A tale of mammoths and a disappearing lake

A Herd of Mammoths. Source: WikimediaCommons/Kira Sokolovskaia

A Herd of Mammoths. Source: WikimediaCommons/Kira Sokolovskaia

A wonderful study revealed a sad story of the St. Paul Island population of woolly mammoths. Using a creative and diverse set of analytical approaches, scientists identified freshwater shortage as the likely cause of their extinction.
A cross-disciplinary collaboration of scientists from The Pennsylvania State University, University of Alaska, and University of California, Santa Cruz, yielded five different types of evidence suggesting that the last mammoth disappeared from St. Paul Island at the same time as the main (or only) freshwater source evaporated.

What goes around, comes around – even extinction

St. Paul Island is a rather small island of 110 km2. It was originally part of the Bering Land Bridge, but became isolated due to rising sea levels at the end of the Ice Age. A population of mammoths was trapped on the island, which strangely saved them from extinction.
When the St. Paul mammoths finally went extinct 5,600 years ago, there was only one other population of mammoths – their distant relatives also struggling for survival about 1,500 kilometers from there, on the much larger but mountainous Wrangel Island.
While we still don’t know what exactly happened to the very last mammoths on Wrangel Island, climate is probably to blame for the St. Paul extinction. The rising sea “consumed” coastal lakes and the increasingly arid conditions between 8,000 and 5,300 years ago led to freshwater shortage.
As the lake was disappearing, mammoths were losing ground, like literally.

“Mammoth activity around Lake Hill probably contributed to the degradation of freshwater quality. As ecosystem engineers, elephants strongly modify their local environments, especially at water holes and other areas of heavy use… Thus, mammoth activity likely denuded the lake margin, accelerated rates of erosion, and contributed to lake infilling and decreased water quality.”

Bulletproof evidence

How did the scientists figure out all the details? Professor Russell Graham and his colleagues performed a very robust analysis of a sedimentary core from the Lake Hill, namely: (1) radiocarbon dating of mammoth samples, (2) mammoth DNA in the lake sediments, (3) three types of coprophilous spore types, (4) stable isotope analysis, and (5) freshwater species composition.
And miraculously all of these indicators pinpoint the extinction to 5,600 years ago. Stable nitrogen isotope values from mammoth bones and stable oxygen isotope values from the lake sediments suggested the increasing aridity from approximately 9,500 years ago. The changing species composition shows how the lake was changing between 7,850 and 5,600 years ago.

“The disappearance of mammoths from St. Paul Island was accompanied by pronounced lake shallowing and increased water turbidity between 7,850 and 5,600 y ago, indicated by significant shifts from pelagic to littoral cladocerans, decreases in planktonic diatoms, and increases in tychoplanktonic diatoms (Fig. 3 and SI Appendix, SI Text). A concurrent rise in conductivity is indicated by an increased abundance of diatoms and cladocerans that are tolerant of high electrolyte concentrations, notably the cladoceran Alona circumfibriata (Fig. 3), which is resilient to fluctuating salinity and can be an indicator of enhanced salinity in lakes (17) (SI Appendix, SI Text).”

Figure 3 showing how the Lake Hill changed before the mammoth's extinction.

Figure 3 showing how the Lake Hill changed before the mammoth’s extinction.

Turbid and saltier water was all the mammoths had left at the end. Radiocarbon dates, mammoth DNA, and coprophilous spores from lake sediments all show presence of mammoths until approximately 5,600 years ago.
It’s almost unbelievable how well all the markers overlap. And that’s also why Graham and colleagues can be so damn sure (as far as you can be sure in the scientific world) that the St. Paul mammoths went extinct due to the freshwater shortage.
Russell W. Graham, Soumaya Belmecheri, Kyungcheol Choy, Brendan J. Culleton, Lauren J. Davies, Duane Froese, Peter D. Heintzman, Carrie Hritz, Joshua D. Kapp, Lee A. Newsom, Ruth Rawcliffe, Émilie Saulnier-Talbot, Beth Shapiro, Yue Wang, John W. Williams, and Matthew J. Wooller (2016) Timing and causes of mid-Holocene mammoth extinction on St. Paul Island, Alaska. PNAS, doi:10.1073/pnas.1604903113

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