Climatic Change versus Overkill
I will lay aside impartiality. I think the overkill theorists have the more convincing argument for what happened in America 10,000 years ago. It seems that the Clovis people spread through the New World and demolished most of the large mammals during a hunters' "blitzkrieg" spanning several centuries.
Despite accumulating evidence that humans caused the megafaunal extinctions, some members of the climate-change school are in deep denial (Grayson and Meltzer 2002, 2003). A cadre of archaeologists, especially those who claim or prefer to believe that people were in the New World before the extinctions began, agrees with them. In addition, many vertebrate paleontologists of my generation, born in the first third of the twentieth century, support the climatic paradigm.* Although details of the process are rarely available, climatic change is often the answer to the question of what accounts for all the numerous extinctions lacing the fossil record.
*Vertebrate paleontologists who have supported climatic extinction models, at least in the past, include Elaine Andersen, Tony Barnosky, Russ Graham, Don Grayson, John Guil-day, Dale Guthrie, Claude Hibbard, Ev Lindsay, Ernie Lundelius, Larry Martin, Jeff Saunders, Bob Slaughter, and Dave Webb. For the Martin-Grayson debate, see "Clovesia the Beautiful," pp. 39-64 in Russell 1996.
The overkill model remains controversial, to say the least. (My department head once inquired, in what I hoped was a friendly tone, "Hey, Paul! How far out on that extinction limb do you think you can go?") On the other hand, while those archaeologists who pin their faith on the existence of pre-Clovis human populations in the New World are especially prone to oppose the concept (for example, Grayson and Meltzer 2003), one or another version of an overkill model finds traction among researchers active in many fields.*
Proponents of various views, including climatic change and overkill, are chapter authors in Martin and Wright 1967 and Martin and Klein 1984; the debate has been with us for some time. It points to some fundamental differences in methodology, outlook, attitude, and training that could be as illuminating as the final answer to the extinction controversy itself. In some cases the approaches taken may be personal, based on views or agendas that go decidedly beyond the narrow issue of extinction and its causes. And some researchers favor a mixture of climate change and human hunting as the functions forcing extinctions in near time. Their approach is politically less risky, but also less testable. I believe it is redundant. If the climates are always changing, climatic change is inescapable. And as John Alroy (1999) and Lyons, Smith, and Brown (2004) show, size made all the difference in terms of which animals went extinct.
Many who support the climate-change theory, like my one-time committee member Claude Hibbard, are vertebrate paleontologists who have thousands of extinctions to account for in the Cenozoic, the majority long predating the time of Homo sapiens. In northern Eurasia and especially in North America, both range changes and animal extinctions occur during or close to the time of the Younger Dryas cold snap, recognized widely at least in the Northern Hemisphere. In addition, archae
*In anthropology these include Fiedel 2003; Fiedel and Haynes 2003; G. Haynes 2002a, 2002b; Mithen 1997; Porcasi, Jones, and Raab 2000; Redman 1999; Waguespack and Surovell 2003. In addition, a partial scan of other disciplines reveals the following publications that elaborate on or apply the model or theory of overkill. The response involves the following fields: biogeography (Brown and Lomolino 1998; Pielou 1991), conservation biology (Kay 1994, 1998; Kay and Simmons 2002), cultural studies (Sayre 2001), economics (V. Smith 1975, 1999), history (Flores 2001; Sheridan 1995), geochemistry (Gillespie 2002), historical ecology (Burney 1993; Diamond 1992, 1997; Lyons, Smith, and Brown 2004), holistic management (Bonnicksen and others 1999; Burkhardt 1996), marine biology (Dayton 1974; Simenstad, Estes, and Kenyon 1978), paleobiology-paleontology (Alroy 1999, 2001; Azzaroli 1992; Boulter 2002; Fisher 1996; Flannery 2001; Holdaway and Jacomb 2000; Merrilees 1968; MacPhee and Marx 1997; Raup 1991; Steadman 1995, n.d.; Ward 1997), popular works (Barlow 2000; Lange 2002; Leakey and Lewin 1995; Wilson 1992), and zoology (Coe 1981, 1982; Janzen 1983; Owen-Smith 1989, 1999).
ologists note that there are no more than a few kill sites of large extinct mammals in North and South America.
