Where Have All the Mammals Gone? by Lynne M. Clos

Introduction

The disappearance at the end of the Pleistocene Epoch, about 10,000 years ago, of a considerable portion of North America's large mammal species has long been the subject of speculation and debate. Among the most popular explanations are:

In this essay I will argue: (1) that, taken in the perspective of the entire Cenozoic Era (the last 65 million years), extinction episodes such as this are not uncommon; and (2) that while all three causes together are probably responsible for the Pleistocene extinctions, the influence of climatic factors and the consequent disruption of floristic communities were of overriding importance.

Mass Extinctions

Mass extinctions have occurred throughout the history of life on earth. They usually accompany large-scale environmental changes to which many species cannot adapt. Rapid turnover is frequently explained by the extinction of major faunal elements, followed by swift adaptive radiation by remaining animals to occupy the vacated environmental niches. This happened 65 million years ago (mya), when the non-avian dinosaurs died out and the ancestors of modern mammals replaced them; the Cenozoic Era had begun.

The adaptive radiation of mammals began with shrew-like creatures and produced a highly diverse fauna by Eocene times (around 40 mya). At the close of the Eocene, a major extinction wiped out large numbers of animals. Diversity recovered during the succeeding Oligocene Epoch, and by mid-Miocene times (about 12 to 15 mya) mammalian variety exceeded that previously attained and was, in fact, the maximum they would achieve (1).

This high diversity among the mammals was fairly well maintained for most of the next 10 million years, with rates of new species origination and extinction closely paralleling one another. Some groups declined and were replaced by others, but on the whole, things were relatively stable. Then during the mid-Pliocene, things again took a downward turn. By the beginning of the Pleistocene (2 mya), both the rates of mammalian species originations and extinctions far exceeded the averages for the rest of the Cenozoic Era, resulting in a very high rate of faunal turnover. In the early Pleistocene, origination rates (new species per million years) exceeded Cenozoic averages by 10 times; by the late Pleistocene, originations were running at 5 times the Cenozoic average. It has been documented in recent ecological studies that overall species diversity within a community tends toward a "saturation diversity capacity" dependent on environmental conditions. (Total diversity is highest in the tropics and decreases with latitude.) Taken in that context, a large fraction of the extinctions which took place in the late Pleistocene may be explained by the natural tendency to return to equilibrium (2).

Changing climate -- the Cenozoic perspective

The earth's climate has undergone an episodic but persistent cooling ever since the middle Cretaceous Period, prior to the rise of the mammalian dynasty. The mammals radiated under cooler conditions than the dinosaurs did; it is now much cooler and drier than it was then; and the past 3 million years have been the severest of all. Several lines of evidence demonstrate this trend, and it can be shown that the times of most rapid climatic deterioration correspond very closely to episodes of mass extinction.

One of the most straightforward methods of evaluating past climates is to compare fossil plants to closely related modern ones. Plants which today are restricted to certain types of environments may be inferred to indicate similar environments in the past. For example, during the Eocene (50-40 mya), Oregon hosted forests similar in composition to those found in Central America today: fig, laurel, cinnamon, and avocado. Temperate forests of redwood, maple, oak, and beech extended from Siberia across Alaska into Greenland. The boreal spruce forest formed a narrow fringe along the banks of the Arctic Ocean (3). Clearly, it was much warmer at high latitudes then than it is now.

A second, less direct method of using fossil plants to infer climate can be applied where a reasonable cross-section of the floristic diversity of a region has been preserved. Studies in modern forests have revealed an empirical relationship between leaf morphology and climate, specifically both the average annual temperature and the amount of seasonal temperature variation. Tropical forests have a predominance of plants with large leaves and entire margins, frequently equipped with "drip tips" which shed rainwater. There is an increasing tendency, as one moves toward temperate regions, for leaves to become smaller and develop toothed margins or a lobate shape (3). The leaf types contained in a fossil assemblage may thus be used as an indicator of both temperature and seasonal variation.

