Image: CLARE FLEMMI
What killed the mammoths and other behemoths that once roamed the Americas? This mammalogist thinks it may have been hyperlethal disease
Image: CLARE FLEMMI
To me these are startling examples of what diseases can do, and the only reason people haven¿t heard about it is because they¿re affecting species that are not all that visible, living in obscure places. It¿s the scientists who are ringing the Klaxon right now. What I think would focus people¿s attention would be if one of these diseases that are now emerging in African wildlife right now caused an outright extinction. African wild dogs in the Serengeti have been essentially wiped out by canine distemper transferred from domestic dogs. The wild dogs still exist in small numbers elsewhere in central Africa, but for a very large part of their original range they¿re bye-bye. It¿s a clear and present danger, and when you start putting these individual examples together, they in fact amount to something.
What relevance does this have for the Pleistocene? If diseases are emerging at a greater rate now, thanks to translocation due to people, then doesn¿t it make sense that when you have people beginning to translocate from Africa and south Asia, where Homo sapiens is ancient, that they¿d be carrying new things with them, in the form of their biological baggage? All kinds of organisms¿including pathogens that they may or may not have known about¿could have been brought to places where humans had never been previously resident. If these examples I¿ve been talking about are meaningful, when immunologically na¿ve species are faced with the sudden introduction of pathogens that they have had no experience with whatsoever, a very typical outcome seems to be very high levels of mortality¿even to the level of complete extinction. Generalize outward from that and there is nothing else in nature that we know about, short of a cometary impact, that could take out the number and kind and distribution of species in North and South America 11,000 years ago that we¿ve been talking about.
If the North American end-Pleistocene extinction was a focal extinction, if it happened only along the western seaboard of North America where people came in first, I could accept that people were completely responsible¿I wouldn¿t see the need to have an alternative explanation. If the extinction event affected single groups¿like all the elephants all over the planet dying out¿I¿d think, Well, that¿s kind of peculiar, but maybe people, for one reason or another, are elephant mad and decided they didn¿t want any more elephants around. But when you look at the continental extinctions in the New World you have upward of 130 species disappearing in a time period of maybe half a millennium or less, all the way from north slope of Alaska down to Tierra del Fuego and in every kind of environment in between. Is there anything on Earth that we know about that could provoke losses on the scale necessary in such a limited time period, to affect populations wherever they existed? Disease is the only thing I know of that is part of the natural landscape that could possibly do it.
In imagining such diseases, ones that can spread very widely and are very host tolerant, I¿m going to the edge of what we know is possible. But we do know from the recent past that there are such diseases. The rinderpest epidemic in East Africa at the beginning of the 20th century took out millions of individual African bovids¿this would include wildebeest, hartebeest, bongos, the whole range. Ecologists have been looking at this and have decided that there were certain kinds of ecological change that were induced by the catastrophic loss of bovids around 1900 from which the forests and other landscapes of East Africa have still not recovered¿which is amazing when you think what that means in terms of the loss of individuals, and the slowness with which the species have bounced back. A number of other diseases show much the same tolerance of hosts; rinderpest is not unique.
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