Day 1 Discussion 2
moderator: Joel L. Cracraft, AMNH
I'm going to ask all the
speakers for this afternoon's session to come up and array yourself,
and we will begin a discussion for about a half an hour. While we're getting all settled, I'll just make a quick observation.
I'd like to raise a couple of issues that I think cut across
almost every one of the talks, and the first one is -- it's sort
of philosophical. Everybody's talking about causation. Now,
I've always been of a mind that pluralism -- or, in David Burney's
term, I think, "synergism" -- is just another word for
fuzzy thinking. So I've always been very thrilled with people
who go right out on a limb and talk causality, as if it's a point
But let me raise one issue -- and I know it's trite, and I know
you all know it, and it's obvious. But species, as discrete entities
-- a species as a discrete entity does not go extinct. Now, by
that I simply mean a species is not an entity that goes extinct.
We all know extinction is when the last individual dies. And
so causation of extinction needs to really make sure we're focusing
on the fact that we're talking about individuals of a population,
of many populations, spread across some sort of landscape. And
so that makes me like the notion of multiple causation -- because
often it's very difficult to see a single thing causing individual
deaths across many populations, across a fairly large landscape.
And one of the things that most of you I don't think addressed
-- but have, I think, in some of your papers -- is that given
that point, the demographics, the population structure, deems,
behavioral ecology of the extinctees becomes very, very important
relative to a particular cause. And, obviously, body size fits
in with that. But now a number of you have championed pretty
much a single cause, and others of you have championed somewhat
of a pluralistic, synergistic approach to it. So who's right?
Which of you brave souls want to go first?
Actually, I would just like to say that extinction doesn't come
with the drawing of the last breath of the last organism. It's
when reproduction is irrevocably broken in a chain. You could
still have hundreds of organisms left -- they could be saguaro
cactuses that can't get to each other, whatever. Well, there's
your demographics for you. Species are reproductive entities,
and if reproduction stops, the species is dead. I don't care
how many organisms are there or how long they live.
As the disease guy, single-cause, obviously-blinkered vision,
I would like to respond to what Joel was saying. It is not that
I believe that disease is what is uniformly and universally responsible
for all extinctions. That would be ludicrous. It is quite clear,
within the space of the last several hundred years, habitat loss,
overexploitation, introduced species have brought undoubtedly
hundreds and hundreds of species, if not more, to the end.
But that's not the question that's being asked here today. The
question that's being asked here today is with respect to extinctions
in "near time," as I like to call them -- ones in the
last 40,000 years -- whether there's something else going on that
we need to know about. To me it has been surprising, since I
started doing this sort of thing 15 or 20 years ago, I could not
accept ab initio that Paul Martin was right. I could not accept
that a bunch of Indians with pointy sticks could run about bringing
down well over 100 species in the space of what now may be 400
years or less. I still disbelieve it. But it was clear to me
that, at another level, he was right -- that, when you look at
pattern, which is all that we have in this inductive framework
that we're talking about. It's not there's any test that's going
to blow us away, and we're going to say: Eureka! In this inductive
framework that we're working in, what you do is take the jumble
of facts, and half-baked notions and confront them with one another,
and you see what comes out at the end. And with every turn of
the screw on the kaleidoscope, you can get the same facts resolving
in a different way. So what should you choose? Well, that's
partly predilection and partly a combination of evidence you're
looking at and seeing what's most successful.
But, to return to the point. What bothers me -- and has bothered
me always -- is not that Paul might be utterly wrong. But even
if he is right, then what does this mean about how easily brought
to an end species were in the premodern period? How could evidently
small numbers of human people -- of humans, rather -- have such
an impact, in such a short period of time, and bring, in the case
of mammals, hundreds and hundreds to extinction? Whereas nowadays,
with our much-improved tool kit, we seem to have a harder job
doing it. And I don't understand why that should be so.
So, in looking for another natural explanation for why this might
be so, disease was the one that was the most obvious. Disease,
by itself, has impacts of a nearly incredible sort on many populations
and species that we already know about. And it seems to me
that it's not implausible to generalize out from there, if we
can accept that condors are going to extinction because they drink
too much battery acid. Then, for God's sake, surely it is not
implausible that, from time to time, diseases can bring populations
down to such a low level that they actually disintegrate, are
no longer functioning in the form that Niles was just talking
about, and then disappear. So that's from the uniformitarian
point of view.
