Emerging Extiction Patterns
Except from an international (USA) symposium by Australia's leading Quaternary palaeontologists
Dr. Timothy F. Flannery, Australian Museum.
I must admit I got a bit of a surprise when I saw the preliminary program for today's schedule. My paper was listed in a session which was called "Catastrophes in Small Places." Well, Australia isn't actually that small, and I decided at that moment maybe I'd better give you people a bit of an outline of the nature of the continent, before I launch into the megafaunal extinctions, which have been particularly dramatic there.
Well, this is the place. This is Australia. It's not that small -- 7.6 million square kilometers; approximately the same size as the contiguous 48 states of the U.S.A. At times of lowered sea level, at glacial maxima, that would have expanded to about 10 million square kilometers. And you could have walked from the northern part of New Guinea right down to Tasmania, uninterrupted by any saltwater. It's a very large piece of land.
It's the smallest of the continents, and probably the strangest. It's certainly had a unique history. It's the only continent with completely passive continental margins. There's no active mountain-building going on anywhere around the edge of the continent, New Guinea excepted, which is a separate island. The main mountain range on the continent runs along the east coast, and it's a very old range. It developed as New Zealand rifted away from Australia -- and we're all very glad it did; but, anyway, it did. Unfortunately, it took the volcanoes and the good soil with it, leaving us with a very low mountain range, and a rather infertile one. It's eroded very little throughout the Tertiary. Dating of basalts along the range suggest that the major river systems have only cut down a few tens of meters in many places over tens of millions of years – so produced very little in the way of soil.
It's also a very ancient land mass. This is a geological map of Australia, which you don't really need to take any notice of at all -- except that this western area here is the Archean plate. It's pre-Cambrian or older -very, very ancient rock -- and basically very little-changed throughout the Cenozoic, in any case.
The ice age had a rather odd effect in Australia. It was very different from North America, where the Laurentide ice sheet formed. Our biggest glacier on mainland Australia at the height of the glacial maximum was down here. It was about 50 square kilometers in extent. It didn't produce any soils that anyone's using today, anyway. Very different from 13 million square kilometers of Laurentide ice sheet across the north of this continent.
What did happen in Australia, though, was that the center of the continent dried out, and a huge anticyclonic system of sand dunes became active throughout the central 40% of the continent, which was basically a vegetationless, howling dry June field. And, again, the New Zealanders won out, because the winds blow that way; and whatever soil we had there, a lot of it ended up on their glaciers, painting their glaciers red, or in the Tasman Sea. So it wasn't a good time to be in Australia, during the last ice age.
The one area where we do have reasonable soils is a little archipelago of volcanoes running down the east side -- very, very small in extent, most of them. And they range in age from, I'd say, about 50 million years through to the last few tens of thousands of years. And they have produced an island archipelago of reasonable soils. The rest of Australian soils are very ancient and highly structured, highly weathered. I suppose they're characterized by having very low levels of soil nutrients, like nitrates and phosphates -- generally about half the level, at best, of equivalent soils overseas. And some recent studies, particularly on phosphates, suggest that Australian soils from this area have about a tenth the level of soil phosphates as equivalent soils in eastern South Africa. And this had very dramatic effects in structuring Australian ecosystems. The soils also have high surface temperature, very poor water-retention ability. They're deficient in trace elements. They're characteristically very thin. Basically, they're pretty awful - especially for a plant trying to grow in them.
This is just an example of an Australian landscape -- a pretty typical western one. This is Mt. Narriyer, in the middle of Western Australia. The oldest rocks on planet earth have been discovered on this mountain - some zircons that are 3.9 billion years old. These are very, very ancient, unchanging landscapes. It looks like that for about two weeks of the year -- the daisies and greenery. The rest of the time it's just pretty bare.
Now, these particular soil conditions, which are so widespread in Australia, have led to the development of rather odd plant communities. Australia is really the home of scleromorphy -- these plants which are generally pygmies. As you can see here, very small -- small, spiny leaves; small internode distances, typically very heavily defended with tannins and phenolics. So nothing eats them -- with the exception of members of the pea family and a couple of others; have very decorporate communities even of folivorous insects, much less mammals. But they're extremely diverse. This is from the southwest of Western Australia. They have tremendous diversity. It's high, in fact, in terms of absolute species. This is the Australian rainforest, or the southeastern Asian rainforest, pretty much. And they're very attractive in spring, covered in flowers - basically, because there's a lot of competition, I think, for pollinators.
