Russell W. Graham, Denver Museum of Natural History

Q: What was the chief cause of prehistoric extinctions during the last 12,000-year period?

RG: I think the primary factor to be considered in the extinction of animals at the end of the ice age -- about 10,000 to 12,000 years ago -- is the climate change that occurred at the same time, as well as the reorganization of biological communities at that time, and the reduction in ranges of many of the species that became extinct.

Q: Did you hear any new interesting factors or interpretations?

RG: I think there were a variety of things that came up at this meeting. Basically you need to look at multiple causes. I've argued one extreme -- that is, climate change. There's another extreme that's frequently argued -- that's human impact, particularly those of prehistoric humans. Another dimension has been disease. Also people looking at previous extinction events -- that is, even earlier than the Pleistocene -- are trying to look at causes, their relationships and patterns. I think all of those things become very important and, in most cases, extinctions are not a simple process. To really understand many of these extinctions, we're going to have to look at more complex solutions. However that makes it very difficult to set up testable hypotheses. Hopefully, we can have a mix of major environmental factors -- like range reduction in species, or human overkill.

Q: Can you give some examples?

RG: I think the idea of multiplicity of causes is an extremely important one. I've argued the climate-change position, because I feel that is an important part of it, and climate change incorporates a variety of things. But there are causes that have been argued by other scholars -- such as disease, human impact, biological reorganization of communities -- and all of these factors have to be taken into account. Let me give you one example of an extinction of a very simple system. An island off the coast of California was studied back in the 1960s by a famous mammologist, W.Z. Liddeker, and he accidentally documented the extinction of the house mouse. One animal, one system, one place. The extinction of that animal required multiple causes. He documented at least six different things that impacted that extinction. So when we started talking about extinctions of animals in 32 genera in North America, I don't think we're going to find the simple answer. It's going to be a whole host of answers.

Q: How would you convince a skeptic that extinction rates are high?

RG: I think one of the best ways is to turn around and look backwards. Look into time itself, look at the paleontological, the archeological record. We know that dinosaurs no longer exist. They did exist 65 million years ago. There was a wide variety of dinosaurs that existed at that time, as well as many other animals and plants that no longer exist today. The cause of that extinction is highly debatable, but there's no question that the extinction occurred. The same thing can be said at the end of the ice age. There are large animals throughout the world that no longer exist. We can go to any museum and look at the bones of those animals. They're there -- and how do you explain why the animals are not here? Because they're extinct. Again, the cause of that extinction is highly debatable. We could argue for climate change, we could argue for human impact. But whatever the cause, it happened.

We can also look at the historical record. We can look at animals that were here when Europeans first came here and those animals are not here. The Carolina parakeet is a good example; the passenger pigeon; the dodo. These were animals that existed on our planet; there's nowhere you can go today to find these animals, except for skins and bones that are maintained in museums. So I don't think there's any question that extinctions have occurred in the past, that they're occurring now, and that they will occur in the future. One of the really important issues for understanding the past -- why should we even look at the past, it's over and done with? -- is that the past can give us a key to some of the processes that may have caused these extinctions. It's hard to predict what the future will be. We can generate computer models for that. But, without the data, the models are meaningless. So we can turn those models around and look at the past, and make predictions about patterns that we would see if certain causes were the reason for the extinctions.

Q: What is the essence of you climate-change argument?

RG: I believe that climate change at the end of the ice age was responsible for the extinction of large mammals throughout the world, especially in North America. We actually have very good evidence for a rapid climate change at the end of the Pleistocene, 11,000 to 12,000 years ago. This climate change caused reorganization in biological communities. This reorganization was what we call "individualistic" -- that is, each species dispersed at its own rate, in different directions and at different times. The result of this is like dealing a hand of cards, in that you get a combination of cards. You turn those cards back in, we shuffle the deck, and you deal the hand again, and you end up with a different combination. This has happened in the past many times. But what we see at the very end of the Pleistocene is the disappearance of what we call "nonanalog faunas and floras," or "nonanalog biotas." These are biotas, or combinations of species, that no longer exist anywhere in the world. So, in a simple fashion, it's habitat destruction. We see the disappearance of old habitats and the appearance of new habitats. This is also coupled with the reduction in the ranges of many of these mammal species. Large mammals require larger home ranges or larger geographic areas, and that's why, I believe, it was preferentially focused on large mammals. The climate change was a trigger, the timing was just right with the geographic distribution of these species, and the system collapsed -- the animals went extinct.

The alternative hypothesis that's been offered to explain this is human overkill, that people impacted these animals. My problems with that model are two. One is just the energetics: I find it very difficult to believe that small groups of people could disperse through two major continental areas -- North America and South America -- with the speed that we see. We know, from our radiocarbon dating that this extinction probably took place in less than 400 radiocarbon years. If you convert that to actual calendar years, we're probably talking about 250 years. There's no question it was a crisis -- a lightning bolt that happened very rapidly -- but we don't find any archeological sites where we find the clear association of humans with many of the species. It's only the mammoth and the mastodon that show clear evidence that humans were utilizing these animals, that they were killing them. Our new radiocarbon dates suggest that the mammoths and mastodons may have been the last animals to survive this extinction. Could Clovis people have arrived on the landscape during an extinction that was already in process due to climate change, and that the last animals they came in contact with -- the mammoth and the mastodon -- are animals that they utilized? They may have played a role in their extinction -- sort of the coup de grâce. But the lack of evidence of human association with other animals leads me to believe that they really did not utilize that resource.

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