I think that it's important that it be mentioned again that one
of the main reasons there aren't a lot of large animals going
extinct in the 20th century is that we're not only very well-adapted
for destroying wildlife, but we also have apparently developed
some techniques which we generally put under the rubric of conservation
biology that possibly have worked in a few cases. And I think
this is a sign of hope. And I seem to be the one who wants to
sound a positive note. But the truth is, in order for us to use
20th-century statistics on extinctions in the same context as
these others, I think we have to acknowledge that difference.
Because humans today are certainly capable of these extinctions.
Now, to move on to the idea of multiple causation. I like beautiful
theories and beautiful hypotheses, but, you know, I'm also one
of these people who loves ugly facts. And the truth is that,
in ecology, I think it's become a bit of a mind-set for ecologists,
in that, you know, we've always had this kind of physics envy. We've always wanted to have mechanistic explanations, and deterministic
kinds of predictions, and make models that really work, and so
forth. And, yet, when ecologists get their hands dirty, over
and over and over they find that, you know, the closer you get
to something, the more you look at it, the more complicated it
seems to get. The more we learn, the less we seem to know.
And I think -- you know, I don't know -- this is very interesting.
New dates in North America suggesting this laserlike -- I believe,
is Ross's wonderful phrase -- extinction event, and that's just
the opposite of what we seem to be seeing on Madagascar now, and
I think the Hawaiian data are another example. Where it looks
like if there was a sort of a blitzkrieg of whatever sort -- whether
driven by disease or by human overhunting -- in either case, it
was this oxymoronic slow blitzkrieg, meaning that things didn't
seem to happen that fast.
And I'm starting to wonder, looking at the New Zealand example,
for instance -- which does seem to be a rather rapid example on an island
-- I'm wondering about the tropics versus the temperate zone, in
particular. Because it certainly looks, from my feeble attempts
at a sort of proxy paleodemography, that in Madagascar it's taken
people quite a long time to populate the island. And this is
maybe a little different twist on the obsession with disease.
But it seems to me that in the rainforests of Madagascar, for
instance, and places with high rainfall that we're in contact
with in Africa -- through the trade routes and so forth -- that human
diseases have probably prevented people from really fitting any
sort of rapid population growth model.
And, as we look at these cases, I think what we're starting to
see is that finding a global model may be the hardest thing.
And the interest in finding a single cause that we can apply globally
may be the most impossible thing of all. And it's not so much
out of a spirit of compromise that I suggest that there's evidence
for several different things going on at once. But I am one of
these folks that does believe that the present is somewhat the
key to the past, and that's obviously what we see when we look
at environmental problems today. There's almost never one little
thing going on. There are a lot of big things, and when they
operate on each other you get this multiplicative effect, which
I'm calling this environmental synergy.
I think the New Zealand situation can, as you say, be illustrative
there. The New Zealand event is actually a collection of every
individual event. We have a species pair in there that was also
instructive. Ross had it up there on the bottom right-hand corner
of one of his slides. The greater short-tail bat is one of a
pair of species which used to be in New Zealand. The two Mysticina
bats are very strange little animals that run around like shrews
and mice, spend a lot of their time on the ground. And they varied --
there were two species -- they varied in size by about 50%.
We know from owl deposits that both of them -- that they were
sympatric in large areas; but they also had their own habitats.
And they're, I think, a good illustration that we might have
to look down into the microecology of these things, because the
greater short-tail was found in European times only on a small
island off Stewart Island. And it was exterminated within two
years when Rattus rattus went ashore. Now, the lesser short-tail
bat is still widespread on both main islands. We've only just
rediscovered it in large areas of the South Island.
Now, the Achilles' heel of the Mysticina bats seems to be in
the width of the entrance of the winter roosting cavities in trees.
In the lesser short-tail bat, this is no greater than 20 millimeters.
In the summer, these guys are feisty enough to resist even Rattus
rattus in the larger-entranced roosts. In winter, each bat takes up to
a minute and a half to get itself in and out, and it's too small
an entrance even for the Pacific rat.