There was a marked change in dominant leaf types at the end of the Eocene Epoch, indicated by such well-preserved floras as those in the Florissant and Yellowstone volcanic ash deposits. Floral diversity plummeted, and irregular-margined leaves replaced entire-margined ones. The change was of such a magnitude as to indicate approximately a 10°F drop in average annual temperature, along with a marked increase in seasonality, in the mid-United States (3, 4; see figure 1). The timing of this episode coincided with a previously mentioned mass extinction among the mammalian fauna of North America. The same trends are also seen in the European fossil record (3).

Corroborating evidence for global cooling comes from the analysis of marine foraminifera. These are small organisms which utilize the oxygen bound up in seawater for constructing their calcium carbonate shells. The proportion of two isotopes of oxygen, 016 and 018, in seawater varies with the temperature of the ocean; when the foraminifera build their shells, they use the oxygen isotopes in the prevailing proportions and thus record the ocean temperature at the time they lived. Later incorporation of their shells into marine sediments preserves a record of water temperatures in the past (5).

Planktonic foraminifer species thus provide a fossil record of sea surface temperatures, and benthic species record the temperature at depth. The evidence indicates that deep ocean waters at 50° latitude averaged 50° to 60°F in the early part of the Cenozoic, versus about 35°F now, and that half this drop occurred within a few million years spanning the Eocene/Oligocene boundary (37 mya) (5). These dramatic changes in global climate are generally attributed to the wanderings of continents and the resultant effects on atmospheric and oceanic currents, which redistribute solar energy from equatorial to polar regions.

Temperatures remained more or less stable at mid-latitudes during most of the next 30 million years, although plant fossils indicate a marked drying trend, accompanied by a shift from forests to savannah in the continental interiors (3). Mammal populations responded with the evolution of large numbers of browsing and grazing animals, and the development of hypsodonty: high-crowned teeth capable of the sustained wear required to cope with a diet of siliceous grasses (4, 6).

During the Pliocene Epoch, about 3 mya, global climate again took a drastic dive. Another 10° F cooling at mid-latitudes, coupled with increased seasonality, pushed the subtropical forests to south of the 40th parallel and the boreal forest to south of Hudson Bay (3). There was no glaciation yet in North America, but the ice ages were not far away.

Ice and immigrations

The cooling and drying trend which characterized the late Pliocene saw the replacement of moist savannah grasslands by less productive steppe over a large portion of the continental interior, and with this, an accelerating decline in the numbers of grazing mammals. Extensive areas of chaparral and desert vegetation developed in the west (3, 4). Tundra-steppe vegetation expanded southward, temperatures embarked upon wide cyclic fluctuations, and there was large-scale disruption of plant communities as vegetative zones retreated toward the equator (3). All of these things created a great environmental stress for the animals living in North America. This instability led to increased faunal turnover via both higher speciation and higher extinction rates as previously discussed (2).

The onset of glaciation only made things worse. Many animals simply left. The Central American land bridge had emerged during the Pliocene (3 mya) and allowed warmth-loving animals from North America to take refuge in the south. Although South America was undergoing the same forest-savannah retreat and expansion of steppe as North America, it still harbored a large area of the tropical forest which had completely vanished north of Mexico (6). Early Pliocene extinctions in South America also left vacant niches which immigrants could occupy (7). More northern species were able to establish themselves in the south, where the climate was relatively mild, than there were South American species able to overcome their tropical origins and colonize North America's increasingly harsh climate. (Half of the land mammal species now living in South America have arrived from North America within the past 3 my (8)). In fact, many mammals usually counted as "Pleistocene extinctions" in North America are well and doing fine in South America: this group includes llamas, spectacled bears, capybaras, peccaries, and sloths (1). If prehistoric man had hunted them to extinction in the north, would he not also have done so in the south?

Throughout the Pleistocene, North America was repeatedly connected to another continent: Asia, via Beringia. But the animals native to northeast Asia -- the ones which hadn't themselves fled south, or become extinct, as the climate cooled -- were adapted to precisely the habitat in North America which was at the other end of the bridge: steppe-tundra. Europe and Asia developed much larger expanses of steppe-tundra during the ice ages than did North America, and hosted a correspondingly wider variety of animals than evolved here (9); rather than marching across Beringia and taking over North America, these tundra animals entered a habitat that was undersaturated with their kind, and found it relatively easy to become established.