Certainly climate changes can reduce, increase, or shift species' ranges; reduce or increase the availability or nutritional quality of forage; change the length of seasons; and otherwise regulate animal populations. Overall, however, the climate-change proponents seem to me to assume their conclusion rather than to prove it. Just saying that "climates change and cause extinctions" does not make it so. In any case, climatic change is invoked whenever it is necessary to explain an otherwise mysterious extinction event, and paleontologists have lots of those. In addition, talk of climate change has the advantage of not distressing those concerned with cultural sensitivity—some racial groups could suffer bad press if the word gets out that their ancestors might have helped to exterminate moas, Megalania, mammoths, or megatheriums. This may be a much more serious problem for social scientists than Earth scientists, although all aspire to cultural sensitivity. In this chapter I will outline some of the climatic evidence, showing why it offers little if any help in accounting for the extinctions in near time. Historically, droughts like the ones in Kenya and other parts of Africa have had a huge impact on numbers of large herbivores. So have poachers. So has disease. The climate is always changing, so we can always bring it into the theater of explanations. I claim that human invasion of empty lands, a unique event in different parts of the world in near time, is an overriding variable that explains much more than these other factors.
Any plausible climatic explanation for these extinctions must meet three criteria. First, the evidence must show that there in fact was significant climate change around the various times of the extinctions in the various places where they occurred. Second, the change (or changes) must, alone or in combination with other factors, have been unique in the Quaternary. A change closely resembling others that the megafauna had repeatedly survived, like drought, is not a good candidate in the search for explanations of extinction. Third, the change must have been one capable of striking large terrestrial mammals while sparing most other terrestrial animals, as well as plants and marine life. Unfortunately for its proponents, the climate-change theory meets none of these criteria.
To take the most basic first, the record simply does not show significant climate change spreading across the globe in synch with the time-trans-gressive extinctions of near time, which, as we have seen, progressed from Australia to the Americas to oceanic islands over a period of roughly 50,000 years.
One change frequently posited to have caused the extinctions is known as the Allerod-Younger Dryas oscillation. As revealed in ice cores, fossil pollen diagrams, and fossil beetle faunas, around 15,000 to 12,000 years ago, the climate of western Europe suddenly switched back to glacial cold (known as the Younger Dryas) after a few thousand years of almost interglacial warmth (the Allerod). For a millennium at most, in Scandinavia during the Younger Dryas glacial ice and periglacial tundra readvanced. (A member of the rose family, Dryas is one of the tundra herbs preserved in late-glacial silts and clays.) Then there was a rapid reverse switch to postglacial warmth, which saw the spread of woodland over the formerly treeless periglacial landscape in Europe, Siberia, and the eastern United States. In brief, the evidence for important vegetation change, as I know it, is strongest in western Europe, where those digging for peat as fuel revealed stratigraphic sections of warmth-loving plants below and above a clay unit with Dryas octopetala and other tundra herbs that supported the argument for a reversal to glacial cold. Nothing this dramatic can be detected in the western United States, where thousands of packrat middens indicate climatic change from cold to warm, without an obvious reversal.
Even in the Old World, where its climatic signal is strongest, the Younger Dryas is not closely tied to sweeping megafaunal extinction. In the arctic and subarctic, the last of numerous records of woolly mammoths, for example, postdate the Younger Dryas by thousands of years and are not aggregated in a way suggesting simultaneous extinctions (MacPhee and others 2002). In their search for ancient pathogens in frozen tissues or well-preserved bones in the Siberian permafrost of the Taimyr Peninsula, various investigators, both Russian and foreign, have accumulated a wealth of new radiocarbon dates on Eurasian megafau-nal extinctions and range changes.
For that matter, it is not clear that the Younger Dryas could have forced any European or Russian megafaunal extinctions. For some time, zoologists viewed the Younger Dryas as having forced the extinction of the Irish elk or giant deer, Megaloceros, which vanished from Ireland, long thought to be its last refuge, around the Younger Dryas (Barnowski 1986; Stuart 1991). Recent fossil finds, however, complicate the story. Giant deer have now been dated to 9,200 radiocarbon years on the Isle of Man and to 9,400 radiocarbon years in Scotland, 1,400 years later than the climate-driven cold millennium of the Younger Dryas (Gonzalez, Kitchener, and Lister 2000). In the subarctic, woolly mammoths and musk oxen lasted for thousands of years after the Younger Dryas (MacPhee and others 2002).