Now, the greater short-tailed bat is twice the mass -- about
30 grams -- and its winter roost-hole entrance would have to be
significantly larger. Now, in the fossil deposits that we have
-- and there are many now -- if there is any more than 5% of Pacific
rat bones in that deposit, Mysticina robusta is not present.
So it seems that, in this instance, a paired comparison -- it
brought to mind the bison in North America. If you hypothesized
disease, then you've got to look at one that takes out some bovids,
but leaves one to become superabundant. I think we've got to
start looking down and teasing down into the microecology of these
things, and not treat them as blanket events.
Could I just -- I think, it's time to pull back a bit from the
details and say that Paul Martin is right -- there's no doubt
about it in my mind, and everyone else is wrong. Because there
are several insurmountable problems to every other hypothesis
that's been put forward. The first, relating to climate arguments
-- what we see is, this is a global extinction phenomenon of large
mammals. It happens before the ice age in Australia; it happens
just after it in North and South America; it happens 800 years
ago in New Zealand -- under times of widely different climatic
conditions. Now, if you argue that it's people and climate working
together, you have to accept that the climatic input is so weak
that it doesn't matter what climate's doing -- all that's important
is people arriving. And that is an insurmountable difficulty
with any argument that relates to climate.
As far as the disease hypothesis goes, we've got equally difficult
conditions, because the extinctions occurred in Australia in the
absence of any commensal species -- it's just humans getting in
there. Things like rabies are unknown in Australia. Many, many
of the diseases which are widespread around the world never arrived
in Australia and were unknown among aboriginal people until the
Europeans arrived. So that is quite a serious difficulty for
the disease hypothesis.
The question of how people could kill things off -- again, the
evidence is before our eyes. We made the savage beast. We might
have tamed animals and domesticated them, but before that we made
them fear us. If any one of you has been to the Antarctic, or
to sub-Antarctic islands, you'll see there animals the size of
elephant seals, that weigh three tons, which show no fear of people.
If you want to kill one, you can take a stake and drive it to
the place where you want to kill it, then knock it on the head.
If you go to an oceanic island, where there's a naïve bird
fauna, you can take a bird, break its legs, and its cries will
attract all of its conspecifics to you, where you can do the same
thing and collect them very, very rapidly. So you have these
naïve faunas, and, presumably, American elephants wouldn't
have to be driven into pits -- they simply wouldn't recognize
you as a problem if you were a hunter. And that makes you a very,
very effective competitor.
This ties in very well, also, with the fact that these extinctions
seem to happen relatively quickly. Even in Madagascar, if it
is a thousand years, that's still quick on the geological time
scale. The fact that there's no extinctions afterwards is because
those animals have already learned -- they already have a behavioral
refuge -- and that's why you get no more extinctions. There's
also a hell of a lot of niche space out there once you've knocked
off all of their competitors. So this, together, seems to me
to be a very coherent case that was first put by Paul Martin,
and I haven't seen anyone knock any holes in this basic hypothesis
May I try?
With respect to the Australian case, I don't purport to be an
expert in it. But, listen -- it is very unlikely that humans
themselves were the carriers, but it is not possible to say that
they didn't bring something in with them. It may not have been
dingos -- it may simply have been parasites that were also parasitic
on humans that could have done it. It's not out of the question,
any more than any of these other alternatives are.
But it's about the notion that it's easy to kill. It may be.
The evidence of this sad and bloody century is that the road
from Armenia to Treblinka to democratic Kampuchea to Kigali could
be the same road that our ancestors took from Meganesia to the
Americas, to Madagascar and so on. The killer ape in us is hard
to deny. But the killer ape is aiming mostly at members of the
To what degree is it possible -- again, with respect to megamammals
-- to easily kill enough to do the job? The example I want to
use is whales, because I think, in many ways, whales -- being
remarkable beasts by themselves -- should be able to tell us something
about what we need to do, if I can put it that way, to actually
Although whaling, over the last 150 to 200 years, was carried
on in staggering proportions, at staggering levels, although hundreds
of thousands of whales from several different species were slaughtered,
the evidence is that there has not been a single whale extinction
in recent times. Now, there are populations that have been brought
to extinction -- that's true, to extirpation. The Atlantic gray
whale would be an example, although it looks like that disappeared
in the 16th century, well, well before commercial whaling was
at its height. It just seems extraordinary to me that, over a
several-hundred-year period, ships out of Europe could have gone
after bowheads and rights, and the whales that they were chiefly
interested in, and slaughter at a level that is nowadays scarcely
imaginable, and still the animals survived. I'm not saying that
they have survived well; I'm not saying that they are happy in
their current condition. I am simply making the point once more that
it seems very difficult, just by human intervention in terms of
this direct-impact notion, to bring things to that very low level
that Norman was talking about this morning.
