Megafauna Extinctions - What (or Who) Killed All the Big Mammals?

Megafaunal extinctions refers to the documented die-off of large-bodied mammals (megafauna) from all over our planet at the end of the last ice age, at about the same time as the human colonization of the last, farthest-flung regions out of Africa. The mass extinctions were neither synchronous nor universal, and the reasons proffered by researchers for those extinctions include (but are not limited to) climate change and human intervention.

The Late Pleistocene megafaunal extinctions occurred during the Last Glacial–Interglacial Transition (LGIT), essentially the last 130,000 years, and it affected mammals, birds, and reptiles. There have been other, much earlier mass extinctions, impacting animals and plants alike. The five largest mass extinction events in the past 500 million years (mya) occurred at the end of the Ordovician (443 ma), the Late Devonian (375–360 mya), the end of the Permian (252 mya), the end of the Triassic (201 mya) and the end of the Cretaceous (66 mya).

Pleistocene Era Extinctions

Before early modern humans left Africa to colonize the rest of the world, all of the continents were already populated by a large and diverse animal population, including our hominid cousins, Neanderthals, Denisovans, and Homo erectus. Animals with body weights greater than 100 pounds (45 kilograms), called megafauna, were abundant. Extinct elephant, horse, emu, wolves, hippos: the fauna varied with the continent, but most of them were plant-eaters, with few predator species. Almost all of these megafauna species are now extinct; almost all of the extinctions occurred around the time of the colonization of those regions by early modern humans.

Before migrating far from Africa, early modern humans and Neanderthals co-existed with megafauna in Africa and Eurasia for several tens of thousands of years. At the time, most of the planet was in steppe or grassland ecosystems, maintained by megaherbivores, massive vegetarians that impeded the colonization of trees, trampled and consumed saplings, and cleared and broke down the organic matter.

Seasonal aridity influenced the availability of rangelands, and climate change involving increases in moisture is documented for the late Pleistocene, which is believed to have exerted extinction pressure on megafaunal rangeland grazers by altering, fragmenting and in some cases replacing the steppes with forests. Climate change, migration of humans, extinction of megafauna: which came first?

Which Came first?

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Giant Mammals of the Cenozoic Era

By Bob Strauss

Despite what you may have read, it is not clear which of these forces—climate change, human migration, and megafaunal extinctions—caused the others, and it is very likely that the three forces worked together to re-sculpt the planet. When our earth became colder, the vegetation changed, and animals that did not adapt rapidly died out. Climate change may well have driven human migrations. People moving into new territories as new predators might have had negative effects on the existing fauna, through overkill of a particularly easy animal prey, or the spread of new diseases.

But it must be remembered that the loss of the mega-herbivores also drove climate change. Enclosure studies have shown that large-bodied mammals such as elephants suppress woody vegetation, accounting for 80% of woody plant loss. The loss of large numbers of browsing, grazing, and grass-eating mega-mammals certainly led or added to the decrease of open vegetation and habitat mosaics, the increased occurrence of fire, and the decline of co-evolved plants. Long-term effects on seed dispersion continue to affect plant species distributions for thousands of years.

This co-occurrence of humans in migration, climate change, and animal die-off is the most recent time in our human history where climate change and human interactions together re-designed the living palette of our planet. Two areas of our planet are the primary focus of the studies of Late Pleistocene megafaunal extinctions: North America and Australia, with some studies continuing in South America and Eurasia. All of these areas were subject to massive changes in temperature, including the variable presence of glacial ice, and plant and animal life; each sustained the arrival of a new predator in the food chain; each saw related decreases and reconfiguration of the available animal and plants. Evidence collected by archaeologists and paleontologists in each of the areas tells a slightly different story.

North America

• Earliest human colonization: 15,000 calendar years ago (cal BP), (pre-Clovis sites)
• Last glacial maximum: ~30,000–14,000 cal BP
• Younger Dryas: 12,900–11,550 cal BP
• Important sites: Rancho La Brea (California, USA), many Clovis and pre-Clovis sites.
• Die-off range: 15% disappeared during Clovis and the Younger Dryas overlap, 13.8–11.4 cal BP
• Species: ~35, 72% of megafauna, including dire wolf (Canis dirus), coyotes (C. latrans), and saber-toothed cats (Smilodon fatalis); American lion, short-faced bear (Arctodus simus), brown bear (Ursus arctos), scimitar-tooth sabercat (Homotherium serum), and dhole (Cuon alpinus)

While the exact date is still under discussion, it is most likely that humans first arrived in North America no later than about 15,000 years ago, and perhaps as long ago as 20,000 years ago, at the end of the last glacial maximum, when entrance into the Americas from Beringia became feasible. The North and South American continents were rapidly colonized, with populations settled in Chile by 14,500, surely within a few hundred years of the first entry into the Americas.

North America lost about 35 genera of mostly large animals during the Late Pleistocene, accounting for perhaps 50% of all mammal species larger than 70 lbs (32 kg), and all species larger than 2,200 lbs (1,000 kg). The ground sloth, American lion, dire wolf, and short-faced bear, wooly mammoth, mastodon and Glyptotherium (a large bodied armadillo) all disappeared. At the same time, 19 genera of birds disappeared; and some animals and birds made radical changes in their habitats, permanently changing their migration patterns. Based on pollen studies, plant distributions also saw a radical change primarily between 13,000 to 10,000 calendar years ago (cal BP).

