Friday, 12 August 2016

Understanding the demise of the last Mammoths of St Paul Island.

Mammoths, Mammuthus spp., disappeared from the mainlands of Eurasia and North America around the end of the last Pleistocene glaciation, between 14 000 and 13 200 years ago, with some populations having possibly persisted as late as 10 500 years ago. However populations of Woolly Mammoths, Mammuthus primigenius, are known to have survived on several Beringian islands long after the end of the Pleistocene, with the last populations having survived on Wrangel Island (north of Siberia) till about 4020 years ago and St. Paul Island (in the Pribilof Islands of the Bering Sea) till around 5600 years ago. The demise of the Mammoths of Wrangel Island is known to have occurred around the time the first Humans arrived on the island, suggesting strongly that they may have become extinct directly due to Human activity. However the first Humans to reach St. Paul Island thought to have arived in the late eighteenth century, long after the demise of the island's last Mammoths, suggesting that another cause must have been responsible for there demise.

In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 1 August 2016 a team of scientists led by Russell Graham of the Department of Geosciences at The Pennsylvania State University describe the results of a study into the demise of the last Mammoths of St Paul Island.

St Paul Island is thought to have been cut off from the Bering Land Bridge between 14 700 and 13 500 years ago, and shrunk rapdily in size until about 9000 years ago, then more slowly untill about 6000 years ago, when it reached approximately its current size.

The previous last recorded date for Mammoths on St Paul Island was about 6480 years ago, but such dates are not generally thought to represent the last actual date a species was present. Graham et al. carried out radiocarbon dating of collagen from 14 newly discovered Mammoth remains from the island, obtaining a latest date of 5530 years ago, ~about 950 years after the previous last date for Mammoths on St Paul Island, but still ~1500 years before the last known date for Mammoths on Wrangel Island.

Map of current continents (dark gray) and the past position of the Bering Land Bridge (light gray) with red boxes indicating Wrangel Island (Upper) and St. Paul Island (Lower). Graham et al. (2016).

Graham et al. then examined a series of sediment cores from Lake Hill, a freshwater lake near the center of the island, and the largest and deepest body of water on the island at about 1.3 m. These were then examined for four different climatic proxies across the period when the Mammoths disappeared. These were the spores of three coprophilous Fungi (species that specialize in growing on animal dung) Sporormiella, Sordaria, and Podospora and the presence and nature of sedimentary DNA. The general environement was reconstructed across this period using a variety of proxies including Cladocerans, Diatoms, pollen, plant macroremains, and stable isotopes, and dated using radiocarbon dates from the plant macroremains and tephrochronology (isotope dates from volcanic ash layers).

Mammoth DNA was present in soil samples till about 5650 years ago, while the coprophilous Fungi Sporormiella and Sordaria disappeared 5680 and 5650 years ago respectively (Podospora disappeared much earlier), supporting the idea that the last Mammoths lived on the island about five and a half milenia ago. 

This date coencides with a marked drying of the environment on the island, as indicated by the environmental proxies. Species of Cladocerans and Diatoms associated with a planktonic lifestyle in clear, deep, lake waters disappear, and are replaced by specues associated with shallow, turbid (muddy) waters. 

This timing also coencides with a period of increased climatic instability recorded in other parts of the Aluetian Islands and Alaska, which would be consistent with reduced rainfall on the island lowering the availability of fresh water. Modern Indian and African Elephants consume between 70 and 200 litres of water per day, and Mammoths are likely to have had similar requirements, suggesting that any such draught could have hit a small island population very hard. Under such circumstances the Mammoths may have been forced to crowd around a few greatly reduced water sources, which would have led to further environmental degredation, causing increased erosion around the pools and degrading the water quality.

See also... Computed Tomography studies of two Woolly Mammoth calves from Russia.            The Woolly Mammoth, Mammuthus primigenius, is thought to have diverged from the earlier Steppe Mammoth Mammuthus trogontherii in northeast... Dwarf Pachyderms of Crete, Mammoths or Straight-Tusked Elephants?                  Fossil Dwarf Elephants are known from a number of small islands around the world; this is not altogether surprising, dwarfism is common in populations of animals cut of on small islands (as is giantism). Animals in such environments often need to adapt to... Nitrogen tells us about the diet of Mammoths.                                             Nitrogen has two stable isotopes, Nitrogen-14 (¹⁴N) and Nitrogen-15 (¹⁵N), which have different atomic weights, but identical chemical properties, and can be incorporated into identical compounds by...
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