Nearly 13,000 years ago, there was a sudden and significant extinction of megafauna that continues to puzzle scientists to this day (PBS). This mass extinction event brought an end to some of the largest and most curious creatures that have ever lived in North America. A sampling of these creatures includes armadillos that were larger than cars and giant marsupials that were the size of hippos (The Guardian). There is a great deal of research done on this topic that attempts to answer just how these species became extinct at this time after living for tens of thousands of years on a changing planet. The suggested explanations for this extinction include overhunting by recently arrived humans, climate change unrelated to human activity, the spread of a mass disease that affected numerous species, a mass catastrophe event that caused great destruction, and some combinations of these theories (PBS). However, the most convincing and comprehensive theory that answers the fundamental question of what happened with the current, limited supply of information is that the human’s migration into the North American environment caused an interruption and upheaval of the food system, resulting in the loss of many megafauna species in a sequence of related events.
It’s hard to deny that humans had some sort of effect on this mass extinction with the widely accepted sequence of events in this time period. With the arrival of the Clovis people, the first humans to come to North America, approximately 13,000 years ago, the extinction of the megafauna in North America occurred within the next two thousand years, a relatively short time span relative to the age of the world (The Guardian). Without any prior knowledge of mass extinctions or paleontology, it would be hard to say that the limited timespan between the events is just a coincidence. It’s important to note that any environmental changes independent of humans could have very well contributed or accelerated the rapid loss of these large mammals in North America, but that they weren’t the primary factor either (Ripple and Van Valkenburgh 2010, pg. 516)
In the first chapter of A Green History of the World, Clive Ponting looks broadly at the interactions of humans and their environment as they began to settle the world and establish civilization in all reaches of the globe. Here, he makes the claim about the megafauna extinction that “there is little doubt that [the extinction] was the outcome of human intervention in some form” (pg. 35). The top-down hypothesis presented by Ripple and Van Valkenburgh builds upon this initial claim and looks closely at just how humans were able to interrupt the sequences of the system and thus cause the extinction of the megafauna. In order for this to happen, there were several preconditions in the ecosystem that allowed for humans to enter and disrupt the ecological balance. One of them is that many of these large megafauna were predator-limited, meaning that there were low numbers of them relative to the continent’s carrying capacity and there were a great deal of predators in relation to their population (Ripple and Van Valkenburgh 2010, pg.517). Another precondition that allowed the food system to collapse is that humans were omnivores and didn’t clash with other large predators like gray wolves and sabertooth cats over food sources frequently (pg.517). With humans’ defensive advantages like fire and congregation in groups along with their sudden introduction into North American food chains, they were able to join the ranks of the predators of North America without significant suffering (pg.517). With a great number of carnivore predators now in the food web, it would only be a matter of time before competition causes the prey to become extinct. Its’s important to note that there far more predators than human hunters and that humans played a small role in the killing and extinction of the megafauna (pg. 516). This is an important difference between the human overkill theory and this theory about the Pleistocene extinction event.
We can see evidence in support of this hypothesis by looking at the fossilized remains of the megafauna because they reveal a wealth of information about the cause of the extinction. One example is the examination of the tusk thickness growth rate of mammoths throughout the Pleistocene period and determining if there is any change in the size of the growth rings (pg. 518). With the understanding that larger rings mean there is plenty of available food and thinner rings mean there is little food available, it is clear from the data provided in the Ripple and Van Valkenburgh article that there was no decline in these growth rings size, showing that the food supply had no effect on this mass extinction (pg.518). We can also look at the teeth of the predators to better understand what their diet was made up of at a specific moment in time (pg.519). If the dental fracture rates are high in the fossils of the apex carnivores, then it is clear sign that there was a limited food supply for the carnivores because they were forced to consume more of the carcass for nourishment, and vice versa (pg. 519). Looking at several predatory carnivores from the late Pleistocene period, the rates of fractured teeth are much higher than any rates we see today, signaling that the food supply for carnivores was quite diminished at this time (pg.520). Both of these close looks into the fossil record give us evidence that points us towards the top-down hypothesis where humans disturbed the environmental balance and caused the megafauna extinction indirectly.
The idea that the entrance of humans into the food system of North America disrupted the delicate and complex balance of predators, foragers, and scavengers draws upon a contemporary understanding of how ecosystems function and the importance of every part of the habitat. An example in a modern-day environment of the effects of the disruption of a food system can be seen in the current food chains of Alaska (pg.517). With the start of some human hunting on moose, moose populations are on the decline along with sheep populations, as wolves had to switch to different prey to continue their survival (pg.517). As a result, the sheep, moose, and wolf populations all decline quickly as they are all either prey with more predators or predators with less prey to feed on. (pg. 517) Humans don’t face much decline in this specific scenario because they are able to switch food sources easily and as omnivores, they don’t depend on these sources of meat for survival.
This is the most convincing answer to this perplexing Pleistocene extinction because it is a simple application of the idea of dependency of every part of a habitat on one another that is prominent in ecology today. The idea of trophic cascades and massive changes occurring in an ecosystem due to a small event or extinction is based in historical observations and can easily be applied to a major extinction event that occurred approximately 11,000 years ago. Relying upon a limited fossil record to determine if a disease or natural disaster occurred is quite difficult as there is simply not enough evidence to back these theories. In addition, the idea that humans played the largest role in killing off the megafauna doesn’t seem plausible given the vast numbers of megafauna in North America and the omnivore diet that humans consume that would limit how often they would need to kill prey for survival.
In sum, the migration of homo sapiens across the Bering Strait approximately 14,000 years ago brought unexpected change to the complex food web of North America and thus unintentionally caused a mass extinction event that may or may not have been influenced by climate change. This answer to this highly contentious question doesn’t rely upon an incomplete fossil record to make an enormous claim, but rather historical observations of how impactful the loss or addition of a predator in an ecosystem can be.
Bibliography
McKie, Robert. “What killed off the giant beasts-climate change or man?” The Guardian, March 15, 2014. http://bioscience.oxfordjournals.org/citmgr?gca=bioscience%3B60%2F7%2F516
Ponting, Clive. A Green History of the World: The Environment and the Collapse of Great Civilizations. New York: St. Martin’s Press, 1991.
Ripple, William J and Blaire Van Valkenburgh. “Linking Top-down Forces to the Pleistocene Megafaunal Extinctions.” BioScience Vol. 50 No.7 (2010).
Tyson, Peter. “End of the Big Beasts.” NOVA/PBS, March 1st, 2009. http://www.pbs.org/wgbh/nova/evolution/end-big-beasts.html .
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