Can I have a response there?
I have to disagree. I mean, in terms of whaling, what happened
was, we drove virtually all stocks into economic extinction --
and whaling is a large-scale economic process. It's not something
a single individual or a small group does. If you read Buller's
account of the sperm-whale industry through the middle of last
century, it was always a difficult business making money in that
thing. And it's just not worth, in that instance. . . .
It was easier for Paleo-Indians?
It was, because you don't have to have a ship, and you don't
have to have a market. You don't have to have large inputs of
capital in order to make money. When you're in that situation,
it's not worth killing the last whale -- you're simply throwing
your money away.
But I would point out, you've also got to go into the Ungava
Peninsula, where hardly anybody lives today, to bang out the last
of the proboscidians. You've got to go into tropical forest to
get out the last of the proboscidians that made it into South
America. You've got to do that simultaneously against horses,
camels, certain pronghorns and so on and so on, all at the same
time. It is believable, in a particularistic way, that climate
change, for example, could have brought to grief many of the large
mammals in North America. It is believable that, singly, humans,
if they were concentratedly working on single species, could have
done the job. But what we're asked to believe in the North American
case is 135 species -- granted, not all hunted under anybody's
scenario, but a high proportion, presumably, would be, Paul would
say -- and you have to do the job everywhere, and you have to
do it with finality, at a level that I don't think we easily approach,
even today with modern methods. And that's the point -- that's
the pattern. The pattern is not the individual entities -- the
pattern is to look at the whole thing globally, as Martin did.
He likes the idea that human interventions alone could have done
it. I don't, and I think that we have to look for something more.
It may well not be disease, but it is something more than Indians
with pointy sticks.
I don't think that you can necessarily transmit
the New Zealand position everywhere, so don't get me wrong on
that. But Stone Age people, without bows and arrows, destroyed
11 species of moa, from 20 kilos up, within probably 200 to 250
years, over a small continental-sized area of land. They destroyed
habitat on half of that, but at least four taxa were widely distributed
along the side of the South Island, and on much of the North Island,
where there was no extensive habitat destruction. And there
certainly wasn't climatic change within that 200-year period.
And they got every last one.
So you would discard the possibility, Richard, which was raised
by Athold Anderson, that disease could have brought some of the
native birds -- not necessarily moa -- but native birds to extinction?
There is plenty of evidence in the archeological record for massive
overhunting of all life-cycle stages. And where there are gaps
in the distribution of archeological sites, these are apparent
rather than real. On the West Coast, in the interior of the North
Island, it's been said that the six taxa of moas there could not
have been hunted out, because there are no sites. On our first
visit, looking for such sites, we found two in one morning. So
it's a matter of application of effort, and Jared Diamond showed
that a single visit every two years to a valley in New Guinea
-- which is rather more rugged than most of the inhabitable parts
of New Zealand -- was sufficient to keep the cassowaries at an
extremely low level, if not exterminated. In New Zealand, the
standing crop of these animals was probably 50,000 -- Athold suggests
50 to 60,000 birds, tops, at any one point in time of all species.
So I think the New Zealand event -- where you have material in
these sites which is sufficient to have sustained extraction by
the railway trunk load for fertilizer -- and has been explained,
half these sites have been lost to coastal erosion. I think just
the magnitude, when you walk under one of those sites, you'll
see photographs of what it was like. The evidence for overkill
there is indisputable.
I think it's a very important and interesting point about the
New Zealand record, where there does seem to have actually --
this is an island case, where there's very good evidence of a
big-game-hunter culture harvesting the moas. And there are tremendous
sites, as Richard says. There are sites that I've read described
as originally covering 90 hectares, with maybe perhaps a meter-deep
sediment -- and these are moa-hunter sites, butchered sites.
