Geo 365 research paper

Jason Condit

Late Pleistocene Megafauna Extinctions The Pleistocene epoch occurred between 2.6 million and 11,000 years ago and was characterized by extreme changes. Dynamic climates, glaciation, and expansion of the modern Homo sapiens all were roles in this time of rapid change. As an effect, large scale extinctions of megafauna characterized the time as well. Studies done on this epoch have begun to explain what caused the multiple climate changes, how glaciers affected large areas, and how a mix between climatic differences and human expansion potentially combined to cause many of these extinctions. We will look at how climate and the extension of humans to other areas may have caused extinctions individually, or if a combination of the two is the most reasonable explanation.

The Pleistocene epoch was characterized by an overall pattern of heating and cooling.

One of the more widely explanations of this pattern is the cyclical changes of earth’s orbit around the sun. This principle includes looking at how the earth’s orbit deviates from a perfect circle, the angle of the earth’s tilt, and the orientation of earth’s axis in the solar system. When scientists look at these factors, they see that the repeated, brief warming periods corresponded with the combination of these factors that allowed the earth to be closest to the sun. However, most agree that these alterations alone do not quite explain the dramatic swings between glacial and interglacial conditions. What may have had just as large of effects are the positive feedback systems that occurred. These include warming from lower albedo levels as glaciers melted, and an increase in CO2 as plants increased in productivity. Despite slight periods of warming due to cycles, glaciation made up the most of the Pleistocene and had its own unique effects (Lomolino, 2010).

During the Pleistocene, there were many glacial and interglacial cycles that played into these heating and cooling patterns. Up in the higher latitudes of the Northern Hemisphere, the glaciers sometimes reached 2 or 3 km thick and at one point covered one-third of the earth’s land surface. Glaciers prevailed during this epoch with glacial periods occurring about 90% of the time. During these glacial periods, average air temperatures were about 4 to 8 degrees Celsius cooler than interglacial periods. Additionally, during these glacial periods, climatic zones shifted toward the equator and prevailing winds and ocean currents changed (Lomolino, 2010).With these great climatic shifts, species needed to be able to adapt. One of the greatest climate changes

occurred in the Late Pleistocene when the Wisconsin ice sheet began to retreat. This interglacial period of warming caused temperatures to become less homogeneous, which, as a result, affected local environments. During this period animals began to expand their range while others began to disappear (Gibbons, 2004).

When looking at all these extreme changes in climate, we begin to see how animals were affected. They were forced to migrate, adapt, and compete for their resources. Changes occurred relatively quick, and if species were not capable of adapting they were faced with the possibility of extinction. The changing climate and expansion of hunter-gatherer Homo sapiens may have combined to cause one of the most large scale megafaunal extinctions.

About 50,000 years ago, continents were inhabited by roughly 150 genera of megafauna; however, by the end of the Pleistocene, about 97 of those were extinct (Barnosky, 2004). Some of these megafauna included species of horses, camels, ground sloths, giant beavers, and many others. One of the first theories to address these extinctions is the role of climate change and how it may have been the key contributor in an epoch riddled with glacial-interglacial cycles. As stated earlier, these cycles, caused by changes in earth’s dynamics, each posed unique climate conditions that species were forced to adapt to. Some initial studies aimed to use these Milankovitch cycles in order to explain the changes. These cycles are quite widely accepted to explain changes in climate, and have been used to explain long-term cooling and glaciation trends during the Pleistocene. However, if these cycles were the only factor, changes in climate would be linear and more predictable, but studies have shown this is not the case (Lisiecki, 2007). One of the explanations for non-linear changes in climate has to do with the presence of positive feedback systems. One of these feedback systems includes albedo levels. In general, white ice has higher albedo, meaning that it reflects a higher percentage of solar radiation back out into the atmosphere; whereas, ocean water is darker and has a lower albedo meaning that it absorbs most of the solar radiation then emits it back into the atmosphere as heat. What happens in this system is when the glaciers begin to melt, it exposes more ocean, which absorbs more solar radiation and warms the earth more, leading to more ice melt and so on. Systems like this where the output also acts as the input can sometimes help explain sudden climate changes that cannot be explained by normal cycles. On a more local scale, we can look at the Wisconsin ice sheet and how it exhibited the effects of glacial and interglacial cycles. During the prolonged

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glacial periods, the Wisconsin ice sheet showed high environmental heterogeneity and supported diverse species; however, when the landscape began to de-glaciate, environmental heterogeneity decreased causing high diversity to be unsupportable. The halts and advances of the Wisconsin ice sheet during extinction periods were never able to restore previous environmental balance, so species were constantly trying to adapt. Primary habitats were eliminated, while others were temporary until the glaciers changed once again (Gibbons, 2004).

