Genetic analysis finds vegetation change around same time as megafauna extinction
Per Möller/Johanna Anjar
For want of mums woolly mammoths were lost.
A genetic analysis of ancient permafrost suggests that after the last Ice Age the Arctic shifted from a landscape dominated by nutritious flowering plants known as forbs to one dominated by hard-to-digest grasses and woody plants. Evolutionary geneticist Eske Willerslev of the University of Copenhagen and his colleagues report the finding in the Feb. 6 Nature. That shift may have helped drive the extinction of large herbivores such as woolly mammoths and woolly rhinoceroses, Willerslev speculates.
The researchers examined 242 permafrost samples from 21 sites in Siberia, Alaska and Canada. Each sample was carbon dated to determine its age. To identify plants in the samples, the researchers sequenced DNA from plant organelles that carry out photosynthesis.
From about 50,000 years ago until around 12,000 years ago, the most abundant plants were forbs that thrived in dry environments. They included relatives of plants such as prairie sagewort, yarrow, chrysanthemums and asters, the researchers discovered.
That means mammoth steppes were probably crazy quilts of blooming plants. Previous studies of ancient pollen had instead suggested the environment was marked by vast grasslands upon which mammoths and other large herbivores grazed.
Between 25,000 and 15,000 years ago, plant diversity plummeted to record lows in the cold, dry climate of the last Ice Age. Forbs still ruled the land, but fewer species could make a go of it.
After temperatures climbed and glaciers receded, the Arctic became wetter. Plant diversity rose again, but instead of reviving pre-Ice Age species, moister soils allowed grasses to rise in prominence and new species such as horsetails, cotton grass, willows and other woody plants to invade.
To find out what prehistoric animals were eating, Willerslev and his colleagues analyzed plant DNA from the guts or fossilized feces of eight large herbivores that lived between 55,000 and 21,000 years ago: four woolly mammoths, two woolly rhinoceroses, a bison and a horse. None of those animals roam the Arctic today. The ancient herbivores dined largely on forbs, the researchers discovered. About 63 percent of plant remains in the samples were forbs, with grasses comprising about 27 percent.
"Our study really questions the whole concept that a grass system was necessary to sustain the megafauna," Willerslev says. "I'm not saying that they weren't eating grasses, but our data suggest the forbs were sustaining them."
Because grasses and shrubs contain fewer nutrients and are harder to digest than forbs, large grazers may have essentially starved to extinction, he speculates. Animals such as reindeer survived by adapting to the vegetation switch, he says.
Missing megafauna may also have led to the decline of the forbs in a vicious cycle, some researchers say.
"All those great big beasts not only ate the vegetation but helped maintain it," says Scott Elias, a paleoecologist at Royal Holloway, University of London. Trampling and grazing would have spurred growth of forbs and kept grasses down.
Other researchers are happy to see a new technique for reconstructing long-gone landscapes but say the conclusion that vegetation changes contributed to mammoth extinction overreaches.
“This is a fantastic dataset, but if you look closer at the data, the story is not as straightforward as they make it,” says evolutionary biologist Michael Hofreiter of the University of Potsdam in Germany. Vegetation patterns changed most dramatically a few thousand years after woolly mammoths and other megafauna went extinct, he says. Mammoths were gone from most of the Arctic by 10,000 years ago.
What’s more, the gut contents and fecal samples all came from animals that lived before or in the early stages of the Ice Age. So the researchers don’t address what animals ate after the glaciers receded, says paleoecologist Jessica Blois of the University of California, Merced.
And an animal’s last meal doesn’t necessarily represent its usual diet, says Pavel Tarasov, a paleoecologist at the Free University of Berlin. He thinks deep snow may have interfered with the animals’ ability to find food.
Willerslev and others say that combining DNA analysis with other methods of reconstructing the past will give a more complete picture of how ecosystems vary along with climate. The result could aid in predicting future changes.
Editor's Note: This story was updated February 25, 2014, to correct the organ sampled (the gut) from the herbivores.
E. Willerslev et al. Fifty thousand years of Arctic vegetation and megafaunal diet. Nature Vol. 506, February 6, 2014, p. 47. doi:10.1038/nature12921.
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