and whether or not it's something we should be worried

about, and how to think about things

like the extinction crisis which is being caused by human

activity.

And I don't want you to forget this extremely simple idea here,

that the impact of humans on the environment basically is a

function of how many people there are on the planet,

times the average amount that each of those people consumes,

multiplied perhaps by some fudge factor to express good or

bad behavior on the part of the humans.

And I think that the Greeks probably could have written that

down 2500 years ago.

It's pretty straightforward.

It's nothing that is particularly surprising.

Nevertheless it is striking to me that this half of the

equation, the upper half of the equation,

which is the population problem,

has pretty much disappeared from public discourse on this

issue, and I think it's a result of

pushback from various groups that see any discussion of the

population problem as inevitably involving contraception or

abortion, which for some groups has

religious issues.

And it's also a pushback from the poor countries of the world,

against the rich countries, saying,

"You can't come down here and tell us what to do after

you've all gone and screwed up your own environment.

Let us get on with our own lives, and that's not our

priority."

Nevertheless, in the long-term,

that's true.

The impact of humans on the environment of the planet is

basically a function of how many people there are on the planet

times how much they're using.

And we can't get away from that.

You will find in the literature on things about biodiversity

problems, extinction problems, an awful lot of fairly refined,

scientific analysis.

You will find people applying cutting edge ecological theory

to the issue.

But, by way of introduction, I never want you to forget that

the problem of human impact on the other species on the planet

is not a problem that we can really solve with scientific

research.

We can only solve it by understanding the incentives

that people encounter to have more babies,

or fewer babies, and the incentives that they

encounter to consume more or to consume less.

This is a really tough nut.

If any of you think this is simple, just think of what's

going on in the global financial crisis.

The people of the world are trying to stimulate consumption.

They're desperate not to be poor.

And that flies right in the face of the environmental

catastrophe.

I don't think there's any easy answer on that.

Yes Blake?

Student: People are interested in

>.

Prof: Right, Bob Wyman teaches a class on

population growth.

By the way, let me just take one of Bob's slides,

which he shared with me recently.

If you go around the planet and you ask people how many babies

do they want, good sociological research

indicates that they want about two-thirds of what they actually

get.

And that suggests that the easiest way to solve the

population problem is not to do anything Draconian,

but just to give every woman on the planet control of her own

reproductive fate; by whatever means.

Okay, I just wanted to make sure I got that message across

up front.

Now today I'm going to discuss extinctions, and I am going to

do it from ecological, economical, evolutionary and

personal points of view.

And within the ecological literature this is an old

chestnut, and basically back oh say about

thirty to fifty years ago there was intuition that suggested

that the more diverse an ecosystem was the more stable it

would be.

And people like stability, they don't like to be hit by

surprises, and so diversity would be good because it

conferred stability.

Then Bob May showed that more diverse communities can be less

stable.

That's not a necessary logical connection, that more diverse

things are more stable; sometimes more diverse

communities are less stable.

Since then there have been a lot of experiments.

There isn't a convincing clear pattern.

More recent theory shows that sometimes diversity can increase

stability.

This one seems to be a moving target,

and I think it's one where we have to be modest and say that,

"You know, in any particular circumstance

we don't really know what would happen."

Stability itself is a fairly abstract term,

and it can either refer to resistance,

which means ability to remain in the same state,

or to resilience, which means ability to return

to the same state following a perturbation;

so bending without breaking.

And that is probably more important in the real world.

Okay?

Now, what do we actually know experimentally about what

species diversity does to ecosystems?

Well there's some evidence, by the way, that most of this

data has to do with plants, or plants and insects.

A richer community appears better able to survive a

drought.

That is probably because the plants in interaction with each

other are actually conserving water locally.

If you look at net primary productivity,

as you increase the number of plant species,

the productivity, in terms of kilograms of carbon

fixed per square meter per year goes up and then levels off;

so there's diminishing returns.

But the more different kinds of things,

at least at the beginning, that you pack into a given

space, because they're partitioning

the environment differently the more they're going to be able to

take out of the sunlight and the C02 and the water that's in the

system, and convert it into

biologically useful materials.

There's some evidence that as you increase the number of

species in the community, the harder it is for an

invasive species to get into that community.