All these rapid changes played large effects on extinctions, especially of megafauna due to their specific needs and ranges. Pleistocene climate change was most likely to cause extinctions as it drove range adjustments in large mammals (Barnosky, 2004). Large animals generally require larger habitats. They require more forage area and more water to maintain health than smaller species. This is why we usually see only a few megafauna in a given area (Gibbons, 2004). As the climate change occurred quickly in the late-Pleistocene, it triggered vegetation changes which, since it is lowest in the food chain, affects the ecosystem and alters where species must go in order to obtain the resources they need (Barnosky, 2004). For larger animals, they were forced to migrate to other areas as climate change shrunk their necessary habitats. As they did, multiple populations of large animals were forced to inhabit the same small areas with limited resources, causing some extinction due to competition. Over specialization, competition, and inability to migrate all were magnified as these primary habitats shrank.

Another cause of extinction associated with unpredictable environment conditions is the disruption of correlation cues that larger species use for reproduction. Megafauna generally have long gestation periods, so they use these cues in order to correlate the timing of their mating and birthing seasons with favorable climatic conditions essential for mother and newborn survival.

When the environment becomes unpredictable, it throws off the correlated cues which as a result lead to lower reproductive rates, which increases the likelihood of extinctions (Gibbons, 2004).

Studies have shown that megafauna extinctions in the northern hemisphere occur in two pulses coincident with climate change; however, these pulses of extinction also coincide with the spread and increase of Homo sapiens populations. Given this, human interaction and hunting is yet another theory of cause for these late-Pleistocene extinctions.

The first solid evidence of a significant human population in the Americas was at 13,400 years before the present which is right near the beginning of the extinction span (Alroy, 2001).

This coincidence is found as significant evidence because additional studies of Homo sapiens have proven their ability to be a top predator as well as prove that meat based protein became a large staple in their diets while located in colder climates (Gibbons, 2004). Other evidence supporting human caused extinction includes cut marks and breakage of fossil bones as well as the survival of slow-breeding megafauna that were nocturnal, alpine, or deep-forest dwellers.

This evidence shows that early Homo sapiens were hunters and were selective where and when they hunted, which correlates with the type of species that went extinct at that time (Barnosky, 2004).

Models of anthropogenic extinction generally include three possibilities including overkill, rapid overkill (blitzkrieg), and indirect cause such as fire, disease, and habitat fragmentation. The hypothesis of overkill suggests that the extinction of megafauna was concordant with the arrival of the modern Homo sapiens. In simulations conducted by scientists, there is evidence that suggests hunting is a huge factor in Pleistocene extinctions. Out of many parameters in the simulation, including population density and overall human range, hunting ability was one of the most important and influential factors. In the simulations, even when factors such as human population density were significantly low, extinctions prevailed as long as the hunting ability variable was high. In all these simulations, about the only way to prevent extinctions as human-hunters were introduced would be if they preferred to hunt smaller game (Alroy, 2001). Given these findings, there are actually very few kill and processing sites in archaeological record connecting humans to the extinct species. This lack of correlation does not necessarily refute overkill, but may suggest rapid overkill called blitzkrieg. This model represents overkill that maximizes speed and intensity of human hunting and impact. As a result, it minimizes the time of overlap between the first human migrator and the disappearance of native, extinct species. This model suggests that hunting and overkill occurred too quickly to leave an extensive amount of evidence (Gibbons, 2004).

Another possibility of human-caused extinction is shown with indirect causes. Since human predation was more than likely, even small amounts of hunting could have played large roles. Once humans are introduced into the food chain, they affect the previously stable ecosystem. What was ignored in the above simulation were the additional indirect effects that humans could play. Human predation would affect herbivores which would in return change

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ecosystems and their vegetation structure, insect habitats, and watershed dynamics (Alroy, 2001).

Adding these factors to the simulations would increase extinction rates and decrease further the human population required. Additionally, effects completely non-related to hunting could have played a role as well. Habitat fragmentation and alteration from human caused fires along with the introduction of diseases would decimate megafauna populations as well. Homo sapiens served as a transport for bacteria and parasites foreign to the newly colonized areas. This introduction very well could have killed off the large megafauna unprepared for the exposure (Gibbons 2004). As one can see, the introduction of humans to new areas played a very important role in extinctions. All this being said, since exact extinction rates are not known for either climatic or human caused extinction, theories based on a combination of the two are more widely accepted.

Both theories of climate change and human expansion and their effects on latePleistocene extinctions are backed up adequately; however, neither has managed to dominate the debate. A large possibility, therefore, exists that both explanations are simultaneously likely.