So something like overkill may well have happened there. But it
is interesting that this left a tremendous archeological record.
And in no other island situation where these extinctions also
occurred did such a record get left behind.
So there's an inevitability to the extinctions. When humans
arrive on the landscape, the extinctions happen. And this encourages
single simple explanations -- because, you know, it almost encourages
bolide impacts, because it always happens when humans arrived.
The sort of thing that makes you want to come up with disease
or overkill. But when you look at each island -- islands are
a wonderful resource, because each case is different. And the
human cultural adaptations, and the ecological conditions that
existed were different. So once we look at these little facts,
no single explanation covers everything.
New Zealand is well-known for the moas -- but, at the same time,
two otarid seals were exterminated from -- one was extirpated
from most of the mainland; another was exterminated on the mainland,
and there is almost no archeological evidence for that. Apart
from tools in the tool kit for sewing skins, which [inaudible],
Martin and Klein, in '84, as being for sewing moa skins, I can't
imagine what it would be like to wear a birdskin like that. And
sealskins -- these animals were breeding as far north as North
Cape. So there we have mammals which were almost certainly destroyed
as populations by people, with almost no published archeological
evidence -- although it is there and it's common.
Could I just say, too, that, I mean, there's no hunting weapons
in these New Zealand sites. People haven't got things like bow
and arrows. There is no evidence of how they killed these animals.
Presumably they just walked up to them, put a noose or something
around their neck and led them back to camp. I mean, there is
just nothing in the way of those hunting instruments.
The other important point is that the rapidity with which these
New Zealand sites have been destroyed -- as we said before, half
of these life sites have been destroyed in a century or so, just
by coastal erosion. So even if you're looking at extinctions
which are 1,500 years old, rather than 800 years old, you may
have already suffered great degradation of evidence -- particularly
if sites are at geographically active places, like river mouths.
I'm going to jump in. Unless it's very, very quick -- because we
want to let the audience. . . . All right. Let's let the audience jump into this. Any questions
for any of our speakers, or a general question for all of them?
From Audience (Russell Graham):
I'll address this to Tim Flannery. One of the things that's
intrigued me about the North American extinction -- if you're
going to invoke people, why do we have megafauna here today? Things
like Ovibos, which went extinct in the Old World but have survived
in the New World, which are very maladaptive to human hunting.
Things like Odocoileus, which showed up here in the . . .
well before people did -- so they should have been naïve
as well. And I've hunted Odocoileus quite a number of times,
and not done too well, myself. So why do these things survive?
Why weren't they killed? And we know they're hunted throughout
the Holocene, in large numbers.
I'm not familiar with the behavior of many of the North American
mammals, but if I can take it back to the fauna I do know, which
is the Australasian fauna, there we get some exceptional survivals,
as well. Koalas, which are relatively large mammals, survive.
But they're very rare until aboriginal numbers decline; then they
become more common. The large kangaroos, as well. Their tibia-femur
ratios are quite high. They're fast animals compared with the
megafauna -- so, presumably, that gave them an edge, and they
managed to get through, despite the fact that they were big.
Wombats live in burrows -- I mean, aborigines used to send little
kids down burrows with spears to get the wombats to come out.
But, despite that, they managed to survive through to the present. So I think the species that did survive, there's some behavioral
trait which just makes them a little bit more difficult to hunt.
But I think, I imagine, in North America it may have been like
Australia, that these large things were actually very, very rare
through the pre-European period.
From Audience (Russell Graham):
Well, we could use that same argument, actually, to ask why bison survived. If you look at the fossil record of bison,
you don't find a large number of bison until post-Clovis times
-- even post-Folsom time, which would be about 10,000 years ago.
So people were hunting these. There are Clovis sites, at least
two of them -- maybe three of them -- that are associated with
bison. They're the only sites where we find a good association
of those animals with the Clovis people when they were hunting
them. And I would agree with you -- with a higher diversity,
that the numbers of bison probably were smaller, until you did
have the extinction, and you had ecological release. And, so,
it seems to me that bison should have been just as vulnerable
as anything else to Clovis hunting, but they came through it.
Oh, just speculate wildly for one second. I mean, they're very,
very social animals. It may be that you can't kill a thousand
of them -- they do get a chance to learn, you know?
