, Research Paper
March 15, 2000
ICE AGE EXTINCTIONS OF THE MEGAFAUNA
During the last Ice Age before humans arrived, the North American continent belonged to various forms of enormous, fantastic creatures. By the end of the Ice Age, most of these large animals had become extinct. Numerous attempts have been made to explain the disappearance of these animals, but there has yet to be a consensus. Among the theories that have been debated, two are predominant. They are the climatic change theory, and the overkill theory. The climatic change theory advocates the idea that the global warming, which brought about the end of the Ice Age, caused the animals to die off. The other is the overkill theory, which maintains that humans, by over hunting the animals, are responsible for the extinctions. It is likely that many of these animals became extinct at the end of the Ice Age due to a combination of climatic change and overkill.
The Ice Age, which began about one and a half million years ago, is also referred to as the Pleistocene Epoch. Much of the Pleistocene falls within the geological time period called the Quaternary stage. We are still in the Quaternary today. The Ice Age consisted of a series of glacial advances and retreats called glaciations and deglaciations. There have been a total of at least twenty glaciations, including four or five that were significant, in the last 2.4 million years.
In the most recent glaciation, thirty percent of the earth was covered in sheets of ice over two miles thick. The last glaciation in North America is called the Wisconsin. The Wisconsin glaciaciation began 122,000 years ago. The advance of the Late Wisconsin began 35,000 years ago. The ice sheets, called the Laurentide and the Cordilleran glaciers stretched southward to the middle of the continent, covering most of what is now Canada and the area in the Pacific Northwest from Alaska to Tacoma. On the eastern half of the continent, the ice covered the area to about the middle of where New York is today. Diverse environmental conditions south of the ice sheets remained as they were, but in the northern half of the continent, regions that had once been surfaced with dense forests, fertile land, and abundant game, turned cold, and barren.
The expanding glaciers in the Ice Age forced sea levels to drop between 330 and 490 feet, opening a passage between Siberia in northeastern Asia and Alaska in North America. This passage was a strip of land, called the Bering Land Bridge. The land bridge still exists today, but is immersed under water. When sea levels were lowered by 330 feet, a 1000 mile wide strip of land was exposed. It surfaced and submerged many times throughout the Ice Age, and is believed to have been exposed for much of the time between 75,000 and 14,400 years ago. New research shows that the land bridge may have even been exposed until as late as 11,000 years ago.
This land bridge was the route of migration for a large variety of mammals crossing between Asia and North America. Some of the animals arrived long before the glaciation of North America. The animals that migrated to this continent adapted to their new surroundings. Many of the animals living on the continent were herbivorous and extremely large compared to the present fauna. These huge, plant eating animals are called the megafauna.
Some of the most well known megafauna are the Probocidea. Animals with trunks, which include the mastodon (Mammut) and the mammoth (Mammuthus) of the Ice Age, and the two modern species of elephants, belong in the order Proboscidia. The mastodon probably traveled alone rather than in a herd, browsing on Spruce limbs in open woodlands. The mammoths migrated into North America by at least 1.7 million years ago (Haynes, 1991). They traveled in herds and generally grazed in vast grasslands. The types of vegetation that these animals consumed, can be determined bye observing the structures of their teeth. A new mammoth species evolved from the original mammoths as they migrated from Eurasia, over the Bering Land Bridge, leaving substantial populations in Beringia as they progressed southward. This new species was called the Columbian mammoth (Mammuthus columbi). The Columbian was a larger mammoth well adapted for the North American grasslands.
While the Proboscidea originated in Asia and Africa and migrated to North America, other animals evolved in the New World and traveled across the land bridge in the opposite direction from North America to Eurasia. Among the genera that originated in the New World was the horse (Equus). Horses migrated to Asia and Europe during a period when the land bridge was exposed. Before the mass extinctions, the horse became one of the most populous mammals in North America. By the end of the Ice Age, the horses had completely died off in the New World, but survived in Eurasia. The horse exists, now and even abounds in the New World, because it was reintroduced in the sixteenth century by Europeans. Other Ice Age animals include such varied genera as the glyptodont (glyptotherium), the ground sloth (Eremotherium), and the giant beaver (Castoroides ohioensis). What is notable and extremely interesting about these animals is that they have many similarities, but are much larger than their modern day counterparts. For example, the glyptodont, similar to a turtle, would have been comparable in size to a Volkswagen Beetle. The ground sloth reached weights of up to three tons, about the size of a mammoth, and the giant beaver was about the size of a black bear (Grayson, 1993).
About 18,000 years ago, the Wisconsin glacier began its retreat. Buried lands began to reveal themselves. Strong winds spread seeds planting various types of new vegetation along the banks of the glaciers. By the time of 10,000 years ago, the glaciers retreated to the size they are today. This point marks the end of the Pleistocene and the beginning of the Holocene.