What about the New World? Here, the paleobotanical evidence for a late-glacial readvance of tundra 11,000 radiocarbon years ago is less apparent than in the Old World. The Allerod-Younger Dryas shift simply may not have been as sharp here, particularly in the arid Southwest. Some regions, such as Arizona and adjacent states, show gradual, not sudden, transformation from glacial to postglacial types of upland vegetation (Martin 1999). In western North America both fossil pollen (Anderson and others 2000; Weng and Jackson 1999) and fossil packrat midden records (Betancourt, Van Devender, and Martin 1990) fail to indicate a sudden sharp switch from warm to cold conditions comparable to the Allerod-Younger Dryas switch in western Europe. In South America, neither microhistology nor pollen analysis of ground sloth dung indicates significant change in local plant ranges around the time of the extinction of the Shasta ground sloth. Though the climate changed more dramatically in northern North America, the number of extinctions was no higher there (Monastersky 1999).
In addition, while the Allerod-Younger Dryas switch at least coincided chronologically with the megafaunal extinctions in continental America, it was out of synch with those on other landmasses. It occurred much too late to account for the extinctions in Australia, Europe, and much of Asia, and left no imprint on the oceanic islands. Unlike the time-transgressive prehistoric arrival of human colonists, climate changes do not, on a global scale, provide a close fit to severe near-time episodes of extinction. The Australian large-animal extinctions, for example, are 30,000 years older than those in the Americas. It is difficult to see how any single climate change could have caused them both, and indeed there is no evidence of such a change; Australia had neither glaciers nor rapid postglacial warming (Ward 1997, 152). On both landmasses, however, the extinctions coincide closely with the widely disparate times of human arrival.
The West Indies provides another test. As discussed above, the fossil record suggests that at least 4,000 years after megafaunal extinction in North America, Cuba and Haiti still harbored diminutive endemic ground sloths. Evidently, then, the hypothetical climatic catastrophe wiping out the full-size ground sloths in both North and South America will not account for the much later extinction of the small Cuban-Haitian sloths (though it is hard to imagine how they could have escaped). A consistent theory would have to invoke a second climatic or environmental crisis that selectively swept away dwarf Cuban and Hispaniolan ground sloths thousands of years later, at a time when few if any extinctions of animals of any size are known on the mainland.
Finally, no global model of large-mammal extinctions corresponding to the Younger Dryas or other cold reversals or cold stages recognized in the Northern Hemisphere yields examples in New Zealand. The last glaciation, known in New Zealand as the Otiran, affected climates and vegetation on both North Island and South Island, and the postglacial warming did so as well. The habitats and ranges of moas shifted accordingly. Nevertheless, there is no record of extinction of moa species until about 500 years ago (Bunce and others 2003). (An earlier extinction episode struck the terrestrial fauna of New Zealand around 2,000 years ago, when Pacific rats arrived; see chapter 6.) If a climatic catastrophe forced extinctions in Australia around 46,000 years ago, and in North and South America 10,000 to 12,000 radiocarbon years ago, neither event registered in New Zealand. The avian extinctions accompanying colonization on eastern Pacific islands also occurred thousands of years after the Younger Dryas and other major late-glacial climate changes (Martin and Steadman 1999). They appear to coincide with Polynesian landfall, sometimes in two stages.
The climate-change model fails not only the test of time-transgres-siveness in the near-time extinction pattern but also that of uniqueness. Quite apart from the Allerod-Younger Dryas switch, the climate was indeed changing in various parts of the world throughout the Quaternary. There is nothing unusual in that. Proxy climatic data from ice cores and other sources show that ice age climates were continually and rapidly fluctuating; the late Quaternary was a flickering switch of climatic change. Indeed, near time and the Quaternary have been so variable that the expression "climatic change" is redundant (Hughen and others 1998).