So these kinds of things are done usually in fairly simple

experimental gardens.

And I think that the application of that to the real

world I have to remain agnostic on;

it might work, it might not.

Recently there was a very nice paper--

this in the Proceedings of the Royal Society this year--

showing that more diverse pollinator communities provide

more reliable service.

So this X axis here is proportion of native vegetation,

and this is the number of pollinator individuals,

and this is again proportion of native vegetation and number of

visits.

And basically what it's showing is that the more diverse the

pollinator community, the more likely it is that the

plants in it are going to get pollinated,

because the different kinds of pollinators are complementary,

they're trading services as they come in.

And this connects a bit to ecosystem function,

because after all one of the things that Mother Nature gives

us in agriculture is the services of the pollinators.

And if we didn't have them, the almond industry and the

apple industry of the world would collapse.

So if we do things that wipe out the pollinator community,

or radically simplify it, this paper suggests we can

expect that we're going to have a decrease in our fruit and our

nut crops.

So a few ecological points about diversity is that it does

seem to improve some ecosystem properties.

There's some evidence that connects species diversity to

resilience and to resistance to invasion.

Not too many people who have been working in the ecosystem

function end of science have been terribly worried about the

diversity of individuals in genes;

they've been looking mostly at species diversity.

But it may very well be that if you have say a group of

pollinators and you look within a single species and you compare

very genetically diverse species of pollinators with very

genetically homogenous species of pollinators that the genetic

diversity may also have a significant impact.

So there are a whole series of levels at which one can ask the

diversity question, and not all of them are equally

well researched.

Now let me just go back to the ecological view of diversity

here and make a general comment on it.

One of the arguments that conservation advocates use is

that biodiversity is important for ecosystem function.

And I'm now about to go into an argument on ecosystem function.

Okay?

And it's about how much it's going to cost us to replace

those services.

And I'm just setting a pointer here,

because I'll come back in a few minutes and point out that if

you claim that you need a lot of biodiversity to maintain clean

air, clean water,

pollination services, everything that Nature gives us

for free, but if Nature is really

redundant, so that you could actually get rid of 90% of the

species on the earth before you even noticed any decrease in

ecosystem function, that you are then involving

yourself in a political argument which is quite dangerous;

and that is that you appear not to know what you're talking

about.

Because your critics could come back year after year saying,

"Oh, we've lost another 10% of the species on the planet

and the ecosystems are still functioning just fine.

You're just crying wolf."

Until, of course, you get down to the point where

you've trimmed away so many that the next ones really do make a

difference to ecosystem function,

and then we're all in deep water.

So there is a real problem here of being able to communicate to

the general public, and the politicians,

the issue that you could have a lot of ecosystem redundancy,

which is buffering you from the extinctions that might otherwise

be affecting ecosystem function, but at some point,

if you've eliminated a lot of species,

you will hit a limit at which there's no redundancy left,

and at that point ecosystems start to collapse.

So let's now go through the economic argument,

and you'll see how much it might cost to replace things.

So there have been attempts, mainly by Bob Costanza and his

group-- he's now at the University of

Vermont-- to estimate what is the value

of ecosystem function.

And before we get into Costanza's, we can look at

something like Habitat II.

So Ed Bass decides twenty years ago or so to build a very

futuristic architectural piece out in the Arizona desert,

which is an attempt to see whether or not you could

actually build an interstellar spacecraft with a self-enclosed,

completely recycling ecosystem, such that people could go in it

and there would be everything in it that people would need to

live forever.

So if you just make one of those things and put it out into

space, potentially humans could colonize other galaxies.

It's a very bold idea, right?

So it costs 9 million per person, and if we wanted to

replace the earth with that, you could figure that it's

about 54 thousand-trillion to replace- to support everybody on

earth, to get them to another galaxy.

Unfortunately Habitat II didn't work.

>

So it wasn't complete, and the people who went in it

and tried to live in it had to bail out after about six months.

So this line of reasoning, which by the way is always

going to end up with very big numbers, is basically a comment

on externalization; and externalization is

economics talk for things which are not my problem,

thank you.

So they are basically what we define,

in our approach to the problem, as being outside the scope of