A new study of Pleistocene megafauna extinctions published in the latest issue of Science received considerable publicity. Most of the media outlets misrepresented the study’s conclusions. Perhaps journalists were confused by the title of the article: “Abrupt Warming Events Drove Late Pleistocene Holarctic Turnover.” The overwhelming majority of media outlets reported that the study suggests climate, not overhunting by man caused the late Pleistocene megafauna extinctions. None of these journalists must have read the abstract because the last sentence of it concedes that man played an important role in megafauna extinctions. The following passage is the last sentence of the abstract (capital letters mine).
“The presence of many cryptic biotic transitions prior to the Pleistocene/Holocene boundary revealed by ancient DNA confirm the importance of climate change in megafaunal population extinctions and suggests that METAPOPULATION STRUCTURES NECESSARY TO SURVIVE SUCH REPEATED AND RAPID CLIMATE SHIFTS WERE SUSCEPTIBLE TO HUMAN IMPACTS.”
This means the authors of this study believe rapid climate change reduced and scattered megafaunal populations, making them more vulnerable to human hunting. They make it clear that humans were ultimately responsible for the megafauna extinctions in the last 2 sentences of the main body of this study.
“Human presence could have had a major and negative impact on metapopulations by interrupting subpopulation connectivity, especially by concentrating on regular pathways between resource-rich zones, potentially leaving minimal sign of direct hunting. By interrupting metapopulation processes (i.g. dispersal, recolonization), humans could have both exacerbated regional extinctions brought on by climate change and allowed them to coalesce, potentially leading to the eventual regime shifts and collapses observed in megafaunal ecosystems. The lack of evidence for longer such ecological shifts during earlier periods of the glacial (i.g. >45 kyr) when interstadial events were known, but humans were not, suggests a synergistic role for humans in exacerbating the impacts of climate change and extinction in terminal events.”
The authors of this study collected no original data. Instead, they analyzed data already available from previous studies. They plotted climate data from Greenland Ice Cores with studies of megafauna DNA that determined changes over time in population structures. (Scientists can determine past average annual temperatures by analyzing oxygen isotope ratios in Greenland ice cores that go back 100,000 years. These ice cores have annual rings from summer glacial melt.) The scientists used genetic data from 31 species, including 2 from North America (mastodon and elk), 3 from Beringia, and 26 from Eurasia. They found a close correlation between sudden warming pulses, known as Dansgaard-Oeschger Events, and reduced populations and local extinctions of megafauna. Dansgaard-Oeschger Events precipitated interstadials (warm climatic phases) within the Ice Age that lasted for 1-3 thousand years. In some regions some species of megafauna became locally extinct. Later, when the climate returned to a cooler more arid stage, different clades of the same species recolonized these regions from relic refugia where they found suitable habitat during the interstadial.
Chart showing Greenland Ice Core data. This data shows past average annual temperature fluctuations. The arrows point to Dansgaard-Oeschger Events–abrupt warming events that caused brief interstadials during the Ice Age. Prior to the appearance of Homo sapiens in Europe, these interstadials were not associated with local extirpations of megafauna species. But after modern man colonized Eurasia, various species of megafauna did suffer local extinctions following Dansgaard-Oeschger events. Sudden climate change made these species more vulnerable to human overhunting, according to the study linked below.
Populations of megafauna remained stable during stadials, the cold phases of the Ice Ages, because the environments they lived in were static. Stadials were longer lasting than interstadials, and open grassy plains prevailed for millennia. The megafauna became adapted to living in these environments. But rapid warming with increased precipitation caused dramatic changes to the environment. Unproductive spruce forests started expanding, and this fragmented and reduced grassland habitat, forcing megafauna to migrate longer distances for suitable pasturage. They followed the same routes between pastures, making them easier for humans to ambush.
I have a couple quibbles with this study. First, I think this study is not applicable to North America. The overwhelming majority of genetic data is from Eurasian populations of megafauna. Just 2 North American species were used–a database that is far too small upon which to base any conclusions. Moreover, I hypothesize mastodon populations likely increased during interstadials because they were a semi-aquatic species that favored wetland habitat. Warm wet interstadials caused wetlands to expand. The scientists involved in this study used genetic data from just a single population of mastodons. I’m extremely skeptical that mastodons declined in abundance when wetland habitat expanded.
Second, the authors note woolly mammoths survived later in Eurasia than in North America, despite having a longer exposure to human hunting there. I assume they were making the point that climate played a greater role than human hunting in regulating megafauna populations. Indeed, a radio-carbon date of 9760 BP from a woolly mammoth rib found in northern Russia is more recent than a 10,800 BP date from the latest mammoth specimen from continental North America. However, the latter very likely does not represent the last mammoth that ever lived in North America, and sedaDNA from Alaska permafrost suggests mammoths still lived in Alaska 9000 calendar years ago. Mammoths may have lasted longer in North America–there just isn’t enough data to know for sure. It’s unlikely that the absolute last mammoth was preserved. Moreover, northern Russia was probably uninhabited or more sparsely inhabited than most of North America during the late Pleistocene, rendering the author’s point moot.
Cooper, Alan; et. al.
“Abrupt Warming Events Drove Late Pleistocene Holarctic Megafaunal Turnover”