Climatic changes earlier in the Quaternary apparently equaled those of the late Quaternary in amplitude (Porter 1989). For example, Greenland ice cores show that climatic changes like the Allerod-Younger Dryas-Preboreal switch, from warm to cold and back again, had repeatedly occurred earlier (Dansgaard and others 1993; Alley 2000). The displacements downward and southward of many species during the last glacial age reflect perhaps a drop of six to nine degrees Fahrenheit (3.3 to 5 degrees Celsius) in mean annual temperature and a decrease in carbon dioxide. These are evident in the fossil record from ice cores, marine cores, pollen diagrams, and changes in sea level due to the expansion or contraction of glacier volume. By 8,000 to 10,000 radiocarbon years ago, most of the present climatic gradient was back in place. The ice cores (Alley 2000) show that over 100,000 years there were more than two dozen reversals like that of the Younger Dryas, with very rapid changes of at least 18 degrees Fahrenheit (10 degrees Celsius) from warm to cold and cold to warm.
Those earlier changes, however, were unaccompanied by extinctions of large mammals, which we may assume evolved to deal with a wide range of climatic variation. New World equids, camelids, and proboscideans, for example, proliferated over tens of millions of years of constantly changing climates. Even the small ground sloths and other insular species of the West Indies would have been selected for survival under widely varying climatic conditions, otherwise they too would have gone extinct during the start of the ice age, long before near time.
The range of some large animals lends further support to this conclusion. Modern-day elephants, for instance, occupy many climate belts in Africa, from the Skeleton Coast and dunes of Namibia to the rain forests of the Congo and the savannas beneath Kilimanjaro. The woolly mammoths occupied northern Eurasia and northern North America; the Columbian mammoth's range was transcontinental, from Alaska south throughout most of the United States, and went from an elevation of 9,000 feet in the mountains of Utah to sea level in Florida and Mexico. It seems unlikely that such adaptable animals could have been totally wiped out by even the most severe weather conditions. Indeed, there is some direct evidence that the climate was not a problem for them; in one case the tusk growth of a mastodon from Michigan suggests favorable environmental conditions close to or at the time of mastodon extinction in that region (Fisher 1996).
Given the large mammals' demonstrated ability to survive the "normal" severe climate swings of the Pleistocene, those favoring climatic change as the primary cause of the extinctions must invoke some unique event (or time-transgressive series of events). Unfortunately for them, nothing unique jumps out from the wildly fluctuating changes reflected in the marine and ice cores of Greenland and Antarctica in near time (Dansgaard and others 1993; Alley 2000). A global catastrophe, something off the scale, would surely have registered in these ice cores. In addition, we should see such a catastrophe in the fossil record, and we do not. The last shift from a glacial to an interglacial climate saw the last major change in the regional distribution of desert plants and animals. However, rather than evidencing a sudden, widespread shift in vegetation types, which might reflect some sort of unique climatic crisis, the midden record changes gradually, over thousands of years, as species favoring cooler climates are replaced by those favoring warmer ones (Be-tancourt, Van Devender, and Martin 1990; Martin 1999). In mountain ous country the late-glacial-age shifts of 2,000 to 3,000 feet shown by fossil pollen and midden records could have been traversed easily by large herbivores in less than a week, if not a day.
In brief, while the environment south of the ice margin fluctuated and at times changed dramatically, there is nothing to suggest that those changes alone could have forced, for example, the extinctions of over 30 genera of large mammals in North America. The best negative evidence comes from the repeated sudden, severe climatic switches, unaccompanied by extinctions, seen in over half a million years of change in ice cores. There were many such switches beginning long before extinctions struck.
The taxonomic selectivity of the late-Quaternary extinctions poses a third problem for the climate-change model. With one possible exception, no tree or shrub extinctions are evident in the near-time fossil record. In the western United States, rich in well-dated fossil deposits of extinct large mammals, no contemporary plant extinctions are reported. The recent discovery and description of an extinct and a highly displaced species of spruce, Picea critchfieldii, from the southeastern United States (Jackson and Weng 1999) is a reminder of how unusual plant extinctions have been in North America since the mid-Cenozoic. In contrast, extinctions or extirpations of numerous plants, including temperate genera of trees, occurred in the Pliocene and early Pleistocene of Western Europe. These are thought to reflect climatic change at the end of the Tertiary, when temperate species were barricaded by the Alps from any easy retreat southward during times of glaciation and the spread of tundra-taiga south to the Alps (Leopold 1967).
