Prof: You guys had a
little bit of fun getting
dressed up and doing some stuff
up on stage last Friday,
and so I though that I would
actually have a little fun
myself.
 
And I was unable,
as you know,
to decide which section had
actually done the best job.
So I decided that you would all
get a reward.
And in order to understand the
reward,
I want to give you the
background, which actually comes
from a very,
of course, deep theoretical
investigation of the kind that I
always want you to know about.
Right?
 
So let me get this up there.
 
Here we go.
 
I want you to understand the
Swiss theorem.
Okay?
 
And the Swiss theorem is really
a very essential part of
population biology.
 
Basically it tells you that the
partial derivative of happiness,
with respect to an increase in
chocolate intake,
is positive across the entire
reaction norm of the human
condition.
 
Okay?
 
So chocolate makes you happier,
right?
And the Swiss,
as is usual,
claim priority for something
which is in fact a contribution
of another major civilization,
which is the Mexicans.
Now, you will notice that I
have had the teaching fellows
divvy up the chocolate onto
paper plates and distribute it
in small qualities around the
room.
This is because the
homogenization of fitness within
groups promotes group benefit
and causes a general increase
in-group success.
 
Because the only way then that
the group can improve is that if
the performance of the
individuals increases the
performance of the whole group.
 
Now, that's about all that I
really want to say,
except that I want to thank you
very much for participating in
that exercise last Friday.
 
I thought it was interesting.
 
I enjoyed it.
 
I hope you've had a chance to
look at it.
And the only issue that now
faces me is whether I should
continue to give the lecture in
this costume.
I have been told by my teaching
fellows that I absolutely have
to do it.
 
Right?
 
But I have difficulty believing
that you'll believe me if I look
like this.
 
Will you believe me?
 
Students:  Yes.
 
Prof: You want me to
leave it on?
Students:  Yes.
 
Prof: All right,
all right.
Let's go.
 
Okay?
 
So we're actually now going to
go back to--
oh by the way,
there is a pedagogical message
in this costume,
and that is if you can't play
like a child,
you can't be creative.
Okay?
 
It's about playfulness.
 
Okay.
 
Have you all got your chocolate?
 
Are you happy?
 
Students:  Yes.
 
Prof: Or let me put it
this way, are you happier?
>
 
It looks like we have some
leftovers, and so you're all
welcome to grab some on the way
out.
The basket, by the way,
is a working basket from a
market in the Ivory Coast,
and when people make baskets to
work with, they make them really
well.
So I don't want the basket to
disappear.
Okay?
 
Now for today's lecture.
 
What we're looking at now is
the impact of space on
communities and on the
distribution of plants and
animals around the planet.
 
So I'm going to talk today
about island biogeography and
metapopulations.
 
And I want you to start
thinking of the world as
fragmented, okay?
 
So, as spatially heterogeneous.
 
There are islands,
there are mountains,
there are lakes and oases.
 
And a lot of the world is
becoming fragmented by humans.
So one of the big ways that
humans are having impact on the
planet, and changing life on the
planet, is by fragmenting the
landscape.
 
So the issue today is what is
determining biodiversity in a
fragment?
 
What happens when we break the
world up?
Here are some sketches of what
happened in Cadiz Township,
Wisconsin, between 1831 and
1950.
So before the settlers go out,
we had an Eastern Hardwood
Forest,
and as the area got
settled--this is 1882,
1902,1950--you can see the
forest disappears and there are
little blocks of tress that are
left as forest fragments,
scattered across the landscape.
And this is where the birds and
the rodents and the coyotes and
everything else now--and the
deer--have to try to make their
living.
 
And if we now look at Wisconsin
from space--
so this is in the early part of
this century--
you can see a picture which
actually has cities in it,
in red, and a landscape,
which is largely agricultural,
with some little dark blocks of
forest scattered through it.
So this is mostly agriculture
here;
this would be forest.
 
Let's take another look.
 