Such an extensive variety of animals, large and small, thrived on the continent until the end of the Pleistocene. At this point most of the megafauna had vanished. Thirty-five genera of mammals that once wandered this land, and nineteen genera of birds are now extinct. Archeological research shows that none of these extinct animals existed after 10,000 years ago. Of the thousands of archeological sites in North America, not one has contained the remains of any extinct animal that proves to be younger than 10,000 years old (Grayson, 1993). It was once assumed by many people that all thirty-five genera became extinct in the short period of time between 10,000 and 12,000 years ago. This presumption emerged when six of the genera were proven to have disappeared in the span of two-thousand years. That was in 1969, and since then, only the extinctions of three more genera have been placed in the same time period (Grayson, 1993). It is possible that the other twenty-six genera became extinct at the same time, but if that is the case, we have a long way to go before it will be proven. The other feasible possibility is that the extinction?s happened thousands, or even millions of years ago.
The climatic change brought about the end of the Ice Age. It resulted in a number of physical environmental changes. The temperature of the earth grew increasingly warm, resulting in a greater difference in seasonal temperatures. The glaciers melted away, causing sea levels to rise and shore lines to recede. The general hypothesis of the climatic change theory is that a mixture of factors, such as environmental and ecological changes that resulted from the warming climate, wiped out the megaphone. Could this climate change of the last glacial retreat be the cause of these extinction?s? For decades, scientists have deliberated the question which is surrounded by controversy. They have considered many different models of explanation that comply with the general hypothesis of the climatic change theory. One idea comes from Peter D. Ward as follows:
As the glaciers receded, a profound reorganization occurred in hydrological, biological, and sedimentological systems. The world?s ice coverage changed from 30% of its surface to the current level of about 10%. This phase change of so much water from solid to liquid was the driving force behind much of the subsequent geological, climatic, and biotic transformation. As the ice melted, water and sediment choked the rivers; they altered the drainage of great lakes and, in breaking the ice dams that had formed some of these lakes, caused cataclysmic floods that themselves changed the face of North America (the channeled scab lands of Washington State and Idaho are examples). The continental shelves changed positions and depth as the shorelines migrated inward under rapidly rising seas. Rainfall patterns shifted; the seasons themselves were lengthened or shortened. Temperatures rose all over the earth. And of course, in the wake of so much physical perturbation to the environment, biotic systems changed. These great changes in the physical environment are at the heart of many scientists? belief that the large-animal extinction at the end of the Ice Age was brought about by physical or environmental changes, not by human hunting. The extinction of many species in a broad environment must involve many variables in both biotic and physical spheres, because ecosystems are complex. Physical changes may bring about the death of one species, and its disappearance then affects other species, such as its predators, prey, and parasites. In this way even a single extinction causes effects that ripple through the ecosystem. At the end of the age of glaciation, many species faced environmental changes, and surely many succumbed before they could adapt or migrate. Habitat destruction is probably the major cause of physically induced extinction.
One may wonder why, if the climatic change ended the existence of the megafauna, did it not affect the smaller animals in such dramatic ways. The likely answer that large animals have more difficulty adapting to new environments than do smaller animals. Larger animals are safer from predators than small animals are, but it is also harder for them to sustain in trying conditions. Many of the larger animals such as mammoths and bison travel in herds, and that, along with their large size, protects them from predators. However, when the predator is Mother Nature, there is virtually no defense. For modern day elephants, drought is the biggest concern. It can take twenty percent of a herd every year, but the elephants continue to exist (Ward, 1997). It would have been harder though, for the mammoths and mastodons to survive a drought. Because of their size and weight, the Ice Age Proboscidea had longer gestation periods, which meant lower birth rates. If too much of the population was lost due to a severe drought, the population would be at risk.
Also in the same time period of 10,000-12,000 years ago, the peopling of North America occurred. Although some archeological clues have hinted at earlier human occupancy, no site has manifested definite proof so let us assume for now that they in fact did arrive in this time period. We know this because there are many archeological sites scattered all over North America that contain human artifacts. The first site from which artifacts dating back to the end of the Pleistocene were uncovered, was discovered in 1932. The site was found in a small town called Clovis in New Mexico. The artifacts that were unearthed were projectile points between two and five inches long, and the bones of extinct mammals. The projectile are curate, bifacial, fluted points. They came to be known as Clovis points, which now mark the earliest known human occupancy of North America, and are characteristic of the sites occupied by these people. The people who fashioned this tool are known as the Clovis People. The Clovis people migrated to North America, as the many animals did, via the Bering Land Bridge.
Although the timing and extent of human contact with the megafauna is ambiguous, it is clear that the Clovis people did encounter and kill the now extinct mammals. The debate is whether or not humans were the sole cause of mass extinctions during the Late Pleistocene. Paul Martin of the University of Arizona hypothesized that with a rapidly growing and spreading population of big game hunters and ?wasteful hunting methods,? the animals became extinct. He claimed hunters would consume only a small portion of the kill leaving much waste. This supposedly resulted in overkill of animal population and subsequent extinctions (Fagan, 1995:85).