Long (not short) time passing

The puzzling suddenness of the Pleistocene extinctions also deserves examination. It is a true statement that "mammoths became extinct about 10,000 years ago" (with the exception of the dwarfs recently discovered on Wrangel Island) or "horses died out in North America at the close of the Pleistocene". True, but extremely misleading. During the Miocene mammalian zenith there were twelve separate genera of horses living on the North American continent. Six million years later, there were half as many. And so on until the last horse vanished at the end of the most recent ice age (1; see figure 2). In other words, horses were going downhill in North America for a long time...long before man evolved, let alone arrived here; and Asian competitors aren't likely to have been the problem, since Asia is where horses did manage to survive. This decline does, however, follow the trend of floristic disruption caused by generally deteriorating climate.

Ditto the mammoths and mastodons. Three species of North American proboscideans bit the dust at the end of the last glacial epoch, but four others had become extinct within the preceding 3 million years (10). Climate probably operated on them via disruption of their feeding habitats. Stomach contents of beasts recovered from the permafrost show that mastodons were primarily spruce browsers, and imperial mammoths were browser/grazers which inhabited grasslands and open pine parklands (11, 12). Radiocarbon dates on fossil bones have indicated that the ranges of both these animals were shrinking for several thousand years prior to their last known appearance. The mastodon's range contracted about a retreating spruce community centered near the Great Lakes region, following the catastrophic collapse of spruce forest in mid-America at the end of the last glacial (12). Imperial mammoths formerly held almost continent-wide distribution, yet by 10,000 years ago they were scattered in a narrow band from Oklahoma to Saskatchewan (11) along the exsolving remnants of the pine forest/grassland edge. Woolly mammoths were steppe-tundra animals, and made their last continental stand in Alaska as increased precipitation turned their lush sedge habitat into lake-dotted sphagnum muskeg (9). Not long ago, mammoth tusks dating to only 4000 years ago were discovered on Wrangel Island in the Arctic Ocean. They belonged to dwarf individuals, the last survivors of a population which held on in an area where steppe-tundra appears to have persisted after it had largely vanished on the mainland (13) (although the absence of human hunters in such a remote location cannot be discounted as contributing to their survival).

This range contraction and dwindling population, caused by disruption of the ecosystems on which the animals depended (14), would have made them particularly vulnerable to extinction. There have been at least two other mass extinction episodes within the Pleistocene, which correspond to the onset of interglacial conditions thought to be particularly severe (1, 2).

It is in the light of these observations that the role of man in the extinctions must be considered. Severe environmental stress at the end of the Pleistocene had put many species close to the brink anyway. It should be noted that Eurasian mammoths died out at the same time as their American cousins, notwithstanding the different dates of human occupation of the two continents. For woolly mammoths in particular, where Clovis spearpoints have been found along with their bones, it may well be that early Amerindians hunted the last few beasts down (11). That is, however, a very different scenario from prehistoric hunters marching across the continent and repeatedly extinguishing one large viable mammal population after another (15). Recent, more accurate radiocarbon dates do not support the "blitzkrieg" model, with its suggestion that large animals died out in a time-transgressive fashion as a wave of humanity swept the Americas from north to south (16). It has been argued that man's role was that of the "straw that broke the camel's back" in that the appearance of Clovis people with the technology to hunt large animals was the deciding factor in why so many animals went extinct at the onset of this particular interglacial and not some other. But it would not have been possible for humans to drive the animals to extinction had not their populations been stressed already by habitat fragmentation. In fact, recent computer models suggest that just a 2% hunting rate per year could push a species teetering on the brink of extinction over the edge, by lowering its reproductive numbers below the threshold from which they could not recover, although actual extinction of the population might take several hundred or thousand years (16). Humans probably did play a role, but it was far from that of wholesale slaughterers.

Or maybe it was the snow that got a lot of them. It is thought that glacial epochs, while colder, were also drier and less seasonal than interglacials (3, 9). Studies have shown that bison can survive bitter cold as long as snow does not accumulate to a depth greater than 15" for prolonged periods of time. If there is too much snow, they cannot paw through it, and they starve. Musk oxen can take high winds and temperatures down to -60° F, but cannot tolerate persistent snow cover of more than 10" (9). Perhaps for some of the many animals whose bones have not been found associated with human artifacts, the answer is that they succumbed to the raging snowstorms that swept the mid-continent after the retreat of the ice.