Another kind of natural
fragmentation,
of course, is archipelagos,
islands.
Now we're getting towards
island biogeography.
This is an observation made by
Robert MacArthur and Ed Wilson
that plots on the Y-axis,
on a log scale,
the number of species on an
island;
and on the X-axis the area of
the island, in square miles.
And this is for the Sunda
Islands;
so this is in Indonesia.
 
And up here you've got the
Philippines and New Guinea,
added to the Sunda Islands.
 
So what you see is that on a
log-log plot,
the bigger the island the more
the species.
And this is the part of the
world we're talking about.
Komodo dragons come from Komodo
here.
The little dwarf Homo erectus
was found on Flores.
This is Timor.
 
New Guinea is going to be off
here somewhere.
Java is over there,
and Krakatoa is down here.
So these are the Sunda Islands.
 
And they have lots of different
surface areas.
I mean, if you just look at
this little one and compare it
to this big one,
you're going to have a lot
fewer birds on there than you
are on there.
So in trying to come up with a
general theory of biogeography,
MacArthur and Wilson did the
standard Cartesian analytical
reduction of saying,
"What are the essential
features of that system?
 
What are the fewest things that
we have to pay attention to,
that will tell us something,
a take-home message,
some key message,
that we can pull out of the
system?"
 
And they thought,
well let's suppose that there
isn't any evolution going on,
and that all of the species
we're considering already exist
on some big continental
landmass,
and that they're getting onto
islands in a process of
immigration,
and they're dying out on
islands in a process of
extinction.
 
And this is the number of
species that are present on the
island.
 
Okay?
 
So they argued that in such a
situation the number of species
on the island will come to an
equilibrium--
there'll be species that are
coming off of the continents,
that are flying,
drifting, getting blown,
whatever, out onto the
islands--and the immigration
rate will start high.
 
When the island is empty,
everything that shows up on it
is a new species.
 
But the immigration rate must
inevitably fall down to zero
when the number of species on
the island equals the number in
the source pool on the mainland.
 
Okay?
 
Because we're just counting
species;
we're not counting number of
individuals arriving.
I mean, if a hundred birds fly
in and their species already
exists on the island,
that doesn't count for
immigration,
because that species is already
there.
 
So that's the way this axis is
constructed.
So this goes down and this goes
up.
The number of species on the
island is going to be affecting
the extinction rate,
probably in two ways.
The simplest is the more
species there are,
the greater the chance that one
of them will go extinct,
just at random.
 
So that curve's going to go up,
just because there are more
species on the island.
 
However, if there are
interactions among the species
on the island,
such that predation,
disease, whatever is going to
drive one to extinction,
you can see that it might bend
upward.
So it's not just going to be
linear, it's going to go up;
so that's how they argued the
curves.
And they said where the curves
intersect,
the number coming in will equal
the number going out,
and that's the number we should
expect to find on the island.
So far so good;
this is all just a priori.
Well what's going to affect the
rates of immigration and
extinction?
 
Well first they argued that
immigration will decrease from
islands that are near the
mainland out to islands that are
far from the mainland.
 
So you could construct a series
of curves;
so this would be the curve for
an island close to the mainland
and this would be the curve for
an island far from the mainland.
And that's just because it's
harder to get out there.
On the other hand,
they argued that extinction
rates will increase as you go
from large islands down to small
islands,
basically because there's more
space on the big island,
more different niches,
more places,
habitats, where organisms can
live;
more different kinds of things
could survive there.
 
But also as you get onto a
small island,
the intensity of the biotic
interactions are going to get
bigger,
and it's going to be harder to
get away from a predator,
or harder to get away from a
parasite.
 
And so you could imagine that
as you shrank an island,
extinction rates would go up.
 
So they predicted,
hey, we could have an
equilibrium over here on an
island which is small and far
from the coast,
or that we could have it here
if it was a small island close
to the continent.
And similarly if we have a
large island which is far away
from the continent,
we might be down here,
and a large island close to the
continent would be here.
A big one close to a continent,
by the way, would be Trinidad.
Trinidad has almost the
biodiversity of neighboring
Venezuela.
 
