The Extinction Enigma

CLIMATE CONUNDRUM

NOVA: So Australia had some pretty unusual animals when people first arrived, didn't it?

Gifford Miller: It's almost beyond comprehension what some of these animals were like. There was a very large, almost hippopotamus-sized, wombat-looking thing. It was the largest marsupial that's ever lived. There were kangaroo-like animals that, instead of being grazers, had arms that went up over their heads and could pull down branches and eat leaves off of trees. There were specialized animals for getting insects out of trees.

And then there were top-level carnivores that used those large animals as prey, such as Thylacoleo, the marsupial lion. It's often called the "drop cat," because its feet are set up in a way that says it simply can't run, and the best reconstructions are that this animal would sit up in a tree and wait for its prey to come around, then leap out of the tree and pounce on its prey.

Thylacoleo is just one of a whole range of animals for which there are simply no modern representatives left. It's a marsupial lineage, which is a different kind of mammal than the placentals that are the dominant animals everywhere else.

How did Australia come to be marsupial heaven, anyway?

Well, Australia has a unique history: when it became isolated from the rest of the world, the only mammalian group that got on board the continent was the marsupials. They came out of South America and migrated across Antarctica and onto Australia, which were all connected then in the supercontinent Gondwana. As Gondwana broke apart, Australia became detached and drifted in isolation over the ocean with only these marsupial lineages on board. Evolution took its course, and over the next 50 or 60 million years Australia evolved this whole array of animals with no relatives anywhere else on the planet.

And over that time the climate changed a lot, right?

Yes, when Australia broke off from Antarctica maybe 50 or 60 million years ago, it was at a high southern latitude, where conditions were relatively moist and cool. It had forest over the whole country, so we had a really damp, cool, forested continent. Then it slowly drifted north towards Asia, and in that process, it moved from those temperate latitudes to the subtropical high-pressure zone, where the continent became both warmer and drier. Consequently, the interior of the continent in particular dried out. And the fossil record shows the vegetation and the fauna adapting to those much more arid conditions.

Superimposed on the long-term shift from that a cool, moist climate into subtropical aridity, the world entered an ice age about two million years ago with glacial/interglacial cycles. Consequently, the last two million years of that 50 million-year journey was a period of high-amplitude climate change, from relatively warm, wet climates to much drier and colder climates, back and forth repeatedly. And these transitions occurred rapidly, many times over a few hundred to a few thousand years. This would have been a very different kind of climate stressor from the more gradual shift towards aridity.

There's no clear evidence that the ice-age cycles themselves led to any of the big extinctions.

But the animals got bigger. Isn't that strange?

It's almost counterintuitive. You might think that big animals would have a hard time surviving on a more impoverished diet. But it actually gives them a certain amount of resiliency, the ability to go through periods of famine if they have to, because they have some body reserves and the ability to eat a large amount of relatively low-nutrient food.

To what degree could these more intense climate changes have brought about the extinctions?

Well, it's really only the recent studies that have come out of cave deposits that have time series long enough to be able to actually track how the animals responded to the high-amplitude ice-age cycles. And those studies clearly show that the assemblages of animals do change—that is, certain animals move out when it gets dry, and when it gets wet again, they come back. But within that we don't see any major extinctions. Indeed, there's no clear evidence that the ice-age cycles themselves led to any of the big extinctions.

So is climate change as the sole cause of the extinctions still a viable argument?

We are now to the point in Australia where there is not a tenable climate argument that can explain the extinctions. We have enough evidence, especially from new studies over the last decade, that has simply, case by case, removed the climate argument as a viable explanation.

That really pushes the question, then, to ask instead, well, if not climate, then what else could it be? And the only real case out there is that there's a human role. The real question now is, what is it that humans might have done that would have resulted in the extinction? Is it overhunting? Is it landscape modification? Did they bring in diseases? Did they hunt mostly juveniles, which eventually led to species extinction through attrition? Was it a long, slow extinction, or was it something that happened in a much shorter period of time?

That's where much of the research right now will be directed, I think, to ask how can we test between these various hypotheses of how human activity might have impacted a continent, and to see whether we can't steadily make some progress in that area.

