During the Pleistocene, between 1.8 Ma and 0.01 Ma, the earth witnessed large climatic fluctuations. Both plants and animals on all continents showed large shifts in their geographical distribution during this period but the extinction levels were not above background values (Alroy 1999). This is true for such disparate taxonomic groups as insects, amphibians, reptiles, birds, and mammals. Yet there is one major exception to this rule: the so-called mammalian megafauna (animals > 44 kg).
Fifty thousand years ago, more than 150 genera of this megafauna were populating the continents, but 10,000 years ago, at least 97 of these genera were extinct (Barnosky et al. 2004). There have been continued debates over whether these extinctions were caused mainly by environmental changes associated with climatic fluctuations, or were the consequences of human impacts. It is undisputed that humans were responsible for the extinction of large mammals and large birds on islands such as Madagascar, Antillean, Mediterranean, East Asian Islands, and New Zealand (Barnosky et al. 2004; Burney and Flannery 2005). Here hunting and habitat fragmentation led to extinctions even in the absence of climatic change.
An important aspect is that mammalian megafaunal species on all continents became extinct but both magnitude and timing of the extinctions differed between continents (Roy 2001; Barnosky et al. 2004). The extinctions were most severe in Australia where 14 out of 16 (88%) giant marsupials succumbed. In addition, all seven genera of megafaunal reptiles and birds went completely extinct (Barnosky et al. 2004). Humans arrived on that continent somewhere between 71,500 and 44,200 years ago, and most megafaunal species became extinct before 40,000 years ago (Barnosky et al. 2004). In North America, 33 genera (72%) went extinct (Roy 2001) within a short time interval between 11,500 and 10,500 years ago, closely correlating with the arrival of Clovis-style hunters (Alroy 1999; Roy 2001; Barnosky et al. 2004). In South America, 50 genera (83%) vanished during the arrival and spread of humans about 12,900 to 10,000 years ago. In Eurasia (excluding southern Asia), 9 genera out of 25 (36%) became extinct during two pulses (45,000-20,000 years ago, 12,000-9000 years ago). These extinction pulses correlate also with the spread and then the population increase in modern humans (Barnosky et al. 2004). In Africa, the losses were relatively mild with only 8 genera (18%). When considering only mammals > 1000 kg, the differences between continents are even more marked. In North America, all four genera were lost, Eurasia saw the demise of four out of five, but in Africa, no such genus went extinct (Roy 2001).
Many phases of human colonization were coeval with marked climate changes. Because earlier, similar climate changes were not accompanied by marked extinctions, hunting by humans (overkill) was proposed as the main mechanism responsible for the extinctions (Martin 1984), either by heavy and selective hunting ("Blitzkrieg") or through habitat fragmentation, nonselective hunting and the introduction of exotic species ("Sitzkrieg"). The current consensus picture for megafaunal extinctions on the continents is that extinctions were most pronounced where a rapid spread and increase in H. sapiens populations coincided with marked climatic shifts (Burney and Flannery 2005). But it was not primarily large sized but rather slow breeding species (this is, in part, correlated with large body size), which were at the highest risk of extinction (Johnson 2002). Even if the proportion of deaths caused by humans was low at any one time, slow breeding megafaunal mammals were driven to extinction.
Yet the story is not over. Since the age of colonialization, the exploitation of nature has reached a new level, and the fate of the dodo (Raphus cucullatus) is just one very sad and telling example of human impact. This flightless bird was discovered in 1598 on Mauritius after Portuguese sailors first reached this island in 1507, and became extinct by 1690, not only by hunting but also by the introduction of domestic species, such as goats, pigs, and rats, which devoured the eggs and the young. The list goes on, and even extremely common species like the passenger pigeon in North America (Ectopistes migratorius) have proven to be no match for intensive hunting humans. Starting during the epoch of industrialization, exploiting nature and destroying natural habitats has proceeded at an ever increasing rate (Wilson 1994). There is no question that current extinction rates for plants and animals have reached a level perhaps 2-3 magnitudes above background rates (Nott et al. 1995; Pimm et al. 1995; Ricketts et al. 2005). Scaling the available estimates of current extinctions up to a magnitude where we can compare them to past Phanerozoic mass extinctions reveals that if species losses continue at the present rate, 96% of the species will be extinct within just a few hundred years (May et al. 1995; Sepkoski 1997; "Sixth'' extinction). This is the maximum estimate for species losses during the most severe of all the Phanerozoic mass extinctions, the one that occurred at the Permian-Triassic boundary! Yet although the end-Permian mass extinction is no longer seen as a crisis spanning millions of years, current estimates are still on the order of a hundred thousand years. There is even further concern. As several episodes of mass extinctions have shown, even a moderate increase in global temperatures of a few degress, if it happened fast enough, has proven fatal for life on the entire planet.
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