A small one,
far away from a continent--
so with a very low
biodiversity--might be something
like,
you would think,
an isolated oceanic island like
Easter Island or Hawaii.
So why is this important?
 
Well for a long time,
from the mid-1960s up to about
1985,
1990, this was the only game in
town,
and there weren't alternative
ways of thinking about these
processes,
and it played a big role in the
design of natural parks and
nature reserves.
 
Essentially it said,
because of the area,
the effect of area on
biodiversity,
it's better to have a big park
than a small one,
and because of the effect of
immigration rate on
biodiversity,
it said it's better to provide
corridors to connect landscape
fragments,
if you possibly can,
so that things can immigrate
and move back and forth.
 
So it was used a lot.
 
However, when we summarize it,
you'll see it's an equilibrium
between colonization and
extinction;
it assumes there's a source
population.
So evolution isn't going on;
there's a source population out
there, that's where all the
species are.
There's no speciation occurring
on islands.
There are just two effects that
you're worried about:
how big the island is and how
far away it is from the
mainland.
 
Extinction is driven by area,
and colonization is driven by
distance.
 
Okay?
 
Now all of these things that
I've just written down here will
be important for you to remember
if you are asked how to
reconstruct the equilibrium
number of species on an island
in a certain circumstance.
 
Okay?
 
I want to emphasize that.
 
That's the kind of thing that
might turn up on a midterm.
If you're on a small island,
a long way from the mainland,
you'll have low species
diversity, and a big island
close to the mainland will have
high species diversity.
And that seems to be an
intuitive point,
but I've given you an
analytical framework from which
to derive it.
 
Now, as you'll see in a minute,
I am now going to blast this
theory out of the water.
 
I'm going to take it apart and
show that it makes a whole
series of assumptions and claims
that are demonstrably not true.
And before I do that,
I want to signal to you that
I'm going to come back,
after I do that,
and say, "Hey,
it was still a good thing."
Okay?
 
So the theory is dealing only
with the number of species,
not with the number of
individuals;
there's no population dynamics.
 
I mean, if you've got ten of
them on the island,
or a thousand of them on the
island, you count them the same
way.
 
That seems to be a little
silly, because the probably of
extinction should be related to
the number that are there.
All the species are considered
together, and there's just kind
of one general immigration rate
and one general extinction rate.
You know, it's kind of fun to
wave my arms when I've got this
gown on.
 
>
 
But the probabilities of
immigration and extinction are
different;
I mean, it's going to be
different for birds and ants and
mosses and paramecia and
elephants and stuff like that.
 
Okay?
 
So they must some ways differ
systematically.
The island biogeographic
theory, kind of like the
Hardy-Weinberg theory,
it's an equilibrium theory.
Okay?
 
It doesn't allow for history.
 
But if we just look at what's
happened in the Western Pacific
in the last 10,000 years,
as the Polynesians came out and
colonized those islands,
we know that 25% of the birds
that were on them have already
gone extinct.
So you send your budding
biogeographer out there to get a
sample from the Sunda Islands or
from Guam or from the
Philippines or Micronesia,
and they come back with a count
of the number of bird species on
the island,
it's a very misleading count
because 25% extinction has
already happened on those
islands.
And that's where those data
came from, that I showed you.
Okay?
 
This effect has not been
compensated for in that dataset.
The theory doesn't allow for
speciation and adaptive
radiation.
 
And you take one pair of
cardueline finches from Central
America and fly them out to
Hawaii,
fifteen or twenty million years
ago,
and voilà,
this is what they produce--
okay?--bunches of species.
 
So that's going on,
and that's not in the theory.
It assumes that the probability
of being able to immigrate
doesn't depend on how many
species are already there.
Okay?
 
But the presence of some
species is probably a
prerequisite for that of others,
and the presence of some may
keep others from coming in.
 
So those are probably effects
to worry about.
It's actually empirically
difficult to decide when an
immigration has occurred.
 