Between 15,000 and 10,000 years ago, biomass burning gradually increased, particularly at the movements of rapid climate change at 13.9, 13.2, and 11.7 thousand years ago. These changes are not currently identified with specific changes in human population density or with the timing of the megafaunal extinction, but that doesn't necessarily mean that they are unrelated—the effects of the loss of large-bodied mammals on vegetation are very long-lasting.

Australian Evidence

• Earliest human colonization: 45,000–50,000 cal BP
• Important sites: Darling Downs, Kings Creek, Lynch's Crater (all in Queensland); Mt Cripps and Mowbray Swamp (Tasmania), Cuddie Springs and Lake Mungo (New South Wales)
• Die-off range: 122,000–7,000 years ago; at least 14 Mammalian genera and 88 species between 50,000–32,000 cal BP
• Species: Procoptodon (giant short-faced kangaroo), Genyornis newtoni, Zygomaturus, Protemnodon, sthenurine kangaroos and T. carnifex

In Australia, several studies of megafaunal extinctions have been conducted of late, but the results of them are contradictory and conclusions must be considered controversial today. One difficulty with the evidence is that the human entrada into Australia occurred so much longer ago than that of the Americas. Most scholars agree that humans reached the Australian continent at least as long as 50,000 years ago; but evidence is sparse, and radiocarbon dating ineffective for dates older than 50,000 years old.

Genyornis newtoni, Zygomaturus, Protemnodon, sthenurine kangaroos and T. carnifex all disappeared at or shortly after the human occupation of the Australian mainland. Twenty or more genera of giant marsupials, monotremes, birds, and reptiles were likely wiped out due to the direct intervention of human populations since they can find no connection to climate change. The local decline in diversity began nearly 75,000 years before human colonization, and thus cannot be the result of human intervention.

South America

Less scholarly research concerning the mass extinctions in South America has been published, at least in the English-language academic press. However, recent investigations suggest that the extinction intensity and timing varied across the South American continent, beginning in the northern latitudes several thousand years before the human occupation, but becoming more intense and rapid in the southern higher latitudes, after humans arrived. Further, the pace of extinction seems to have accelerated about 1,000 years after the humans arrived, coinciding with regional cold reversals, the South American equivalent of Younger Dryas.

Some scholars have noted patterns of stadial/interstadial differences between North and South America, and have concluded that although there is no evidence for the "blitzkrieg model"—that is to say, mass-killing by humans--the human presence in combination with the rapid expansion of forests and environmental changes seems to have led to the collapse of the megafaunal ecosystem within a few hundred years.

• Earliest human colonization: 14,500 cal BP (Monte Verde, Chile)
• Last glacial maximum: 12,500-11,800 cal BP, in Patagonia
• Cold Reversal (Roughly equivalent to the Younger Dryas): 15,500-11,800 cal BP (Varies across the continent)
• Important sites: Lapa da Escrivânia 5(Brazil), Campo La Borde (Argentina), Monte Verde (Chile), Pedra Pintada (Brazil), Cueva del Milodón, Fell's Cave (Patagonia)
• Die-off: 18,000 to 11,000 cal BP
• Species: 52 genera or 83% of all megafauna; Holmesina, Glyptodon, Haplomastodon, prior to human colonization; Cuvieronius, Gomphotheres, Glossotherium, Equus, Hippidion, Mylodon, Eremotherium and Toxodon about 1,000 years after initial human colonization; Smilodon, Catonyx, Megatherium, and Doedicurus, late Holocene

Recently, evidence of the survival of several species of giant ground sloth has been discovered in the West Indies, to as late as 5,000 years ago, coincident with the arrival of humans in the region.

Selected Sources

• Barnosky, Anthony D., et al. "Variable Impact of Late-Quaternary Megafaunal Extinction in Causing Ecological State Shifts in North and South America." Proceedings of the National Academy of Sciences 113.4 (2016): 856–61. 
• DeSantis, Larisa R. G., et al. "Dietary Responses of Sahul (Pleistocene Australia–New Guinea) Megafauna to Climate and Environmental Change." Paleobiology 43.2 (2017): 181–95. 
• Galetti, Mauro, et al. "Ecological and Evolutionary Legacy of Megafauna Extinctions." Biological Reviews 93.2 (2018): 845–62. 
• Metcalf, Jessica L., et al. "Synergistic Roles of Climate Warming and Human Occupation in Patagonian Megafaunal Extinctions During the Last Deglaciation." Science Advances 2.6 (2016). 
• Rabanus-Wallace, M. Timothy, et al. "Megafaunal Isotopes Reveal Role of Increased Moisture on Rangeland During Late Pleistocene Extinctions." Nature Ecology & Evolution 1 (2017): 0125. 
• Tóth, Anikó B., et al. "Reorganization of Surviving Mammal Communities after the End-Pleistocene Megafaunal Extinction." Science 365.6459 (2019): 1305–08. 
• van der Kaars, Sander, et al. "Humans Rather Than Climate the Primary Cause of Pleistocene Megafaunal Extinction in Australia." Nature Communications 8 (2017): 14142.