What is known for sure is that many species with large body sizes, as well as low reproductive rates, were hit hard, and the extinctions all bunch up in a time between 11.5 and 10 thousand RCBP which was a time of both Homo sapiens expansion and marked climate change (Barnosky, 2004). What supports such a combination of views is the diversity of extinctions and their supporting evidence. In North America, for example, where extinctions were rapid and pronounced, evidence has shown that climate change and first human contact occurred simultaneously. Likewise in Eurasia, extinction evidence supports interplay between human impacts and a changing climate. Also, in North America, there were species found that were indisputably hunted, while others located in Alaska went extinct without any significant human presence. Additionally, in Siberia and some islands, species of mammoths and giant Irish deer were found extinct in areas with no human life (Barnosky, 2004). With all these observed variances, the question is what are the linkages and combinations that could have caused such massive extinctions in such widespread areas.

One of the critiques that supports a combination view has to deal with the environmental evidence regarding the glacial-interglacial cycles. According to (Kaushik, 2010), there is no compelling evidence found that shows the last glacial-interglacial transition was any different

from previous ones to the point that it would cause large-scale extinctions, especially given that many species had survived them before. Additionally, it questions how climate change would have spared slow-breeding arboreal and nocturnal animals. The combined hypothesis suggested fills these gaps. The overall trend of combined theories is that climate change caused stress on animals and their range, but the new human expansion finished them off. For example, one theory is that new humans may have killed off mega-herbivores. In addition to range stress already caused by climate change, this loss of the main ecosystem herbivore would alter the vegetation and landscape and disrupt the whole ecosystem. Also, carnivores were sometimes forced to switch prey as competition with humans diminished their preferred prey resource which once again, could have detrimental effects on ecosystems (Kaushik, 2010). Since humans and carnivores both have the ability to change to alternative food sources, they were able to coexist without much competition. Additionally, even as prey sources diminished, humans would still be able to place constant pressure given that they could survive on fish and plants during poor hunting periods (Ripple, 2010). These factors combined with climate change put continuous stress on megafauna and is what may have driven many of them to extinction.

Overall, the possibility of climate and Homo sapiens working together to produce massive extinction is overwhelming. The changing climate caused rising temperatures, desiccation of the environment, and the retreat of the Wisconsin ice sheet; all this combining to push large land animal populations into small ranges with limited resources due to the accompanying environmental pressures. Then, hunting practiced by the newly introduced Homo sapiens, as well as their indirect effects, could have pushed the stresses over the edge leading to large scale extinctions. In this reasonable model of megafauna extinction, both theories are factors but neither is strong enough to have sole responsibility. If human groups were not present, populations of megafauna species may have been able to slowly recover after the unstable climate period. On the other hand, if climate change did not already reduce population sizes of megafauna, the relatively small number of migrating Homo sapiens would not have been capable of eliminating entire species (Gibbons, 2004). Now we’ll look at two case studies to once again see the variance of causes of extinctions in two widespread areas.

During the late-Pleistocene, Beringia was a subcontinent which connected the east and the west of the planet. This area, like many at that time, saw various large events including

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climate change and a mass extinction of megafauna. Using brown bear DNA preserved in permafrost remains, scientist have attempted to study change in large-mammal populations during the Pleistocene. These bears were chosen because of their extensive distribution in the Northern Hemisphere as well as their introduction to the area occurring during the Wisconsin Glaciation.

As fossil records were recorded, a gap in their presence occurred between 35 and 21 thousand years BP. This gap was thought to be formed from one of two reasons: either it was a true species absence, or taphonomic biases. Taphonomic exclusion of fossils was later ruled to be unlikely because many radiocarbon dates have been done for mammals in the same area during the same time and no gaps occurred. All this being said, the study concluded that if these bears were distributed in time the same as the other large mammals, then the gaps represent a true local extinction. Humans were not largely present in this area at this time, so the more possible reason for extirpation was climate change. Climate change no doubt occurred during that time period; however, the brown bear is generally regarded as very climate adaptable, so it seems unlikely that a cyclical climate change would cause extinction. After further explanations were explored, scientists found an inverse correlation between brown bear populations and the larger, carnivorous short-faced bears. As chronology was observed, short-faced bear fossil dates were concentrated between 35 to 21 thousand years BP; the same time that brown bears went locally extinct. On the other hand, the time that brown bears began to recolonize, right around 21 thousand years BP, the last record of short-faced bear was recorded in that area (Barnes, 2002).

This evidence shows how important a factor competition was as a result of climate change. As climate began to shift, megafauna were forced to migrate in order to find an area large enough and with enough resources to support them. Since climate change reduced the area of these primary habitats, many megafauna were forced to share space and resources. As we can see with the brown bear and short-faced bear in Beringia, this leads to competition of resources where the overpowering species survives and the inferior dies off.