Martin also brought up the point that if the changing climate was the cause, horses would not be able to survive in the present. The reason being that the climate, and environment at the end of the Ice Age was much like that of today, and if the horse could not sustain then because of climate alone, they would not be able to sustain, let alone flourish as they do presently in North America (Grayson, 1993).
Twelve sites in North America that bear Clovis points in stratigraphical order with mammoth bones have been discovered. One of these is the Lehner site. It is located in the San Pedro Valley in Cochise County of southeastern Arizona. Ed Lehner discovered mammoth bones in 1952 on property that he later purchased (Grayson, 1993). Emil Haury, an archeologist of the University of Arizona began excavating the site in 1955. In the San Pedro Valley, a layer of black organic clay called the ?black mat? lies at the base of Holocene sediment. The clay contains remnants of a prehistoric marsh, and its lowermost layer dates back to 10,800 years ago. The artifacts were found deeply embedded in a stream bank under the ?black mat,? and date to 10,900 years ago. In total, the bones of thirteen mammoths, one bison, and twenty-one stone tools have been unearthed at Lehner. Eight of the tools were probably used as expedient cutting devices, and the others were projectile points. Eleven of the thirteen points were found in close proximity of the bones, indicating that they were used to kill the animals. The dry streambed near which the bones have been found, is named Mammoth Kill Creek. From the name and the findings at Lehner, it might seem like this site supports Paul Martin?s theory of overkill and wasteful hunting, but it is not even definite that all of the mammoths here were killed by man. The land around the stream used to be desert grasslands, and Mammoth Kill Creek was probably a watering spot for the animals. At this spot the some of the mammoths may have died natural deaths, and other were killed by humans. This was probably not a mass kill, because Clovis people generally would track herds and not take more than one animal of the herd.
Both the case for climatic change and its role in extinction, and the case for overkill and its involvement, present some compelling evidence. There is also room in either theory for debate. The most likely and seemingly obvious conclusion is that Mother Nature and humans worked together in the extinction process. Suppose the change in climatic conditions did affect the megafaunal population. Considering all of the changes that occurred, such as rising temperatures, receding glaciers, changing rainfall patterns, and rising sea levels from which large tracts of land became immersed, it is hard to imagine that the climate could not have had any affect, even if indirectly, on the Ice Age animal populations. Most plants are characteristic of the environments they live in. By looking at a particular type of plant, we can tell a lot about where it came from and in what kind of conditions it lived. Because plants are so specific to their environment, they do not adapt well to climatic changes. Therefore, the changes mentioned above must have affected the Late Pleistocene vegetational environments.
Mammoths and Mastodons were some of the largest mammals of the Ice Age, required large amounts of food. These, and many other animals were herbivores. When there was not enough vegetation, or enough of the right kind of vegetation to eat, the animals had to either migrate or adapt. If they were not able to migrate or adapt quickly enough, they would die. In order for any species to survive, the population must grow faster than its members are dying. This means, for the Ice Age mammals to resist extinction, the birth rate would have to have been higher than the mortality rate, so any other factors increasing the rate of mortality would seriously jeopardize the existence of the animals.
Gary Haynes studied the two living species of present day Proboscidea (Loxodonta africana and Elephas maximus) to find out more about their prehistoric counterparts (Haynes, 1991). First, he compared them to each other, finding them ?…different in physical appearance, but very similar in social behavior and biology,? (Haynes, 1991:10). He then related them to the Mammut and Mammuthus of the Ice Age. He found, through allometric scaling, that elephants and mastodons seem to have similar biological histories. The larger bone structures indicate that the mammut, although it doesn?t differ much in actual size, would have weighed more than those of today. From this information, Haynes was able to deduce an assumption about reproductive patterns in the mastodon.. The longer gestation periods in the prehistoric animals, because of their heavier weight, implies slower reproduction. Haynes also discovered useful information about the ability of these animals to subsist in specific conditions. According to the studies, not only do they need a large amount of food, they also need water to aid in digesting the food, so both or either one of those missing factors would have impacted the mammoth and mastodon. It is likely that towards the end of the Ice Age, megafaunal populations were seriously diminished, and perhaps for some genera, already extinct.
The climate began to change 18,000 years ago, and humans didn?t arrive until about 12,000 years ago. This meant that the changes were still occurring at the time of human arrival, and the animal populations would not yet have had a chance to recover. From the archeological sites containing projectile points together with bones of the megafauna, we know for sure that humans did kill the animals. People may have been the final component in the in extinction of the megafauna, but most likely not the only one. If climate change had not affected the faunal populations, the amount of animals on the continent when early people arrived would have been abundant. It is improbable that man alone could have killed off thirty-five genera of animals in the period of two-thousand years. If climate change had been the only factor in depleting the megafaunal population, survival rates would likely have been high enough to sustain. Therefore, it is highly probable that the changing climate began the diminution of the megafaunal population, but that the process was extenuated by humans.
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