So you're sitting here,
out in the Thimble Islands,
okay, off Branford,
Connecticut,
and it's spring,
and a Bay-breasted Warbler
comes flying through.
 
Do you count it?
 
Well it's going to just stop
off, eat a few insects,
and fly on to Canada.
 
It's just passing through.
 
Is that immigration?
 
Probably not.
 
So you have to actually find
the species that breed on the
island;
that's not so easy.
We assume the system's at
equilibrium.
But how can we recognize an
equilibrium when we've got one?
There's not a clear prediction
on how fast this turnover would
occur.
 
Or do we have to wait ten
generation, a hundred
generations, a thousand
generations?
And hey, what about the problem
that generation time is fast for
little things and slow for big
things?
That makes that issue pretty
complicated.
Then if it is at equilibrium,
the assumption is that every
time a new species comes in,
one that was already there goes
extinct.
 
Well that seems to be a little
unrealistic.
That's a very tight coupling of
immigration and extinction,
and the real relationship's
weaker.
So the major assumption of the
theory,
which is that there's a
turnover of species that
produces an equilibrium between
immigration and extinction is
correct.
 
That's been tested
experimentally on small islands.
But the observed turnover is
often of casual species,
not the ones that breed with
established populations,
and there's nothing in the
theory to tell us what
proportion should be in each
category.
So the theory is a failure;
the theory is a failure if the
goal is to be right.
 
Okay?
 
But hey, the goal is not to be
right;
the goal is to try to explore
Nature in such a way that we
discover the truth.
 
And the theory is a great
success if that's what we want
to know.
 
We can only operate with a
working hypothesis--
for a long time this was the
only game in town--
and the criterion of success is
how much work gets stimulated by
the idea-- okay?;
you'll see shortly that a lot
was stimulated by it--and how
rapidly can it be constructively
falsified and replaced with a
better theory?
The pages of science are
littered with the corpses of
dead theories.
 
And sometimes a theory has the
delightful adaptation that it
contains within itself the seeds
of its own destruction.
And the seeds of destruction of
a theory are often how
fascinated people get by it and
how hard they're willing to work
to try to test it.
 
And that's what happened to
island biogeography;
it was a good one.
 
So we can look at MacArthur as
a Dionysiac, a creative
enthusiast.
 
We can look at Mark Williamson
as an Apollonian objective
critic.
 
And if you like that dichotomy,
then you ought to go back and
read Friedrich Nietzsche's
The Birth of Tragedy,
which he wrote when he was
still a Ph.D.
student and only
twenty-three-years-old.
That was long before,
by the way, he went crazy.
Okay, so that's one view of the
universe;
that's the island biogeography
view of the universe.
Now I want to do
metapopulations,
because this is the other major
alternative way of looking at
spatial dynamics of species and
populations.
So a metapopulation is a set of
local populations that are
linked by movement.
 
And, as with island
biogeography,
the dynamics of metapopulations
are driven by extinction and re-
colonization,
or immigration.
So I'm going to be talking a
bit about some of these
species--
okay?--frogs,
butterflies,
thyme plants,
pathogen populations living in
us, things like that.
So here's the basic conceptual
framework.
Here we have a local population
and it's got reproduction going
on in it.
 
It's producing an excess of
organisms,
and they're moving out into the
environment,
because it's getting crowded
locally and they want to find a
place to live.
 
They go out and they can find
an empty patch to colonize.
And sometimes,
for one reason or another,
their population will go
extinct in a local patch.
So if you just take a big
sample of patches across the
landscape,
each of which is a population,
you will find some of them with
thriving biology going on,
and some of them are empty;
and they can be empty because
they went extinct or they can be
empty because they were never
colonized.
 
And all of them have the
conditions that the organisms
need to survive in.
 
So if you build a simple
metapopulation model,
you can pull out some pretty
important, straightforward
messages.
 
One of them is this.
 
Even if every single local
population is likely to go
extinct, the metapopulation can
survive in a balance between
extinction and colonization.
 