Birds, small terrestrial mammals, marine mammals, and most beetles were much less vulnerable to whatever caused the extinctions of the large terrestrial mammals. Beyond scavengers of large mammals, the extinction wave left no imprint on the fossil record of birds or small mammals, both well represented in Stanton's Cave and many other deposits in the Grand Canyon and the Colorado Plateau. There were no marine extinctions in near time, though these typified mass extinctions in deep time. On islands off California, pinnipeds did suffer local depletions when their rookeries were hunted out (Porcasi, Jones, and Raab 2000). This depletion was episodic, not systemic, and the seals and sea lions recolonized after human hunters left to seek resources elsewhere. The rich fossil record of Quaternary beetles is sensitive to climatic change but lacks much evidence of extinction. The few extinct genera (Coope 1995) are mainly of coprophiles (scarab or dung beetles), suggesting coextinction of scarab beetles and megafauna. That is, when megafauna declined globally, es pecially in the Americas and Australia, there was a major reduction in dung deposition and thus a major shrinkage of dung beetle habitat.
Large animals are more vulnerable to hunting than smaller ones—they are generally easier to track, locate, and spear, and they reproduce more slowly, making it more difficult to replace their losses. In short, the fact that near-time extinctions struck almost exclusively large terrestrial mammals is in accord with the view that early hunters, not some climatic crisis, were mainly responsible.
Some have attempted to address the selectivity problem by arguing that climate change favored forage plants that were less nutritious for the large herbivores. As those herbivores died out, their predators and scavengers followed. A sudden, wrenching change in climate might hy-pothetically have reduced North American populations of trees and shrubs, replacing them with grasses and forbs. Such a massive disturbance might have reduced foraging opportunity for mastodons, which generally preferred woodland, riparian corridors, or forested habitat. At the same time, it would have favored mammoths, horses, pronghorn antelope, and other grazers. (Even mastodons, for that matter, ate some grasses.) Moreover, some of the known near-time changes in forage simply replaced one type of woodland with another, both suitable for the browsers of the time (Martin 1986, 124). And in general, large mammals find disturbed or successional vegetation, to be expected during postglacial warming, to be suitable forage (Martin 1990, 196).
Before the diets of large herbivores were well known, the extinctions were thought to have accompanied a shift from winter-rain grasses to summer-rain grasses resulting from the postglacial increase in summer rains (Kurten and Anderson 1980). This theory is no longer accepted, however. Whether they require winter or summer precipitation does not compromise the forage value of the grasses; both types can provide good pasture. Mammoths, for example, ate both; they and other grass eaters were foraging on summer-rain plants in Arizona (Connin, Betancourt, and Quade 1998) and Florida (Koch, Hoppe, and Webb 1998) long before extinction struck. Summer-rain grasses evolved over five million years ago and are widely consumed by large herbivores, wild and domestic, in tropical and temperate latitudes north into Canada. Theoretically, then, climate change favoring either winter- or summer-rain grasses would be tolerable to large grazers.
Another indication that a shift in grasses does not explain the extinctions comes from Australia. Based on a variety of fossil eggshells of Geny-ornis (a flightless bird larger than an emu), Giff Miller and others (1999)
date its extinction at 50,000 ± 5,000 years. They report that near that time, Genyornis in different parts of its range fed on either winter-rain grasses, shrubs, and trees or on summer-rain grasses.
Finally, it is difficult to imagine a single climate switch that would have affected all the large mammals in any given location simultaneously. Despite overlapping ranges, different species had different climatic preferences. For example, ground sloths were originally tropical, Harrington's goats boreal. A warmer climate should have been suitable for the sloths, a cooler one for the goats. Yet in the Grand Canyon they vanished at the same time. Climate-change proponents have offered various hypotheses to address such objections. Those with which I am familiar do not hold up to logical analysis. One, which argues for a shift from continental to maritime climates and back again, is well illustrated by a trio of related examples: the extirpations of tapirs (Tapirus) and vampire bats (Desmodus) in the United States and the simultaneous northward range shift of marmots.
Fossil records show that in the late Quaternary, marmots were found south of their present range, as at Rampart Cave, where they occurred at 1,500 feet, a very low elevation for marmot habitat even then. To climate-change proponents, this indicates that conditions were cool enough to bring marmots south from northern New Mexico and Utah, where they occur at present. In fact, fossil packrat midden records (Be-tancourt, Van Devender, and Martin 1990) do indicate an overall cooler condition during the last 40,000 years, until the early Holocene, 8,000 years ago.
