.
 
Very good.
 
Okay.
 
So the take-home points from
this--
and by the way,
I don't think the important
thing about the Lotka-Volterra
equations is that they're an
accurate description of reality.
 
>
 
Reality is a lot messier than
those equations.
What they are is a good
analytical tool that helps you
to realize that hey,
we're probably dealing with
four cases,
and there are some nice
qualitative ways to simplify the
analysis of complex systems of
differential equations so that
you don't have to do all the
numerical calculations.
 
And if you put those equations
into something like Mathematica,
on the computer,
you can draw out the vector
fields,
over the entire phase space,
and you'll get arrows pointing
in all possible directions,
and they'll be pretty and
they'll look nice.
But the essential thing is not
that pretty picture,
but the general qualitative
result,
and to know where things are
generally going in each parts of
the phase space.
 
Okay?
 
So on the one hand it's a
method of simplifying a complex
reality, and on the other hand
it's a neat analytical tool;
and that's why I present it to
you.
It's not necessarily the way
Nature actually works;
that's another issue.
 
That's why we do experiments.
 
Okay, so you don't really have
to be able to solve numerically
a differential equation to
understand its outcomes.
There are four outcomes in this
case: one wins,
two wins, or there's an
unstable or a stable
equilibrium.
 
And even the very simplest
analysis of competition shows
that coexistence is possible
when intra-specific competition
is stronger than inter-specific
competition.
I'll give you an example of a
system where that works.
Go to Panama,
look around on the floor of the
rainforest in Panama.
 
Often you'll find yourself
under a fig tree;
fig trees are very important in
rainforests.
Each time a fig tree drops to
the ground, it is colonized by a
fruit fly species.
 
If a fruit fly arrives first,
at the fig, it will start to
display, and then mating will go
on;
it will attract other members
of its own species to that fig.
However, different individual
figs, just by random chance,
will attract different fruit
fly species;
you're just bumbling along
through the forest,
buzz buzz buzz buzz,
and you hit a fig,
and then you say, "Hey,
I found some places for our
babies to grow up.
 
Come over here and mate with
me."
And so, that's what happens.
 
And because the resources are
discrete and they're scattered
across the floor of the
Panamanian rainforest,
what you have is a
concentration of intra-specific
competition within each fig.
 
The mating behavior attracts
more of your own species to the
place where your larvae are
going to grow up,
and the larvae are competing
with each other.
Okay?
 
So this process generates a
situation in which you get
stable coexistence.
 
And there are about 17 species
of fruit flies that can live on
the same fig tree;
the same fruit from the same
fig tree,
in Panama, and it's very
probably because the process
I've just described is
guaranteeing that intra-specific
competition will be stronger
than inter-specific competition.
 
So you can see that the
analysis of the Lotka-Volterra
model is giving us kind of a key
criterion to look for,
that in fact is reflected in a
very complex natural situation;
and it might not have occurred
to us if we hadn't been through
the simplicity of the
Lotka-Volterra analysis.
So now I'd like to go back to
the field.
And this is a habitat that I
like a lot.
This is the Alpine habitat.
 
This is probably the first week
of July.
You're looking at a flowering
meadow.
It's a complicated environment.
 
You've got grasses,
you've got flowering plants.
There are orchids coming up in
here.
In ten square meters you might
have 50 or 100 species.
Below the surface there are
mycorrhizae down in the soil.
There are earthworms that are
splitting up the subsurface
habitat into three or four
different niches;
all kinds of stuff is going on
here.
It's complicated.
 
There's a very interesting
result from plant competition
experiments that have been done
by taking seeds off of
individual plants in a meadow
like that.
If you go in and you determine
which are the next door
neighbors of a particular plant,
in that meadow--so you take one
individual plant and you ask,
"Is it next to species 1,
species 2, species 3?"
 
Normally it's not going to have
more than three or four
neighbors.
 
And you take its seeds back,
and you test it against a
random selection of species,
in the greenhouse.
So you take its seeds,
and you ask,
"Are these seeds doing the
best against the neighbors I
found out there in Nature,
or just a random selection that
I took out of the same
meadow?"
Okay?
 
And what you find is that they
do better in competition with
the neighbors than with the
average of the larger collection
of all species in the meadow.
 
And you ask yourself,
"Well how could that
happen?"
 
That is something that's really
a rather remarkable pattern,
because it's almost as though
they had been selected to do
better against those neighbors.
 
But what process could do that?
 
Well the answer is that there's
a seed bank,
and that there are seeds of
many individuals,
from many different species,
that are in the soil,
all over that meadow,
and in the spring when they
sprout out,
there's selection,
and that selection is mediated
by competition,
and it's acting on seedlings in
juvenile stages,
and both intra- and
inter-specific competition are
interacting to determine who
survives to be adults.
And the ones that we see as
adults are the ones that have
made it through that competition
screen,
and if there were twenty or
thirty or fifty possible
seedlings that were staring up,
to yield just one plant,
surviving,
you can see how that pattern
would be generated.
 
And so in every generation,
out in a meadow,
inter-specific competition is
mediating the distribution of
the plants that we see,
and the fact that selection is
effective is an indicator that
there's a lot of genetic
variation within each of those
species for competitive ability.
This selection couldn't work
unless there were genetic
variation among individuals in
each species.
So that each species consists
of a whole variety of possible
competitive mechanisms.
 
Some of them do well against
one neighbor,
some of them do well against
another;
as a result of which you get
the maintenance of quite a few
different species in the meadow
and quite a few different
genotypes,
within a species, both.
Okay, so to sum up
inter-specific competition.
It really does occur.
 
It does help to shape
communities in Nature;
it's not the only thing shaping
communities in Nature,
but it's one of the things that
shapes them.
And it is often asymmetric.
 
So often you have a situation
where the bully wins.
It's frequently been
demonstrated,
both in field and in lab,
and the best way to demonstrate
it in the field is to remove one
species and see what happens to
the other.
 
There is an analytical
framework called the
Lotka-Volterra model,
that helps us to understand and
pull together the results of
these experiments,
and it points us to a key
conclusion.
And the key conclusion is that
you'll get coexistence when
intra-specific competition is
stronger than inter-specific
competition.
 
This kind of model,
and others, predict that both
competitive exclusion and
competitive coexistence are
possible, depending on the
circumstances.
And so it was probably a
mistake to enunciate the
competitive exclusion principle
as something that must apply at
all times in all places in
ecology,
because under the particular
circumstance of strong
intra-specific competition,
coexistence is possible;
exclusion is not the only
logical outcome.
Okay, next time predation,
disease;
and I'm going to be very
interested to see what you do
with it.
 
