-
9 Extinction
- Indiana University Press
- Chapter
- Additional Information
317 9 Extinction It might be said that humans should be as concerned with the extinction of species as much as with death. They are expressions of the same phenomenon at different scales of the hierarchy of life. To add appeal to the subject, the suggestions of extraterrestrial causes, such as asteroid or comet impacts, first to explain the demise of nonavian dinosaurs and then other groups of organisms, are in marked contrast to the more traditional models, such as gradual climate change, advanced to make sense of extinction (Alvarez et al., 1980; Pope et al., 1996). Although science strives for consensus (most scientists today accept that a single extraterrestrial impact scenario was the main trigger leading to the end of nonavian dinosaurs), it seldom achieves unanimity: Keller et al. (2009) found evidence in the Brazos River, Texas, USA, that the extinction occurred over several hundred thousand years at the end of the Cretaceous, a hypothesis recently strengthened by the identification of a second crater in Ukraine (Jolley et al., 2010). In any case, such events serve as strong reminders that a catastrophe from the far reaches of outer space and far beyond our collective human experience has occurred in the geologic past and that, given time, will occur again. The recent controversies and debates over which models best explain the nearly wholesale disappearance of organisms is a main reason why the study of mass extinctions has since experienced a surge of interest, both among scientists and the general public, and new ideas are being developed to understand causes, processes, and patterns. However, extraterrestrial impacts are not considered by many scientists as the main reason for the devastation most continents suffered during the last several dozen thousands of years, during which large (>44 kg, Martin and Klein, 1984) and very large mammals (>1000 kg, i.e., megamammals) were largely wiped from the face of earth except in Africa and, to a lesser extent, southern Asia (Cione et al., 2009). South and North America were particularly affected, and the splendid fauna this book deals with completely disappeared 10,000 years ago. Although this extinction episode saw the fall of many mammals, it does not fit the typical pattern of mass extinctions because it was only the large mammals that were affected; with the exception of a few smaller mammals, no other animal or plant became extinct (Cione et al., 2009). The various proposals put forth to explain the extinction events that occurred at the end of the late Pleistocene of South America fall into several categories: exclusively or primarily climate driven, exclusively or primarily human driven, some combination of climate and human activity, and extraterrestrial (a relative newcomer to the game, and generally discounted). These will be critically discussed later in this chapter , but we will first deal with one of the suggested biotic causes: the commonly held view of superiority of the North American fauna. This topic specifically addresses the reasons why mammals of North American origin Megafauna 318 seem to have done so much better than their South American counterparts, and although it also involves extinction, it does not provide explanations for the wholesale disappearance of megafauna from the Americas at the end of the Pleistocene. We noted in Chapter 5 several of the variables that have been used to assess the relative success of the North American immigrants (NAI) versus the South American immigrants (SAI). In that discussion, it was made clear that the manner in which we tabulate the number of clades (ranked traditionally as families) that headed either north or south has a strong influence on our interpretations of determining success. We also made the point that the discrepancies between NAI and SAI essentially disappear if we choose (as we do) to include primates and tree sloths in our evaluation of success in migration. However, migration success is not the sole basis on which earlier interpretations of the inferiority of southern mammals were set forth. The other main factor has to do with what eventually happened to the SAI versus the NAI and the contributions they made to the composition of their adopted homeland at and subsequent to the end of the Pleistocene. We have noted in other chapters of this book that there was a major extinction episode near the end of the Pleistocene. It has long been recognized that the extinction episode was unequal and that the South American fauna itself (as opposed, in this case, to the SAI) was hit harder and replaced...