So basically you should think
of it as, oh my heavens,
we're about to go extinct
locally;
I'd better get up and fly out,
and go find a new place,
and just keep hopping.
 
And if a population manages to
do that, it can keep itself
going, even though it leaves
behind a long trail of
consistent extinctions.
 
The landscape is important in
this, and of course that's very
attractive because it gets
people into landscape ecology;
it gets them into photographs
taken from space;
it gets them into geographic
information systems.
And it's actually created--that
whole area is now a new field of
analysis.
 
So the landscape features that
are going to effect extinction
and colonization then are going
to be things that are very
important for regional
persistence.
So, you know,
if we're dealing with say
Daphnia living in ponds in
Connecticut,
and in any particular pond in
Connecticut Daphnia is likely to
go extinct,
but we look across the whole
state and we see that there are
100,000 ponds,
Daphnia actually ends up
probably doing just fine in
Connecticut.
 
And if you doubt that,
I suggest you just take a
beaker of pond water--
not city water,
city water's got a lot of
chlorine in it--
take a beaker of good natural
pond water and put it on top of
your residential college in
downtown New Haven.
And go back six months later,
you will find in it rotifers,
algae and copepods,
and they will have fallen into
it out of the air,
because these guys fly around
in the air;
you might not think so but they
manage to get up there.
 
Another message is that there
is a ratio between colonization
and extinction,
above which a metapopulation
can exist.
 
So if you're concerned with the
question,
is there an analytically
determinable threshold for our
daphnia population in the State
of Connecticut that will tell us
how much they have to move
around in order to stay here in
the long run?
 
The model will give us that.
 
Okay?
 
That's the threshold ratio
between colonization and
extinction.
 
And it's kind of a simple
number.
It tells us those are the rates
we have to worry about.
And you can interpret that in
terms of the proportion of
patches that are occupied and
average patch size.
So there's actually something
you can go out and measure that
will give you an estimate of
this ratio,
which will tell you will the
thing persist or not?
So if you're concerned with
population viability analysis,
if you're concerned with
conservation and with the
threats to biodiversity,
this is something that you can
actually go out to measure,
and then construct an argument
with;
and you can back your argument
up with a literature that has
now some rather impressive logic
in it.
 
So here are a few insights.
 
It's perfectly normal to have
some local empty patches.
You know, in my village in
Switzerland they were very
worried about their carabid
beetles, because that's all they
have left;
and they had some carabid
beetles, they had a few toads
and they had some salamanders,
and there wasn't very much left
in the forest.
And locally people would get
very desperate about their local
pond not having any salamanders
in it anymore or something like
that,
whereas if they would back up
and they would look at say a
chunk of landscape that was 100
kilometers on a side,
they could relax,
because local things are often
going extinct but then being
re-colonized.
 
And you actually need to be
able to back up and look at it,
at a fairly large spatial
scale, and a pretty long
timescale,
before you can discern the
overall trend.
 
This places a big demand on
data collection,
but it leads to a lot more
realism in making forecasts.
So you need to look at the
region and the landscape,
rather than the local
population.
But there is something that's
hard to measure and that's the
migration rate.
 
Okay?
 
It's just hard to see.
 
After all, if I were a
salamander, I would do it on a
rainy night--right?--at about
two o'clock in the morning;
and who's going to be out there
tracking me around?
So this is hard to measure.
 
Is there evidence that in fact
Nature is organized this way?
Well we know that population
size is significantly affected
by migration,
and we can see that in both the
source effect and the sink
effect.
So you put a fence around a
population, and if it's a source
it will increase,
and if it's a sink it will
disappear.
 
And that can only happen if the
source would normally be
exporting migrants,
immigrants that are going out,
and the sink would only happen,
it would only disappear,
when the fence is put around
it,
if it had been previously
maintained by stuff coming in
from sources.
 
So that's experimentally
demonstrable.
We know that population density
is affected by the area and the
isolation of the patch.
 
Big patches tend to have
slightly higher densities,
and distant patches tend to
have lower densities.
If it's really a
metapopulation,
then population density should
be going up and down,
out of synchrony.
 
