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.
<<Students confer with one another>>
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.