Conclusions

The most significant factor in the extinction of large numbers of mammal species in North America at the end of the Pleistocene appears to be rapid climatic change. Annual temperatures at high latitudes have been declining for millions of years, and since northern hemisphere glaciation began, violent, periodic fluctuations have been the rule. With the onset of the Holocene interglacial, extremely rapid warming and increased seasonality served to shatter the floristic associations on which many mammals depended, leading to their demise.

References

1. Webb, S. D. 1984. Ten million years of mammal extinctions in North America. Ch. 9 in Martin, P. S. and Klein, R. G. (eds), Quaternary Extinctions: A Prehistoric Revolution. U of Arizona Press, Tucson.

2. Gingerich, P. D. 1984. Pleistocene extinctions in the context of origination-extinction equilibria in Cenozoic mammals. Ch. 10 in Martin, P. S. and Klein, R. G. (eds), Quaternary Extinctions: A Prehistoric Revolution. U of Arizona Press, Tucson.

3. Dorf, E. 1976. Climatic changes of the past and present. Ch. 28 in Ross, C. A. (ed), Paleobiogeography. Dowden, Hutchinson, & Ross, Inc., Stroudsbury, Pa.

4. Webb, S. D. 1985. Main pathways of mammalian diversification in North America. Ch. 7 in Stehli, F. G. and Webb, S. D. (eds), The Great American Biotic Interchange. Plenum Press, New York City.

5. Douglas, R. G. and Savin, S. A. 1985. Sea level, climate, and the Central American land bridge. Ch. 12 in Stehli, F. G. and Webb, S. D. (eds), The Great American Biotic Interchange. Plenum Press, New York City.

6. Pascual, R. et al. 1985. Main pathways of mammalian diversification in South America. Ch. 8 in Stehli, F. G. and Webb, S. D. (eds), The Great American Biotic Interchange. Plenum Press, New York City.

7. Cifelli, R. L. 1985. South American ungulate evolution and extinction. Ch. 9 in Stehli, F. G. and Webb, S. D. (eds), The Great American Biotic Interchange. Plenum Press, New York City.

8. Webb, S. D. 1985. Late Cenozoic mammal dispersals between the Americas. Ch. 14 in Stehli, F. G. and Webb, S. D. (eds), The Great American Biotic Interchange. Plenum Press, New York City.

9. Baryshnikov, G. F. and Vereschagin, N. K. 1984. Quaternary mammalian extinctions in northern Eurasia. Ch. 22 in Martin, P. S. and Klein, R. G. (eds), Quaternary Extinctions: A Prehistoric Revolution. U of Arizona Press, Tucson.

10. Kurten, B. and Anderson, E. 1980. Pleistocene Mammals of North America. Columbia University Press.

11. Agenbroad, L. 1984. New world mammoth distribution. Ch. 3 in Martin, P. S. and Klein, R. G. (eds), Quaternary Extinctions: A Prehistoric Revolution. U of Arizona Press, Tucson.

12. King, J. E. and Saunders, J. J. 1984. Environmental insularity and the extinction of the American mastodont. Ch. 15 in Martin, P. S. and Klein, R. G. (eds), Quaternary Extinctions: A Prehistoric Revolution. U of Arizona Press, Tucson.

13. Discovery Channel, 199x. Mammoths of the Ice Age.

14. Graham, R. W. and Lundelius, E. L. Jr. 1984. Coevolutionary disequilibrium and Pleistocene extinctions. Ch. 11 in Martin, P. S. and Klein, R. G. (eds), Quaternary Extinctions: A Prehistoric Revolution. U of Arizona Press, Tucson.

15. Martin, P. S. 1984. Prehistoric overkill: the global model. Ch. 17 in Martin, P. S. and Klein, R. G. (eds), Quaternary Extinctions: A Prehistoric Revolution. U of Arizona Press, Tucson.

16. Ward, P. D. 1997. The Call of Distant Mammoths: Why the Ice Age Mammals Disappeared. Copernicus books.