The core question is, "Had humans never colonized Australia, would most of the megafauna still be present on the continent?" I think the consensus response to that question is "Probably, yes." Was climate a contributing factor? Maybe. Animals are always stressed by temperature extremes and moisture stress as well as from predation and competition and a variety of other factors. But as far back as we can see, the megafauna adjusted to these stressors without extinction. Only when humans are added to the equation do we see the large extinction occur.

A BURNING QUESTION

You've argued that humans' use of fire may have contributed to the extinctions on Australia. How so exactly?

When humans colonized Australia, we know they had fire on demand—we've had that for maybe a half a million years—and at least in Australia, humans burn for a whole range of reasons. They burn to clear the land so that they can move across it. They hunt along the fire front. They signal distant bands. Fire promotes the growth of beneficial plants that sprout after burning. So there are many reasons why humans might be burning the landscape in a way that they thought would have a positive impact.

The Australian landscape is unique in that it has extremely low nutrients in the soils, because it's a very ancient, flat landscape, and most of the nutrients have already been removed from the soil. So it has a high sensitivity to burning having a big impact. One of the possibilities is that the cumulative effect of localized burning by small bands of people in a patchwork fashion may have converted an ecosystem that had a fairly highly productive set of plants into a much less productive, fire-adapted landscape, such as we see today.

You wouldn't want to share your camp with a 25-foot-long carnivorous lizard.

Might the ecosystem have collapsed under such pressure?

Yes, if the landscape itself has a sensitivity to burning, then a change in the fire regime might lead to an ecosystem collapse. Now, Australia has always burnt. It has a high incidence of lightning strikes, and almost all ecosystems there are adapted to burning. Some of them, in fact, are extreme fire promoters, where the only way their seeds can germinate is through fire. So you can't imagine that when people first set fire to Australia that it had never seen fire before. That's certainly not the case.

What humans can do, though, is they can burn at a different time of year than the natural fire regime, and they can burn at a different fire frequency. That is, they can start fires with a smaller fuel load than would be required for lightning-strike fires. There are many ecosystems where you can demonstrate that they are quite comfortable being burnt once every 20 years, every 50 years, and recovering with no problem. But if they're burnt back to back two or three years apart, that's more than they can withstand, and those ecosystems die.

With disastrous results for the megafauna presumably.

Right. Clearly the animals depend on a minimum volume of nutritious food to sustain them. The current vegetation across much of the arid and semi-arid zone consists of almost unpalatable desert scrub, so there's very little food value out there now. We know these big animals were living out there in the past, and they certainly weren't eating what's growing there now. We hypothesize that there must have been a much different ecosystem that included nutritious grasses, which don't exist there today, as well as some interspersed woodland leafy plants that would have had sufficient nutrition to sustain these large animals.

Studies from my research group demonstrate that prior to human colonization, large animals in the interior of the continent were eating just such a mixture—feasting on nutritious grasses in years with good rains, and depending on trees and shrubs in drier years. After 45,000 years ago, those nutritious grasses were nearly gone, and those animals that hadn't gone extinct were restricted to dominantly tree and shrub dietary sources.

Those ecosystems that flourished before humans arrived may have been susceptible to a changed fire regime, in which case the sustained burning patterns practiced by modern humans could have altered that woodland grassy ecosystem to the present desert scrub. The large animals simply would not have had sufficient food resources to sustain themselves.

A HAND IN IT?

Could people have deliberately wiped them out?

Well, when humans first colonize a new landscape, they have to look after their basic needs. They have to feed and protect their families, and they have to stay warm and have shelter. So what's the footprint of a successful human colonization? My general sense is that there's certainly no deliberate attempt to try to remove a large segment of the fauna, although you wouldn't want to share your camp with a 25-foot-long carnivorous lizard or a large cat-like creature that might drop on you from an overhanging branch.

You could imagine that people would want to protect themselves by removing the big predators, but in terms of the much more abundant large herbivores, which are a stable food source, it's unlikely that they're deliberately trying to exterminate those. The lesson we might draw is that modifications to make landscapes more suitable to our lifestyle, or to give ourselves shelter, may inadvertently lead to the extinction of what had been a major food source—these large marsupials that could have been a nice meal for a band of humans.

How might humans have affected top predators like Thylacoleo?