From Audience (Russell Graham):
Proboscidians are very social animals.
But not a thousand -- maybe a hundred.
Can I jump in just for a second? Because, you know, Paul Martin
should really be talking about this. But I've just read this
wonderful manuscript of his that he's been passing around, in
which he's gone through the Lewis and Clark journals. And, you
know, Lewis and Clark, and a couple of people with them, kept
quite detailed records, day-by-day, of what animals they killed,
and also how hungry they got at times. And it was very interesting
in the context of what you just asked, because it seems to demonstrate
very well that the Native Americans were a kind of keystone predator,
even for those animals that they didn't drive completely to extinction.
Because the clear pattern -- I felt Paul's article was very
convincing, and you all should certainly read this when it comes
out, and buy the book when he turns it into a book, hopefully
-- is that you did not see, in so-called "wild America"
at the time of Lewis and Clark an entire region across the Columbia
Plateau, and so forth, that was full of, you know, bison and pronghorns,
and this sort of thing. Even deer were scarce in many places,.. elk. And what you in fact saw as a pattern was where the
Indians had made peace among themselves, and there was no warfare
going on, and population densities were going up, because they
were supplemented with salmon and with agriculture -- there was
no game. Lewis and Clark and their group nearly starved to death.
They ate dogs, they ate many horses. They had a very tough time
in supposedly wild America until they got to the areas where
the Blackfeet had expanded recently, and there was a kind of war
zone. This was one of several examples in the West that Lewis
and Clark went through, and the wildlife was marvelous there.
And they said that often they would float down the river and
never be out of sight of elk for days -- things of this sort -- and talked
about the herds of 20,000 bison, and so forth.
So it's possible [inaudible] get along so well. And we have modern examples, which he mentions
in here, of the demilitarized zone between North and South Korea,
for instance, where wildlife has made a dramatic return. So it's possible that what we're talking about here is two things,
really: one is extirpation, local extirpation, and the other
is extinction. And the pattern we see in the fossil record is
really just a reflection of half of it, which is the full extinction.
From Audience (Paul Martin):
I have a small remark. We have talked about human impact,
and there are different humans -- that is, Homo sapiens -- and
you have also Homo erectus. And there are a few very good cases
of Homo erectus on islands that you can see, and the pattern is
somewhat different from the pattern you find when Homo sapiens
enters the island at the end of the Pleistocene.
We have the case of about, in the early Pleistocene, that Java
was an island, and then there were islands at about a large mastodon,
and hippopotamus, deer, and giant tortoise. And then there
became a land bridge with Asia, and then the whole Asiatic fauna
came to the former island, and you got an extinction of all the
large island mammals. And, of course, it is not clear which caused
the extinction -- it might be the tiger which came in.
But one thing is clear -- that the giant tortoise was killed by
the humans, by Homo erectus.
Then we have the case of the island of Flores, where humans came
-- and that must be Homo erectus, 700,000 years ago. You had
a pygmy stegodont, and you had giant tortoise, and you had the
Komodo dragon, and a giant rat. And from those, the dwarf stegodont
and the giant tortoise became extinct, and the others survived. Probably a man survived on the food which was in the form of the giant rat. You see small modification and a recovery of it.
But I just wanted -- there is some difference between the Middle
and the Lower Pleistocene, when humans entered in isolated often
Thanks, Paul. Any other quick questions? Karl.
From Audience (Karl Koopman):
There's been a good deal of distinction made between people causing
extinction and habitat destruction. I would say that you can't
separate them that easily -- because habitat destruction is usually
brought about by people. And the Paleo-Indians, the first settlers
of North America, also made various modifications of the habitat.
The mere fact that they did introduce fire, and used this extensively,
for instance, in hunting, it seems to me could have had all kinds
of different consequences.
So I think that this business of saying: If man was responsible,
it had to be direct hunting; it had to be people with pointed
sticks sticking them into animals -- that was only part of the
story. The whole ecological impact of a new, very aggressive
species entering the New World I think could have had all kinds
of different impacts.
That's a good point. I have a few quick announcements. First,
I would like to thank all of the speakers this afternoon for a
wonderful job. Very stimulating, intellectually. It will cause
a lot of discussion the rest of the evening and tomorrow, I'm
sure. So thank you very much.