by Mary Harrsch © 2025
In Part II we evaluated the potential impact of continuing climate change, a lack of evolutionary experience with human hunters and few options for migration or adaptation on the megafauna in the Americas as factors leading to their extinction which, in turn, resulted in additional dispersal of the small number of humans in the hemisphere, limiting interaction with other groups and subsequent innovation.
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| A mammoth reproduction that I photographed at The Natural History Museum of Los Angeles County, Los Angeles, California. |
But, could the impact of relatively few humans have actually impacted megafauna populations that much?
The small number of people present may not have single-handedly caused megafaunal extinctions, but their arrival could have tipped already stressed populations over the edge, especially in combination with rapid climate changes.
Scholars have proposed even a few thousand skilled hunters could have disrupted breeding populations of slow-reproducing megafauna like mammoths and giant sloths, especially in North America. Extinctions in South America, however, are harder to explain solely by human hunting since megafauna disappeared even in regions with relatively low human population densities.
Alternative Theories
If human overhunting in combination with climate stress wasn’t enough to tip the scales, what other factors might explain the extinctions:
• Disease Introduction – Some researchers propose that humans or their dogs may have introduced new diseases to vulnerable megafaunal populations.
• Ecosystem Collapse – A cascade of environmental changes (e.g., shifts in plant communities, wildfires) may have made it difficult for large herbivores to survive.
Considering these alternative theories, I found the introduction of disease by human hunters and/or their dogs among the least likely. I pointed out that even today, viral or bacterial diseases that are or become transmittable between species are rare as we’ve recently seen with the bird flu.
ChatGPT agreed: “You're absolutely right to be skeptical about the disease introduction hypothesis for megafaunal extinctions. While novel diseases can have devastating effects (e.g., smallpox on Indigenous American populations after European contact), the idea that human migrants or their domesticated animals introduced a pathogen that wiped out dozens of megafaunal species is highly speculative and lacks direct evidence.
Why the Disease Hypothesis Is Unlikely
1. Zoonotic Transmission is Rare Across Distantly Related Species
o Most viruses and bacteria are species-specific and do not easily jump between distantly related animals.
o Even when a disease does jump species (e.g., SARS-CoV-2, rabies), it usually requires close and repeated contact, which was unlikely between early human hunters and megafaunal species.
o Many extinct megafauna were herbivores, making them less likely to share diseases with humans or dogs than carnivores might be.
2. Domesticated Animals Were Almost Nonexistent in Early America
o In later historical extinctions, domesticated animals (like European livestock) introduced diseases to wild populations.
o But early Beringian migrants had no livestock—they were hunter-gatherers, traveling in small bands.
o The only domesticated animal potentially present was the dog, but there’s no clear evidence that ancient American dogs carried a megafauna-killing disease.
3. No Paleopathological Evidence of Disease in Megafauna
o Fossilized remains show no widespread signs of infection or disease in the bones of extinct megafauna.
o In contrast, when disease drives extinction (as seen in the Tasmanian devil facial tumor disease or amphibian chytrid fungus), we often find clear evidence of pathology in preserved specimens.
What About a “Hyperdisease” Hypothesis?
• Some researchers have suggested that a single virulent pathogen ("hyperdisease") could have wiped out American megafauna rapidly, similar to how myxomatosis decimated Australian rabbits.
• However, for such a disease to explain the near-simultaneous extinction of dozens of unrelated species (mammoths, giant sloths, saber-toothed cats, horses, etc.), it would need to:
o Jump across vastly different species.
o Kill at an extreme rate without allowing hosts to develop immunity.
o Leave no survivors or immune populations to recover.
No known disease behaves this way at such a large scale.
More Plausible Explanations for Megafaunal Extinction
If disease was not a major driver, other factors become more compelling:
Climate Change – Rapid warming (~14,700 BCE and again ~12,900 BCE) altered food sources, disrupted ecosystems, and increased drought stress. Human Hunting – While not a sole cause, even low-level predation by humans could have tipped struggling populations over the edge. Ecosystem Disruption – The disappearance of one keystone species (e.g., mammoths shaping grasslands) may have triggered cascading extinctions.So, I asked ChatGPT to explore the Ecosystem Disruption theory further. For example, how would the disappearance of mammoths have changed the environment so much as to create cascading extinctions?
ChatGPT replied: How Did Mammoths Shape Their Environment?
Mammoths (and other large herbivores like mastodons and giant ground sloths) played a major role in maintaining the Ice Age steppe ecosystem, which stretched across North America, Eurasia, and Beringia. Their activities significantly shaped the landscape:
1. Grassland Maintenance ("Mammoth Steppe")
o Mammoths acted like natural bulldozers, trampling and uprooting trees, preventing forests from encroaching on open grasslands.
o By grazing heavily, they maintained a mosaic of open steppe and tundra ecosystems that supported many other species.
o When mammoths disappeared, grasslands were no longer maintained, allowing shrubs and forests to take over, fundamentally altering the ecosystem.
2. Soil and Nutrient Cycling
o Large herbivores like mammoths helped cycle nutrients by breaking down vegetation and spreading fertile dung, which sustained the growth of grasses.
o Without them, nutrient flow slowed, leading to lower soil fertility and changes in plant composition.