Now, these infertile soils -- they may actually have contributed to any coherence that's in my speech today -- because they also grow excellent wines. And we drank several bottles of it last night, and didn't get to bed till very late. So they have many, many effects. Anyway. . . . Here's some more plants from what's called "Kwongan ecosystems" in the southwest of Western Australia. As I said, tremendous diversity and very spectacular plants, but they just lack mammalian herbivores. Well, they're opportunistic, coming in after fire. And here is some more marginal environments.
Curiously enough, where there's not enough rain to sustain these quongan faunas -- you get these ephemeral daisy fields. And these are, in fact, even much more productive, even though they're only operating for a few weeks of the year. These actually support kangaroos and things, whereas the heath-plants really don't.
Layered on top of these rather peculiar soils that we have in Australia is a very strange climatic pattern called the southern oscillation. And you people here, you really do have genuine seasons; and productivity here seems to me to be dictated by the seasons pretty much. We seem to have seasons in Australia -- it gets colder in winter and warmer in summer - but that doesn't dictate productivity. What dictates productivity in particularly eastern Australia is rainfall, which is brought with the southern oscillation.
This is a basic diagram of the southern oscillation and how it works. Every two to eight years, this cycle alters, and during an El Niño year, we get cold water building up off the coast of Australia, and that brings
drought. And you can see here, this brown area shows just how much of Australia was affected in the 1982-1983 El Niño event. About two-thirds of the continent, the eastern two-thirds, is profoundly affected by the southern oscillation -- and if you'll look at the other continents, you see that, on a percentage basis, there's nowhere else that's as profoundly affected by this as Australia.
The typical patterns is that you'll have droughts, such as illustrated here, which can be several years in duration. And then the warm water floods back across the Pacific, flows back this way, and you'll then have the droughts broken by very spectacular floods. And then, if you're lucky, a year or two later, you'll get the kind of average year – the statistically average year -- which is when your kind of Australian cow cropper makes a bit of money and thinks the good times have come back. But, sooner or later -- whether it be two years or eight years – the cycle starts again. So very irregular productivity - it's not seasonal.
And, of course, that explains why our indigenous people never had agriculture - or, at least, did away with what agriculture they had. There's simply no point storing food if you don't know whether you're storing it for six months or 10 years. We don't have any hibernating mammal for the same reason, or very, very few. Some of our species in the east respond to food shortage, but not really true hibernation. And there's a number of other things which are affected by this. We have very few deciduous plants, for example, for the same reason: Why throw away your leaf when you don't know when you're going to want it back again? Very strange systems.
And this is just to show you roughly the periodicity of major rains caused by the southern oscillation. You can see here, running from 1870 to 1970, about a century's worth of record for central Australia. And through that time we've had four major episodes of high rainfall - followed by fire, followed by rabbit plagues, and also, if we're lucky, successful recruitment into some of the plant populations.
So that is the nature of the continent - very, very different from North America. It did have a megafauna, but quite a peculiar one. At some stage in the Late Pleistocene, the exact timing of which I'll return to later, Australia lost everything that lived on the continent which was larger than a human; and there was quite a large fauna on the continent prior to that. We lost in all about 55 species. The majority of those lost were large marsupials, but there were also some very spectacular large reptiles, some large flightless birds, and even some smaller flightless birds - all lost at the same time.
Now, it really is a rather odd assemblage, and I'll just show you some species - or reconstructions. . . . Ah -- before I do that - I knew there was something else I had to do -- talk about New Guinea, Australia's neighbor island, because it is the mirror through which you can understand a little bit more about Australian ecosystems.
New Guinea is a about one-ninth the size of Australia, and it has a mammal fauna of about the same magnitude as Australia's -- about the same number of species. But New Guinea is very different. It's a mountainous island; it's covered in rainforest; some areas get up to 11 meters of rain a year. It's affected by the southern oscillation, but not really so much through rainfall as through frosts -- recurrent frosts which occur during cloudless times of the year -- which are tied in with the El Niño part of the cycle.
