Some Pleistocene Megafauna may have Survived in the Yukon until the Mid-Holocene

One of the first entries I wrote for this blog over 10 years ago highlighted a study of seda-DNA (short for sediment DNA) from cores taken in Alaskan permafrost. Permafrost preserves DNA of local animals that were shedding hairs, urinating, and defecating on the landscape. Different levels of the core were radio-carbon-dated, and scientists came to a surprising conclusion–mammoths and horses survived thousands of years after the youngest known sub-fossil evidence. Mammoths lived on mainland Alaska until 9700 years BP, and horses survived there until 7000 years ago. Recently, some of the same scientists conducted a similar study in the Klondike region of the Yukon, and they came to a similar conclusion. This study of 4 sites in the region was more extensive and also kept track of plant DNA, so changes in the environment could be detected. Apparently, mammoth, horses, and bison persisted in small refugial populations in the region thousands of years after the youngest known dated specimens in the paleontological record. They call this a temporal ghost range. They detected a DNA signal of these extinct and extirpated species from 9200 years BP to 5700 years BP, while paleontological evidence indicated they had disappeared from the region ~12,000 years ago. The authors of this study concede older sediment could have mixed with younger sediment, causing a mistaken observation, but they think this is unlikely because the samples were the same from 21 different cores taken from 3 different sites, and changes in plant composition were consistent with what they expected from unmixed sediment.

Chart showing temporal abundance of megafauna and shifts in climate. From the below reference.
Chart showing abundance of seda-DNA of plants and animals from 4 permafrost core sites in the Yukon. Most species of megafauna were more abundant when grassy steppe was widespread, but they seem to have still occurred in refugial populations long after the shift from grassy steppe to more mesic shrub and forest habitat. Also from the below reference.

Scientists hoped the study could shed light on why the grassy steppe ecosystem of the Late Pleistocene collapsed. There are 2 schools of thought. Dr. Guthrie believes increased precipitation and cloud cover brought on by climate change changed the environment from grassy steppe to mesic peat marsh, willow scrub, and spruce forest; thus, depriving grass-eating animals of their primary food source and causing their extinction or extirpation. Dr. Zimov believes the disappearance of the megafauna itself caused the transformation of the landscape. He thinks herds of large animals trampling, grazing, and defecating suppressed woody growth and maintained the grasslands. Humans overhunted megafauna into extinction in this scenario.

Data from this study can be used to support both arguments. The biggest decline in mammoth populations occurred about 20,000 years ago–long before the transformation of the mammoth steppe into present day environments. There is ephemeral archaeological evidence of people in North America then, and they might have started reducing mammoth herds. Also, mammoths, horses and bison declined about the same time Homo sapiens became more common. However, the final significant decline in megafauna populations did occur when the grassy mammoth steppe was in transition to a landscape dominated by woody vegetation.

I’ve long been convinced humans are completely responsible for the extinction of most, if not all, Pleistocene megafauna, even in this remote region. I think populations of grazing megafauna did decline in this region due to changes in climate. But grassy environments never completely disappeared, and in some areas these refuges were still capable of supporting smaller populations of grazers which did maintain small grasslands with their activities. These refugial populations could have expanded to repopulate the region given favorable changes in climate, like those that occurred periodically throughout the Pleistocene. However, man wiped out these interglacial refugial populations of mammoths, bison, and horses. If not for man, I think there still would be local populations of these species in the region, but they just wouldn’t be as abundant as they were during Ice Ages. They were not picky feeders and could subsist on some woody vegetation. Incidentally, there is fossil evidence of steppe bison (Bison antiquus) from central Canada (not exactly in the region but not on the other side of the continent either), dating to the mid-Holocene. See:

Reference:

Murchie, T. et. al.

“Collapse of the Mammoth Steppe in Central Yukon as Revealed by Ancient Environmental DNA”

Nature Communications Dec 2021

https://www.nature.com/articles/s41467-021-27439-6

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One Response to “Some Pleistocene Megafauna may have Survived in the Yukon until the Mid-Holocene”

  1. Nice charts. – Stephen Bodio Says:

    […] A shorter blog version. […]

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