One tries to imagine what it would have been like to be the first humans in Australia and to see not only these large animals that you could eat, but also these large animals that might eat you. The question then is, why didn't the Thylacoleo win? Why didn't these giant lizards that are 25 feet long win?

The reality is that humans don't really have to hunt these big animals to lead to their extinction. All they have to do is reduce the prey, the large browsers and grazers on which the big carnivores depend. Carnivores are very sensitive to prey abundance. If humans reduce the number of potential prey below a threshold, then large predators like Thylacoleo and the giant lizard Megalania will likely starve to death.

NEW FINDINGS

What light do the finds from the Nullarbor caves [as shown in "Bone Diggers"] shed on the extinction question?

The Nullarbor caves, besides offering this beautiful array of well-preserved fossils that are themselves interesting from the simple curiosity standpoint, also provide the first look back in time at how these big marsupials adapted to an arid environment. The Nullarbor is an arid zone—there's not much vegetation there.

The Australian story says we should proceed with caution.

It's long been thought that the big marsupials weren't well-adapted to extreme aridity, because they originated in a moist climate regime. As the continent moved farther into the zone of aridity, and especially with ice-age cycles of extreme aridity, this would have stressed them, assuming they were not well-adapted to such climate extremes. In contrast to this paradigm is the new record from the Nullarbor caves, which goes back almost a million years and shows that these animals were quite comfortable living in an arid zone during times of fluctuating climates. They were apparently already well-adapted to the changed moisture regime.

One of the most exciting additional aspects of the Nullarbor caves study is that the research team was also able to reconstruct what the climate was like at the time the large animals were living there. So we know, independently of the fauna, something about the climate. Their results tell us that the climate then was much like today: it was an arid climate, and for a million years these animals were living successfully in the arid zone. So that eliminates, really, the argument that aridity was a major stressor on them, because they were living there without significant extinctions for a million years.

The other thing that's so important from this find, is that it tells us—independently, now, from the climate—what the vegetation was like. So we have three pieces: we know what animals were living there, we know what the rainfall was, and we know the vegetation. The rainfall was like today, but the vegetation was completely different, much more diverse, with many more palatable elements in the ecosystem. So we have evidence here, quite independent of any other evidence from Australia, that says large marsupials were well-adapted to the arid zone but lived under a very different vegetation assemblage than at present.

That, then, adds a very important piece to our question, "How might humans have impacted large animals across Australia?" Human activity may have altered ecosystems so that the vegetation present today is fundamentally different from the vegetation of the past, despite similar rainfall. The Nullarbor cave evidence tells us that in the past there was an arid climate, very diverse vegetation, and large marsupials, whereas the present day has the same climate but impoverished vegetation and few large marsupials.

What would you ideally like to find to help solve this mystery?

What one would like to find is a continuous series of fauna right through the extinction event. Possibly there is a pitfall cave somewhere whose accumulation spans the extinction window. That would allow us to better evaluate both the rate of extinction and whether extinction and ecosystem change are coincident in time or not; this would be stunning information. Somewhere out there, I expect that cave will eventually be found. It would help us understand whether all the species that make up the megafauna persist right up to the extinction window and then are gone, or whether some of those elements fall out a little bit earlier.

It's a rare event, to get the preservation of one of these big marsupials. So we are limited by our sample size, but by finding more of these pristine caves, we should be able to flesh out that part of the story.

In closing, what lessons do you take from these mass extinctions for us today?

Well, one of the lessons from the Australian story, I think, is a two-fold one about life. One is, there's tremendous resiliency—flora and fauna are able to find ways to adapt to big climate changes. But two, there are thresholds, that despite having survived major climate changes, ice-age cycles, with everybody adjusting and surviving, at some point, extinction claimed the large marsupials (and many smaller ones). It doesn't actually matter, for this lesson, what the cause was. There is some sort of a threshold that was exceeded, beyond which these big animals simply couldn't survive.

You can shift that lesson to the modern world. We're modifying landscapes at unprecedented rates. The interconnections between all these life activities are tremendously complex, and we don't really understand them completely. So the Australian story says we should proceed with caution, because despite life's tremendous resiliency, there are thresholds in the system that when crossed yield distinctly non-linear results, where changes take place suddenly that would not have been anticipated by looking at what went on in the past.