Okay?
 
If it's really tightly linked,
and there's a tremendous amount
of immigration,
then you could just treat the
thing as just one big
population.
But if it's a metapopulation,
and some things are doing okay
and some things are going
extinct,
then doing okay means going up,
and going extinct means going
down,
at the same time.
So they would be asynchronous;
and that is often observed.
Is there a population turnover?
 
Do local populations go extinct
and then get re-colonized from a
source?
 
And that's been observed,
at least in one case,
for snails living in ponds in
the U.K.
And that's done by taking
samples of the mud in the bottom
of the ponds and going back over
many years and looking for the
presence of snails.
 
And they disappear and they
come back, and they disappear
and they come back.
 
If you're a good naturalist and
you know where your beast likes
to live, then one often sees
that suitable habitat is
present, but it's empty.
 
And both in a plant population
and in a butterfly
metapopulation,
both of these metapopulations
persist, despite lots of local
extinction.
So the butterfly population is
in Finland, and the plant
population is in Provence;
its thyme in Provence.
And both of these--by the way,
if you like metapopulation
biology, you get to do lots of
neat field biology,
and do it in wonderful
circumstances.
And since the French use the
thyme in their cooking,
and the butterflies are
beautiful, this is a nice
sensory experience as well.
 
The risk of extinction in a
metapopulation does depend on
patch size.
 
If you're in a small one,
it's much more likely that
you'll go extinct than if you're
in a large patch.
So the evidence for that is
pretty strong.
And the colonization rate
depends on patch isolation;
and that's true for most
species.
I would like to note here that
if you take the Max
Planck-Gesselschaft A330 up for
a little tour around the globe
at 35,000 feet,
and you put out a--you slow the
plane down enough so you can put
out a plankton net,
and you troll for aerial
plankton up in the stratosphere,
you will find baby spiders and
fern spores covering the planet,
up in the stratosphere.
 
You can do that over the South
Pole,
at 35,000 feet,
and you will find,
in a state,
believe me, of deep
hibernation, little tiny baby
spiders rafting along at -70
degrees Celsius,
and they're still alive.
Fern spores will do the same
thing.
So, in fact,
these are exceptions.
The colonization rate,
depending on patch isolation,
however, would be very
important for elephants,
rhinoceroses,
bears, stuff like that.
Okay?
 
So you can see that there's a
gradation based on dispersal
ability, body size;
lots of biology.
Small isolated patches are
likely to be empty.
Big connected patches are
likely to be full.
Lots of evidence for that;
that's certainly a
straightforward prediction of
the model.
And a fugitive competitor can
exist in a metapopulation.
A fugitive competitor is one
that if you just put them into a
local equilibrium population,
they will get beaten out by the
other species that's there.
 
But if they are better at
dispersing, while the other one
is better at competing,
they can keep jumping out and
getting ahead of that.
 
And that's been well
investigated with Daphnia in the
Finish Archipelago.
 
Furthermore,
I'm going to show you a shot of
a prey species that would go
extinct in a local population,
but it can co-exist with its
predator in a metapopulation;
and I'll show that for two
mites living in greenhouses.
Now, the point that is made
here is that when you shift from
the equilibrium local population
perspective,
up to the metapopulation
perspective,
the complexities of spatial
distribution will allow a lot
more things to co-exist with
each other.
And that's true both for
competition theory and it's true
for predation theory.
 
So here's the Finnish
Archipelago.
It is, by the way,
continuing to emerge from the
water, because there's glacial
rebound.
After the Pleistocene,
glaciers melted.
They had depressed the
underlying continental crust,
and it is now rebounding and
rising up.
So these islands continue to
come out of the water.
And they are a lovely kind of
fairytale kind of landscape,
filled with all sorts of
interesting biology.
They have on them,
for example,
six-foot long water snakes,
that are about that thick,
and they have lots of birds and
other things.
And on them they have little
pools of fresh water,
surrounded by this part of the
Baltic, which is not really
seawater, it's kind of brackish.
 
