Reorganization of Late Quaternary Mammal Faunas and Causes of Mass Extinction

John Alroy, Smithsonian Institution

Today I'm going to discuss the big picture with respect to the issue of the terminal Pleistocene mass extinction event in North America. And when I say «big picture», I mean it in three different ways……a)…, I'm going to discuss the very large spatial scale of the continent of North America – not just one place or another, but as much of it as we can discuss…..b)…, I'm going to discuss the very long temporal scale of the Cenozoic Era, all 65 million years of it. And, …..c)…, I'm going to discuss all of the mammals – not just a few species, not just the victims of the extinction, but also the survivors.

Now, I'm going to focus on two specific points. And this is going to involve two different data sets and two different methods of analysis. So, … d) …I'm going to discuss extinction intensity in the Late Pleistocene, and I'm going to try to answer the question of whether the intensity, and also the selectivity of that extinction, was unusual in the context of the entire Cenozoic – the big picture.

…..e)…, which will be much briefer, I'm going to discuss the issue not of extinction per second, but of organization of mammal communities.

So first I'm just going to summarize some points that I believe most of the speakers this morning can agree on. We've already seen that the North American event was coincident with both the deglaciation that was very rapid and extreme and with the first appearance of the Clovis hunting technology. That might not mean first appearance of humans, but it certainly is an interesting archeological event. We know that the extinction was probably very rapid -- perhaps took a few hundred years, perhaps a little more than a thousand. We know that there was intense selectivity targeted at large herbivores -- the larger, the more extinction. We know that the event occurred everywhere in the continent, from all the way in the north to Florida, to Arizona, and that a similar event happened all the way down through the Americas, all the way to Patagonia. …

And the other point that's been raised by Paul Martin already is that this deglaciation event was not unique in the Late Pleistocene. …

So, to me, at least, this last point is a fatal problem for those who believe that climate and habitat change, by itself, was responsible for the extinction event.

That's not to say that climate and habitat change didn't play any role -- just that I don't believe they played the sole role in the event. …

So the basic idea is, we take these data and we turn them into an index of disharmony. The more red lines, the higher the index. We know that this means something – it isn't just counting angels on the head of a pin – because there's a lot of disjunction and a lot of conjunction in the recent. …. So, relatively speaking, disharmony is a very rare pattern. We see the same thing throughout the six time slices, going from oldest to youngest. That pattern does not change very much, even across the mass extinction event here. It persists into the Late Holocene. …. And we can show that by doing the same analyses on all pairs of the intervals. We see the same types of correlations over and over again, so essentially we see the same patterns persisting through time. And what that means is that disharmony is not an unnatural feature of the last glacial – it's a natural feature of the Quaternary. And, …..f)…, what might be truly remarkable is range contractions in the last 500 years. Those might be responsible for the patterns of disharmony, and, essentially, the disharmony issue might be a red herring and not really germane to the extinction event.

So,…..g)… all of these points, the extinction rates curves show that the end-Pleistocene event was among the worst ever in the Cenozoic. No other event before the end-Pleistocene was strongly selective for any group, such as large herbivores. We know that patterns of disharmony are real. They're relatively rare, although absolutely common. They're very predictable from one time to another; they're very constant through time. And what that means is, that the survivors of the event were not reorganized spatially – they just kept on trucking -- and that makes it very hard to understand how habitat and climate changes could have done very much to the victims, whose ranges were changed and were, in fact, reduced to zero.

Task 14. Some linking words and phrases are missed. Fill in the gaps:

1. First of all; 2. in other words; 3. secondly; 4. in the second half of the talk; 5. finally; 6. to sum up; 7. in the first half of the talk.

Task 15.

a) Find the sentences which define the purposes of the work.

b) What useful phrases are used to find main points of the presentation?

c) What words are used to summarize the ideas?

Date: 2016-01-05; view: 1665