On the other side of the world in Australia, more than 85 percent of land mammals exceeding a mass of 44 kilograms became extinct during the Pleistocene. Of these animals, one in particular was the large, flightless bird Genyornis newtoni. Based on 700 dates of their eggshells, their sudden disappearance was viewed about 50,000 years ago. Scientists dated

eggshells of both Genyornis and Dromaius in various climatic regions to not only compare the reactions of different birds, but in response to climate change as well. Additionally, the two bird species occur together in almost all regions showing that they coexisted and required similar resources.

After all dates of the eggshells were collected, scientists found that Genyornis disappear suddenly about 50,000 years ago while Dromaius eggshells were still found in the relative areas and continued to exist. This shows that absence of eggshells wasn’t due to lack of preservation, but rather due to extinction. Due to the evidence that Genyornis survived in multiple climates, as well as climate change at the time of their extinction being rather mild, the explanation of extinction caused by climate change is quite unlikely. Therefore, the presence of human populations must be looked at to assess the possibility of extinctions. Humans have been known to have impacts on large birds in places like New Zealand, for example, so this theory was not impossible. Genyornis were about two times the size of Dromaius and as a result less agile. Due to this, it is safe to assume that Genyornis were more likely to be targets for hunting because of their large frame and lack of speed. This also would explain the disappearance of Genyornis in the same areas where Dromaius survived; however, evidence of direct hunting of Genyornis is only found in one location.

The alternate explanation is that indirect human activity caused ecosystem destruction which then affected large birds. Studies of the Genyornis beak shows that it was a vegetation browser and was dependent on shrubland. Due to this, the burning practices of humans at that time could have cause unnatural wild fires, disrupting ecosystems and as a result, burning up the resources Genyornis needed to survive. These fires and disruption were just added stress that could have driven the large bird to extinction (Miller, 1999). This study, as opposed to the Beringia bears, shows the possibility of direct and indirect human causes of extinction.

Additionally, it further illustrates that the extinctions of the Pleistocene were various in both how they occurred and where.

As one can see again, there is significant evidence supporting the multiple theories of extinction of megafauna in the Plesitocene. Climate change and human expansion have played individual roles in these extinctions, but some of the most reasonable explanations include a combination of the both. If we look at modern experiences right now, we can recognize that

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habitat disruption from both anthropogenic climate change as well as an overall increase in human population are the primary factors contributing to the current endangerment and extinction of species. As ranges are being broken up and diminished, each population becomes more vulnerable to extinction from other factors (University of Maryland, 2014). Not only can these observations give us insight on how extinctions of the late Pleistocene megafauna may have occurred, but it should also serve as a warning of what may come if our current actions continue. With the knowledge we are continually gaining from scientific research, we need to put it to use in order to hopefully limit another mass extinction.

Reference List Alroy, John. (2001). A Multispecies Overkill Simulation of the End-Pleistocene Megafaunal Mass Extinction. Science, vol. 292. Retrieved 7 Nov. 2014.

Barnes, I., Matheus, P., Shapiro, B. (2002). Dynamics of Pleistocene Population Extinctions in Beringian Brown Bears. Science, vol. 295. Retrieved 7 Nov. 2014.

Barnosky, Anthony D. (2004). Assessing the Causes of Late Pleistocene Extinctions on the Continents. Science, vol. 306. Retrieved 6 Nov. 2014.

Gibbons, Robin. (2004). Examining the Extinction of the Pleistocene Megafauna. Stanford University. Retrieved 7 Nov. 2014.

Kaushik, N., Tiway, S. (2010). Extinction in the Late Quaternary Period. Academia.edu.

Retrieved 13 Nov. 2014.

Lisiecki, L., Raymo, M. (2007). Pilo-Pleistocene Climate Evolution: Trends and Transitions in Glacial Cycle Dynamics. Quaternary Science Reviews, vol 26. Retrieved 13 Nov. 2014.

Lomolino, M., Riddle, B., Whittaker, R., Brown, J. (2010). Biogeography. Sunderland, MA: Sinaur Associates, Inc. Miller, G., Magee, J., Johnson, B. (1999). Pleistocene Extinction of Genyornis newtoni: Human Impact on Australian Megafauna. Science, vol. 283. Retrieved 7 Nov. 2014.

Ripple, W., Valkenburgh, B. (2010). Linking Top-down Forces to the Pleistocene Megafaunal Extinctions. Biosciene, vol. 60. Retrieved 13 Nov. 2013.

University of Maryland Department of Geography. (2014). The Call of Distant Mammoths: The Pleistocene Megafaunal Extinctions. University of Maryland. Retrieved 1 Dec. 2014.