On the other hand, the late-Quaternary disappearance of tapirs from Florida, California, Kansas, and Arizona (from the Sonoran lowlands to over 6,000 feet on the Colorado Plateau) is often attributed to a climate change in the opposite direction (E. Anderson 1984; Kurten and Anderson 1980). Three species of tapir and vampire bats survive in the tropics of southern Mexico, through Central and into South America. Assuming that the present is the key to the past, many vertebrate paleontologists have surmised that for tapirs to have reached temperate latitudes in near time, the climate must have been warmer and the winters frost-free, as they are where many tapirs live now. The fossil presence of vampire bats in Arizona and California, north of their historic range (which covers tropical Mexico and points south), has also been taken to indicate late-Quaternary warming. Some of these bats were Desmodus stockii, an extinct species or population roughly 10 percent larger than the living D. rotundus.
To accommodate the movements of all three animals, paleontologists favoring climatic explanations have had to indulge in some contortions: the marmots came south because the summers became cooler. The bats moved north because the winters became warmer.
But neither vampire bats nor marmots live in coastal California now, in a climate free of the hot summers and cold winters of the continental interior. (In the late Quaternary vampire bats occurred on one of the Channel Islands off the coast near Santa Barbara, along with dwarf mammoths.) Moreover, plant fossils from packrat middens in Arizona and adjacent states do not suggest a more maritime climate in the late Quaternary. None of the packrat midden deposits at Rampart or nearby caves harbors plant assemblages typical of the California coast, such as soft chaparral. There was also no widespread northerly penetration of frostsensitive tropical plants to match the range expansion of tapirs and vampire bats. Indeed, all the middens found in the Southwest (postglacial, late glacial, and full glacial) reflect arid conditions and a climate that while cooler was just as continental as it is now (Betancourt, Van Devender, and Martin 1990). When vampire bats haunted Rampart Cave and tapirs roamed the Colorado Plateau, they did not bring the rest of the tropics with them.
Some attribute the megafaunal extinctions to a shift from hypothetical warmer winters and cooler summers to the reverse (Hibbard 1958). The model shares the failings of other climate-change theories. It requires a unique climatic event, and one to which not only small but also large animals would have been vulnerable. In addition, it implies that large mammals were intolerant of continental as opposed to maritime climates. This is discordant with the present ranges of large mammals, which include some of the more continental climates known on Earth, such as those of the Himalayan and Mongolian plateaus.
The extinction of the large mammals, the likely source of the vampire bats' blood diet, probably did affect the vampire bats that once lived in Arizona and California. The miracle is that the vampire bats survived in the tropics. Very likely they multiplied and expanded their range with the historic introduction of livestock. As for the tapirs, it is more reasonable to model their late-Quaternary reduction in range as the result of confinement of refugial populations to tropical forest, where they were less vulnerable to new predators (people) than tapirs in more open country in temperate latitudes. A similar explanation may account for the fact that capybara (Hydrochoerus) and the spectacled bear (Tremarctos) are now limited to the tropics.
Finally, what about the marmots? Why did they not survive in Arizona and southern New Mexico? By the Holocene those regions had sizeable permanent human populations, and marmots are known to be popular prey historically. Historic range expansions northward of opossum, armadillos, and javelina, all attractive prey, may reflect similar change, with range expansion accompanying relaxation of hunting pressures since contact. I suspect that if introduced from the Rocky Mountains, marmots could be reestablished in Arizona.
The outstanding revolutions in the history of life—the global extinction of dinosaurs at the Cretaceous-Tertiary boundary roughly 65 million years ago and the extinction of many large mammals at the end of the Quaternary—were both long believed to reflect changes in global climate. Both can now be attributed to catastrophic perturbations unrelated, or only weakly related, to intrinsic climate change originating on our planet. John Alroy finds that "a consensus is forming that the end-Quaternary extinctions were caused largely, or possibly solely, by human impacts" (Alroy 1999). I totally agree.
