NS: Looking outwards from Africa, where we still have a wide diversity of large mammals, I would note the disappearance of those very large mammals elsewhere in the world. Around the end of the Pleistocene -- perhaps 10,000 years ago, or earlier than that -- these species, which were once formerly abundant, no longer occurred.

Q: What are the chief kinds of things people do to cause extinctions?

NS: There are two main effects that humans have. The one that was very prevalent in the past was, of course, as hunters; people depended upon those animals as food sources. Humans also have many other effects -- modifying habitats through use of fire, changing fire effects, and recently by occupying habitats formerly available to species, land that was once available for wildlife.

Q: What is the relevance of understanding prehistoric extinctions to our current biodiversity crisis?

NS: It's really important for us, in trying to explain what's happening now and in the future, to explain what has happened in the past.

Q: Did you hear anything at the symposium that you thought was interesting new data or interpretation?

NS: I think what the meeting has reinforced is the awareness of the central role of humans in the extinctions, particularly as supported by the timing of when those extinctions occurred in different parts of the world. In Australia, they now appreciate that the extinctions occurred at an earlier period in time, and therefore much closer to the first arrival of humans in Australia. But in looking at the role of humans in those extinctions, I think of the very large mammals -- the so-called "megaherbivores," weighing more than a metric ton, the elephants, rhinos, species of that size -- while their role as predators of those species seems uncontestable. Those species were not able to sustain hunting pressure from a predator, when in the past they had not had a predator preying upon adult animals. Those species were vulnerable.

Nevertheless, there were other factors involved, particularly in the timing of the extinctions, which occurred at a time of rapid climatic change at the end of the last glacial. Of course, that transition, the climatic transition, doesn't seem to have been any different from that of previous changes from glacial, interglacial, conditions. And, therefore, the climatic change alone doesn't seem a sufficient argument. That's why I felt one also had to look at some other consequences of what might have followed the elimination of the megaherbivores by human hunting. In particular, awareness of how great an impact those species can have on habitat conditions for other species. They are these species that can function as ecosystem engineers and change woodland conditions, change grasslands, in a way that may actually have been beneficial to other species in creating a mosaic diversity of habitats. In fact, the paleontological record does seem to suggest that habitats in the past were more diverse than they are at present, and it's difficult to ascribe that change in diversity to climatic change alone.

Q: What reasons are there to hope to stem the current extinction rate?

NS: That's actually a major challenge, largely because of the preemption of habitats by human population growth. To the extent to which that population growth remains unchecked, and people spread out across the landscape, we face a major problem. I think the only way of really attacking that problem is to draw people out of rural areas into city environments, where they exert less direct pressure on habitats of other species, and they also are open to education and awareness of problems that they would not be aware of living in remote rural areas.

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