Here's a representation of the mammal fauna - the non-volant mammal fauna of New Guinea. Don't worry about the shapes, particularly, but the black blobs at the top are the things that became extinct, the megafauna that became extinct in New Guinea. As you can see, none of them are very big - the biggest was probably 300 or 400 kilograms in weight. And out of a fauna of over 220 species, New Guinea lost just six species. And we're now building up a good fossil record for the island; and this is becoming
evident, that the megafauna just wasn't much more diverse than that. So I'd say that once you leave Afro-Eurasia, this would have to be the smallest level of impact of megafaunal extinction seen anywhere on the planet.
The six species that were lost occurred in all environments in - well, they ranged from the alpine tundra, which surrounds the glaciers in the highest parts of New Guinea. We've found fossils from periglacial deposits with megafauna in them. We've found fossils of other species in the mid-montane, and yet other species in the lowland jungle. So here it didn't matter where you lived in New Guinea -- what really mattered was just how big you are. And you can see here that these things are all ordered according to size, and there's a distinct cutoff there. Anything that weighed more than about 10 kilograms went. If you were smaller, it didn't matter where you lived. You survived.
And I should say that there's no conceivable kind of a climatic event which could have altered significantly, or could have brought drought to New Guinea as we know it. The bryophytes and everything else have survived there; the small mammals have survived. There are many things which are much more water-sensitive than large mammals, so something else, I think, is going on in New Guinea.
I just wanted to show you something to convince you that marsupials really are cute. This is one of the largest things that survived in New Guinea. It's a tree-kangaroo, but it's a species which has come out of the trees and now lives on the ground. It lives above the tree line in New Guinea, in some of the very coldest parts of the country, and it was only discovered about three years ago. So there's still lots to be done, even as far as the modern mammal fauna of New Guinea goes. That's the same beast. Very cute, very tame.
Now here is the equivalent picture for the Australian megafauna. The black dots, again, are those which went sometime in the Pleistocene. And you can see there, there's many, many more species lost than in New Guinea. Most of these are herbivores -- most of the species you're looking at here. Most of them are browsers, curiously; most of the lost species are browsers. There's very, very few grazers, or clearly identified grazing mammals which were lost during the Australian Pleistocene event. It's rather odd and quite different to the other continents in that regard.
You can see the smaller species that were lost were about 10 kilograms in weight; the largest, perhaps a couple of tons. And to understand the Australian fauna, in general, it's good to keep in mind a thing that I call a "third rule," and that is really that if it's an Australian marsupial you're looking at, it's probably only going to be about a third the body mass of its ecological vicar elsewhere. So our big marsupials - our elephant equivalents -- weighed one or two tons, compared with five tons or more for elephants . . . (tape ends) . . . You see this repeatedly. Our equivalent of hyenas, the Tasmanian devil, you could safely sit one on top of this lectern -- as long as you weren't bitten - whereas a hyena, of course, is a much, much bigger animal. So that general rule seems to apply. And, again, think this is due to low nutrient levels in Australian ecosystems, as much as anything else.
Among the non-mammalian species lost were some large birds. This is one of them, a bird called Genyornis-- a large herbivorous bird. Seemed to be very widespread before its extinction. This is Diprotodon, our largest marsupial. People, even in Australia, don't understand marsupials very well. This model is fleeing from a very large goanna, that I'll speak about in a second - and the preparators at the museum were very upset at putting this on, because they said it looked like it was having a spontaneous abortion from terror, from running away - not realizing it's a marsupial, it has a pouch, and the pouches point backwards on these things. But, anyway . . . rather strange reactions. So these animals are a ton or two in weight and was probably semi-arid adapted-- it's probably living in the drier parts of the country.
A large browsing wallaby, of which there are a dozen or so species in Australia and New Guinea - next to a person, so you can see the size of these things. Again, these are browsers. We actually have stomach contents of some of these species that show very coarse browse, preserved. This is an example here of some stomach contents from an Early Pleistocene browser - and you can see there's skin impressions even preserved, and that there is some quite nice Pleistocene fossils from Australia. But these remain unstudied, though discovered 15 or 20 years ago; and, as yet, no one has actually looked at those stomach contents, beyond the kind of very coarse microscopic identification of them as browse.