Of course, the overkill model need not imply that climate change has never forced an extinction. In North America, for example, over the last 65 million years (the age of mammals after extinction of their Mesozoic prototypes), roughly 2,000 or more mammalian genera went extinct before our species arrived on the continent. Climate change is commonly invoked as the cause of these extinctions, and that may well be true in many cases. There are exceptions. Storrs Olson and David Wingate (2001) regard rise in sea level and associated drowning of the sizeable Bermuda platform as the cause of extinction of a newly described large flightless rail in the King Rail-Clapper Rail group, Rallus recessus. Their conclusion is supported by the absence of any historic accounts of the bird, and of any indication that prehistoric voyagers discovered and colonized the Bermuda platform.
Whatever exceptions those of us supporting the overkill model may be willing to make based on the field data, defenders of the climate school seem to feel they must warn the good people of Hamlin that Pied Pipers are dancing off with their intellectual children. For example, Don Grayson (2001) insists that those who study late-Quaternary extinctions most intimately, namely his generation of vertebrate paleontologists, strongly support a climatic as opposed to a cultural or some other explanation. Grayson's earlier objections intrigued one of his colleagues, paleontologist Peter Ward. In his book The Call of Distant Mammoths, Ward quotes Grayson as follows:
The results of that search [for reliable radiometric age dates on the extinct mammals] strongly suggest that overkill could not have been the force that Martin has claimed. The differential appearance of kill sites (only proboscideans, and within the proboscideans, almost only mammoth) and the strong hints that many of the taxa involved may have been on their way to extinction, if not already gone, by 12,000 years ago imply a far lesser human role in the extinction than the overkill model allows. The climatic models account not only for the extinctions, but for the histories of smaller mammals during the Pleistocene. With greater explanatory power, most scientists studying the extinctions issue accept climatic, not overkill, accounts, while recognizing that far more precision is needed in these accounts. This does not mean that people played no role in causing the extinctions. A multivariate explanation may yet provide the best account of the extinctions. But no matter what the human role might have been, overkill was not the prime cause of the extinctions. That cause rather clearly lies in the massive climate change that marks the end of the Pleistocene. (Ward 1997, 161)
Ward then adds:
In the mid-1990s, I was struck by the almost eerie similarity between Grayson's arguments [against overkill] and those being leveled against the impact theory for the disappearance of the dinosaurs. The proponents of both arguments believe that the victims—the Quaternary megamammals and the Late Cretaceous dinosaurs—were dwindling in diversity and abundance well before their extinction. Both assume that there would be a "bone bed" or that there would be more kill sites if the sudden, catastrophic explanation were correct. Both cite last occurrence dates (the time when the last known individual of any species occurs in the fossil record) for the victims as being well before their supposed final extinction. And both imply that they are correct . . . because those best acquainted with the facts agree it was not catastrophic. (Ward 1997, 161-162)
I am delighted with Ward's insight. Certainly there is a much greater opportunity to test for decline in range or numbers of near-time megafauna in America and Australia just prior to widespread human invasion than there is in the case of late-Cretaceous dinosaurs prior to the extraterrestrial impact that left the Chicxulub crater. Yet no reduction in the deposition of ground sloth dung or other common fossils of extinct species is evident. Mammoths, for example, apparently saw no decline in number of animals, number of taxa, or range right up until they went extinct (Martin and Steadman 1999, 42). The dates on the fossils of Harrington's goats also do not suggest a declining population (Mead, Martin, and others 1986). Early returns on dwarf ground sloth extinction in the Greater Antilles, a crucial test of the climatic model which predicts synchronicity, indicate survival of the dwarf ground sloths for thousands of years after extinction of their continental relatives. A simple climatic explanation of ground sloth extinction will not account for the extinction chronology.
In an exchange between Grayson and Dave Meltzer (2002, 2003) and Stuart Fiedel and Gary Haynes (2003), Grayson and Meltzer decry the scarcity of archaeological deposits associated with mammoths or other creatures from the more than 30 genera of large animals that vanished close to the time of human arrival. Fiedel and Haynes, on the other hand, think "there is far more support for overkill than for climate change as the principal cause of the extinctions." These four archaeologists do agree that the total number of unambiguous associations of human interactions with now-extinct mammals is represented by 14 proboscidean kill sites. I am glad to see that Grayson and Meltzer accept human impact as the cause of thousands of flightless bird and sea bird extinctions on oceanic islands. Only 40 years ago virtually no one, with the exception of Charles Fleming, attributed moa extinction to humans.
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