But the Daphnia can't really
survive in the sea;
the salinity is still too high
for them.
So they move around among
pools, on these islands,
in places like this;
this little island here might
have ten or twenty pools on it.
 
And they are moving around,
among these islands,
probably having their ephippia,
which are their resting stages,
borne on the feet of shorebirds
that are flying.
And there are at least two
species of Daphnia that live in
the Finish Archipelago.
 
One of them is a better
competitor,
one of them is a better
disperser, and they co-exist
because what one lacks in
competition ability,
it makes up in dispersal
ability.
So if you have that kind of a
tradeoff, you can generate a
persistent metapopulation in a
system like that.
I strongly recommend,
by the way, if you're ever in
Stockholm or in Helsinki,
that you go out into the Baltic
Archipelagos;
they are just beautiful
landscapes.
 
The other is mites in a
greenhouse.
And this is Carl Huffaker's
experiment,
and he had a brilliant idea for
a model system in which you
could investigate the impact of
spatial structure on
predator/prey interactions.
 
He had a herbivorous mite that
likes to eat oranges;
and these are oranges, okay?
 
And he had a predatory mite
that eats the herbivorous mite.
And what he did was he
constructed a model ecosystem
that consisted of a whole bunch
of oranges, interspersed with
billiard balls;
well obviously the herbivorous
mite can't eat a billiard ball,
but it can eat an orange.
And then he altered the
migration rates of the species
by putting grease down,
in between the two.
So he had a system that you
could actually put in your
kitchen cabinet,
that had a complete spatial
ecosystem in it,
and he could play with the
parameters.
 
And what Huffaker discovered is
that he could have persistence
of a predator and prey with
spatial structure where if they
were confined to a single orange
they would both go extinct;
first the prey would go extinct
and then the predator would
start.
 
Okay, so back to a comparison
between the two ways of looking
at the world.
 
If you look at the number of
publications per year,
using say Web of Science,
you can see that interest in
island biogeography peaked in
the mid-80s and then has
declined;
it's not gone to zero,
but it's going down.
 
But since 1985,
there's been an explosion of
interest in metapopulations.
 
And there are really two
reasons for that.
One of them is that it is
obvious that the landscape
really is fragmented,
and so metapopulation theory
has become an organizing concept
in conservation biology,
where people try to maintain
biodiversity at a landscape
scale.
 
We can see it all around us.
 
It's much easier to study and
manipulate a metapopulation than
it is to manipulate an
archipelago.
So there's been a lot more
progress doing experiments,
like the ones that I showed you
that Carl Huffaker did.
That was actually before the
theory came up;
he was sort of a prophet way
ahead of his time.
And there's an analogy to
epidemiology,
and it's quite compelling;
and we know that epidemiology
works.
 
So I now want to show you the
analogy with epidemiology.
And this is a connection now
between ecology and infectious
disease.
 
So the host is a local patch.
 
Here's a local patch;
that looks like a good one to
infect.
 
Here's another one.
 
Boy am I going to get her in
the dorm.
You know?
 
Pathogens have local
populations within hosts.
Now what would constitute
extinction?
Well extinction would be either
you kill your host,
so you die with it,
or the host develops an immune
response.
 
Okay?
 
So you get--you can go extinct
for either reason;
and by the way,
if you're a pathogen,
either is equally bad.
 
The disease transmission rate
is equivalent to the migration
rate.
 
And if we look at that--I'm now
going to go back and I'm going
to re-rehearse this issue of
measles on islands and in big
cities.
 
It was mentioned last Friday,
briefly, but it's an important
example, and I feel entirely
unashamed about mentioning an
important example twice;
there might even be a better
chance of it being remembered a
week or twenty years later.
So measles in big cities are a
huge metapopulation with a
continual input of young hosts
that don't have any immune
defense: babies.
 
Okay?
 
Luscious little susceptible
babies, ripe for infection.
On islands--an island is a tiny
little metapopulation.
You know, look at the Falkland
Islands or the Orkneys or
something like that;
Pitcairn Island, Easter Island.
Very few hosts.
 