Short-faced kangaroos-- at least 20 species, all browsers. A very complex and rapid radiation of rather bizarre-looking kangaroos, every one of which is now extinct. And onto the carnivores, which are very peculiar in Australia. This is kind of a wombat-turned carnivore, if you want – a rather strange thing called the marsupial lion. It's from herbivorous ancestry. It was our only catlike predator in the Pleistocene. And it's rather curious that we have one catlike predator, one doglike predator - the Tasmanian tiger; one scavenger, which is the Tasmanian devil – and one or two weasel-like things. Whereas these guilds overseas, or niches overseas, are filled by a large variety of species. The catlike niche, for example - you may have five, six or seven species on a continent; the doglike niche likewise, that are divided according to size and to habitat preferences. In Australia we just have one kind of each. And this was the largest homeothermid carnivore ever to have existed in Australia. It probably weighed about 40 kilograms-- so humans are, in effect, the largest warm-blooded predator ever to have existed in Australia ecosystems.
What we did have though, at the top of the food pyramid, was some things that look a bit like they're out of "Jurassic Park." This is a skeleton of a seven-meter-long goanna, which would have weighed several tons. So this was our top-order predator. And probably not since the days of tyrannosaurus was anything quite so horrible seen on any landscape anywhere. But it was only one of a number of reptilian species which dominated the large-carnivore niche. We also had terrestrial crocodiles, which would have weighed several hundred kilograms and been three or four meters long. We had some now-extinct freshwater crocodiles. We had a whole subfamily of gigantic snakes, which lived at quite high latitude -- which would have been six or seven meters long and as thick around as me, perhaps – very large. So the reptiles were what were dominating the carnivore niche in Australia - again, I think, a reflection of just low levels of nutrient availability and therefore productivity. You're much better off being a cold-blooded carnivore in Australia, because you can survive from one El Niño cycle to the next, and you don't need to eat all that much, anyway. Now, that's just to give you an idea of the size of this goanna . . . a rather horrible creature. Okay, let me just stop there, briefly.
So when did all of this fauna become extinct? I guess this is the great question. And if you'd asked me that some five years ago, I would have said: Oh, it all survived until about 25,000 years ago. We had some sites which we, as a scientific community in Australia, believe genuinely reflected the survival of this megafauna to 25,000 years ago. At that same time, we had very good evidence of people in Australia at least for 35,000 years. It's now looking like it may be 50,000 to 60,000 years - but then at least 35,000 years. And in support of that we had cave deposits from the far southwest of the continent - from Tasmania, from the southeast - with tens of radiocarbon dates all stacked up and conforming with each other, which reflected a human presence at least that deep. We had a couple of open sites with what we thought was megafauna at 25,000.
In the last couple of years, though, things have changed dramatically. Those sites that we thought reflected late survival of megafauna have all been reexamined. And we can now say that if you apply the criteria of Meltzer and Mead, say, in terms of analyzing dates on megafauna in Australia, we don't have a single instance of a dated megafauna in Australia -- dated with 14C or any Late Pleistocene dating. The guess is now that these things all became extinct before about 40,000 years ago in Australia - certainly in New Guinea. We have five sites there where we have articulated or associated megafaunal remains, all of them which are dated -- all five are dated. They date to beyond 38,000 - and that date's probably just a residual radiocarbon date. These sites may be older than that.
So what we have in Australasia, I think, is a very early invasion by humans. Certainly it happened by 35,000 years ago -- possibly by 60,000-- and very early extinction of everything bigger than a human in these Australian ecosystems. We have no evidence of any late survivals, it seemed. Oh, the other thing I should say is, we have no kill sites – we have nowhere where people and megafauna can be demonstrated to be coexisting. In fact, I mean, really, there are not - what should I say? There's a few potential sites where this kind of evidence may turn up. One has been discovered recently in New South Wales, which may have evidence of overlap between humans and megafauna - but the lack of this evidence actually isn't all that surprising. Because if you consider that these extinctions happened before the Last Glacial maximum - and in Australia the glacial maximum brought such dramatic reorganization of landscapes.
There's a lot of reworking; a lot of sites were simply eroded away. Just to consider how rapidly this data can be lost, you consider the case of New Zealand, where moa became extinct only perhaps 500 or 600 years ago. There are the major moa-hunting sites, which were found in the 1850s – many of them have lost up to 50% of their surface area just because they're at river mouths, and there's a lot of reworking of sediments gone on. So you've lost half there over a century. We're looking back at Australia perhaps 40,000 years, with a major glacial event in the middle of it. You don't expect to find a lot of evidence under those conditions.