Real Audio Recording   

Joel Cracraft:

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 solution.

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?

Niles Eldredge:

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.

Ross MacPhee:

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.

David Burney:

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.

Richard Holdaway:

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.

Tim Flannery:

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 today.

Ross MacPhee:

May I try?

Tim Flannery:

Yes.

Ross MacPhee:

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 same group.

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 cause extinction.

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.

Tim Flannery:

Can I have a response there?

Ross MacPhee:

Of course.

Tim Flannery:

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. . . .

Ross MacPhee:

It was easier for Paleo-Indians?

Tim Flannery:

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.

Ross MacPhee:

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.

Richard Holdaway:

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.

Ross MacPhee:

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?

Richard Holdaway:

Yes.

Ross MacPhee:

I see.

Richard Holdaway:

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.

Helen James:

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.

Richard Holdaway:

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.

Tim Flannery:

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.

Joel Cracraft:

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.

Tim Flannery:

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.

Tim Flannery:

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.

Tim Flannery:

But not a thousand -- maybe a hundred.

David Burney:

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 (inaudible) environment.

Joel Cracraft:

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.

Joel Cracraft:

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.