And if measles could get onto
an island like that,
or any other infectious disease
that causes a sterilizing immune
response,
as measles does,
then it'll sweep in a wave
through that island,
and everybody will become
immune before enough babies can
be born to maintain the disease.
 
So repeated extinction occurs.
 
Here is the incidence of
measles in big cities and on
islands between 1921 and 1940--
so this is before measles
vaccine, when you could study
this as a natural process--
and zero years with a month of
no cases in the big cities.
And as we go from fairly large
islands,
down to smaller islands,
we have more and more months
with no cases,
until you get to the Falkland
Islands,
and over that nineteen year
period there wasn't a single
case of measles in the Falkland
Islands.
 
They must've been pretty
worried about a ship coming in
that had somebody with measles
on it, but there was no case
during that nineteen years.
 
So here's a guy with measles.
 
Here's the pathogen.
 
This is the situation in a big
city, and here is the Falkland
Islands.
 
As you can see,
the density that you have in a
big city just makes for
wonderful transmission
possibilities;
fantastic.
So diseases will tend to go
extinct on little islands,
and host populations will then
lose both their acquired and
their inherited resistance.
 
And if then after many years
the disease is reintroduced,
the epidemic can really be
catastrophic.
So I think you already know
that on Hispaniola,
between--that's the Dominican
Republic and Haiti--
between 1492 and the
late-1500s, a population of
about half a million indigenous
Americans was reduced to 300,
by measles and by other
diseases.
Also when the Conquistadores
landed at Veracruz and started
marching on Mexico City,
the wave, the epidemic preceded
them;
so the Aztec army was being
decimated by disease when they
got there.
But, you know,
the Aztecs really weren't--it's
not the only explanation--the
Aztecs were not very well liked;
this habit of ripping people's
hearts out and eating them on
altars hadn't endeared them to
the captives that they got,
and the subject peoples.
 
And so actually it only took
900 Conquistadores to defeat the
Aztec army,
because they had 200,000 local
allies that said,
"Yeah, we want to beat 'em
up too."
 
So it was both that effect,
and the disease effect,
that allowed the conquest of
Mexico.
In a city what's going on is
that the pathogen population is
being rescued by the
colonization of empty habitat.
So this is the rescue effect in
a metapopulation and it's the
rescue effect in epidemiology.
 
And that--basically the
pathogens are being rescued by
babies, and the babies are born
susceptible;
they do not yet have an
acquired immune reaction,
they haven't built up the
population of cells that will
target that particular pathogen.
 
And so that rescues the disease
before it goes extinct.
And there are enough of these
babies coming in,
in a city, so that the
colonization rate,
the transmission rate,
and the number of occupiable
sites,
is high enough to keep that
population going.
 
So the take-home point on this
lecture,
besides the fact that the
professor is crazy and wearing a
mask,
is that geography is very
important in ecology,
and there have been a number of
pretty big attempts to create
analytical systems to deal with
it.
 
A lot of the world is
fragmented.
I think even the Abyssal Plain
is getting fragmented as
trawlers start going deeper and
deeper,
and if we undertake mining
operations in the North Pacific,
to pick up little modules of
molybdenum and things like that,
we're going to just continue to
disrupt the entire planet.
Movement by organisms,
among fragments,
creates a dynamic across the
whole landscape.
And local extinctions and
re-colonizations may be an
entirely normal thing,
and you can't really see that
until you look at a big enough
chunk of space,
in a long enough period of
time, to establish a
metapopulation dynamic.
 
And then finally I'd like to
emphasize that the spread of
disease,
epidemiology,
can be viewed as a
metapopulation dynamic,
and can be viewed as a model
system within which to test
metapopulation assumptions.
 
And when you do that,
it seems to work pretty well.
Okay, so next time I'm going to
talk about the flow of energy
and matter through ecosystems.
 
And if I can get my costume
off, I am available to go to
lunch today.
 
