By Rebecca Morelle
2 August 2016
(BBC News) – One of the last known groups of woolly mammoths died out because of a lack of drinking water, scientists believe.
The Ice Age beasts were living on a remote island off the coast of Alaska, and scientists have dated their demise to about 5,600 years ago.
They believe that a warming climate caused lakes to become shallower, leaving the animals unable to quench their thirst.
Most of the world's woolly mammoths had died out by about 10,500 years ago.
Scientists believe that human hunting and environmental changes played a role in their extinction.
But the group living on St Paul Island, which is located in the Bering Sea, managed to cling on for another 5,000 years.
This study in the Proceedings of the National Academy of Sciences suggests that these animals faced a different threat from their mainland cousins.
As the Earth warmed up after the Ice Age, sea levels rose, causing the mammoths' island home to shrink in size.
This meant that some lakes were lost to the ocean, and as salt water flowed into the remaining reservoirs, freshwater diminished further.
The fur-covered giants were forced to share the ever-scarcer watering holes. But their over-use also caused a major problem.
Lead author Prof Russell Graham, from Pennsylvania State University, said: "As the other lakes dried up, the animals congregated around the water holes.
"They were milling around, which would destroy the vegetation - we see this with modern elephants.
"And this allows for the erosion of sediments to go into the lake, which is creating less and less fresh water.
"The mammoths were contributing to their own demise." [more]
ABSTRACT: Relict woolly mammoth (Mammuthus primigenius) populations survived on several small Beringian islands for thousands of years after mainland populations went extinct. Here we present multiproxy paleoenvironmental records to investigate the timing, causes, and consequences of mammoth disappearance from St. Paul Island, Alaska. Five independent indicators of extinction show that mammoths survived on St. Paul until 5,600 ± 100 y ago. Vegetation composition remained stable during the extinction window, and there is no evidence of human presence on the island before 1787 CE, suggesting that these factors were not extinction drivers. Instead, the extinction coincided with declining freshwater resources and drier climates between 7,850 and 5,600 y ago, as inferred from sedimentary magnetic susceptibility, oxygen isotopes, and diatom and cladoceran assemblages in a sediment core from a freshwater lake on the island, and stable nitrogen isotopes from mammoth remains. Contrary to other extinction models for the St. Paul mammoth population, this evidence indicates that this mammoth population died out because of the synergistic effects of shrinking island area and freshwater scarcity caused by rising sea levels and regional climate change. Degradation of water quality by intensified mammoth activity around the lake likely exacerbated the situation. The St. Paul mammoth demise is now one of the best-dated prehistoric extinctions, highlighting freshwater limitation as an overlooked extinction driver and underscoring the vulnerability of small island populations to environmental change, even in the absence of human influence.
SIGNIFICANCE: St. Paul Island, Alaska, is famous for its late-surviving population of woolly mammoth. The puzzle of mid-Holocene extinction is solved via multiple independent paleoenvironmental proxies that tightly constrain the timing of extinction to 5,600 ± 100 y ago and strongly point to the effects of sea-level rise and drier climates on freshwater scarcity as the primary extinction driver. Likely ecosystem effects of the mega-herbivore extinction include reduced rates of watershed erosion by elimination of crowding around water holes and a vegetation shift toward increased abundances of herbaceous taxa. Freshwater availability may be an underappreciated driver of island extinction. This study reinforces 21st-century concerns about the vulnerability of island populations, including humans, to future warming, freshwater availability, and sea level rise.