3. Permafrost and Climate Regulation
o Recent studies suggest that mammoths helped keep permafrost intact by trampling snow, preventing it from acting as an insulating blanket.
o With fewer megafauna compacting the snow, permafrost thawed more easily, releasing carbon and methane, potentially accelerating climate warming.
Cascading Extinctions: How Losing Mammoths Affected Other Species
1. Loss of Grazing Herbivores → Loss of Predators
o The disappearance of mammoths and other megafauna meant a dramatic loss of prey for large predators like:
American lions (Panthera atrox)
Short-faced bears (Arctodus simus)
Dire wolves (Aenocyon dirus)
Saber-toothed cats (Smilodon fatalis)
o As prey populations declined, these apex predators faced starvation and eventual extinction.
2. Shrubland Expansion → Habitat Loss for Smaller Grazers
o With mammoths no longer maintaining open grasslands, many areas became overgrown with shrubs and trees.
o Species like horses and bison, which depended on vast open spaces, may have struggled with the changing vegetation.
o This shift may have contributed to the eventual extinction of North American horses and camelids, which once thrived but vanished from the continent.
3. Increased Fire Frequency → Habitat Destruction
o Studies suggest that when large herbivores disappeared, the build-up of dry, uneaten vegetation led to more frequent and intense wildfires.
o This could have further altered ecosystems, making them less hospitable for remaining large mammals.
Modern Evidence for Keystone Megafauna Effects
We can see similar processes happening today when large herbivores are removed:
• Elephants in Africa and Asia:
o Elephants uproot trees and maintain grasslands; when they are removed, forests encroach, changing entire ecosystems.
• American Bison:
o Bison grazing prevents shrub overgrowth and maintains prairies. Their near-extinction led to major habitat changes in North America.
• Rewilding Experiments:
o In Pleistocene Park (Siberia), scientists are reintroducing large herbivores to restore Ice Age-like ecosystems, showing that they can slow permafrost thaw and maintain grasslands.
Conclusion
The loss of mammoths and other megafauna likely caused a chain reaction of ecological disruptions:
• Grasslands turned into shrublands and forests, reducing food for herbivores.
• Nutrient cycles slowed, making the environment less productive.
• Large predators lost prey and went extinct.
• Fires and permafrost thawing further destabilized ecosystems.
While climate change and human hunting both played roles in megafaunal extinction, this ecosystem collapse may have ensured that once populations declined, they could never recover.
So, I followed up with “If the human hunters concentrated on mammoths as their chief prey, could the rather small numbers of humans then trigger the ecological collapse?”
ChatGPT replied, “Yes, it's very possible that even small numbers of human hunters could have triggered ecological collapse if they focused heavily on mammoths. This aligns with the idea of a keystone species-driven extinction cascade, where the removal of a single dominant species (mammoths) disrupts the entire ecosystem.
How Could a Small Population of Hunters Cause a Big Impact?
1. Mammoths Had Slow Reproduction Rates
o Like modern elephants, mammoths likely had long gestation periods (up to 22 months) and low birth rates (one calf every 4–6 years).
o Even a small increase in mortality from human hunting could outpace the natural birth rate, leading to population decline and eventual extinction.
o If mammoth populations were already stressed by climate change, a small additional pressure from hunting could have been the tipping point.
2. Targeting Large, Predictable Prey
o Humans would have likely preferred mammoths over faster, smaller, or more elusive prey:
Mammoths provided huge amounts of meat, fat, and bones for tools and shelter. They moved in herds and followed predictable migration routes, making them easier to locate than smaller, scattered animals. Evidence from sites like Wrangel Island suggests humans may have continued to hunt isolated mammoth populations to extinction.o Even occasional hunting of mammoths could cause a gradual population decline over generations.
3. Domino Effect: How Mammoth Loss Triggered Ecological Collapse
If humans hunted mammoths to extinction, the ripple effects could have been disproportionately large:
o Loss of Grasslands → Shrubs and forests expanded, reducing habitat for other grazers (horses, bison).
o Predator Starvation → Saber-toothed cats, American lions, and dire wolves lost a key food source.
o Nutrient Decline → Fewer large herbivores meant less manure fertilizing the landscape, reducing plant productivity.
o More Frequent Wildfires → Without mammoths trampling and eating vegetation, dry plant material built up, increasing fires.
Would This Have Been Enough to Collapse the Whole Ecosystem?
Possibly, but the process was likely gradual and complex.
• Mammoths were already struggling with habitat shifts due to climate warming.
• A slow, long-term decline (not an instant collapse) may have allowed some species to adapt, while others perished.
• Humans may have hunted opportunistically, but if they continued hunting faster than mammoth populations could recover, they may have sealed the species’ fate.
Modern Parallels: Can Small Human Populations Collapse Ecosystems?
Yes, we’ve seen similar patterns in historical and modern times:
• Overhunting of North Atlantic cod – Even with limited early technology, human fishing pressures were enough to cause a collapse in the cod population.
• Elephant poaching today – Poaching alone hasn’t wiped out elephants, but in some areas, elephant loss has transformed landscapes, turning savannas into forests.
• Island extinctions (New Zealand, Madagascar, Hawaii) – Small human populations caused massive extinctions of flightless birds and other animals.
Conclusion
Even a relatively small number of humans—if they primarily targeted mammoths—could have triggered an extinction cascade. The mammoth’s role as a keystone herbivore meant its loss reshaped the ecosystem, leading to further declines in other species.
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