All in the timing: Decline of big beasts triggered ecological chain reaction

All in all, the period since the ice age abated about 15,000 years ago has been pretty interesting. Melting ice raised the oceans, flooding the Bering Strait land bridge across which the Americas were populated. Temperatures rose around the globe, leading to the invention of cities, armies, writing and bacon. Here’s an enduring question. Why were the giant mammals that made the Americas more zoologically diverse than Africa all exterminated within a few thousand years after the big melt-down? Bye-bye beavers as big as black bears, giant sloths, saber-toothed cats, and the elephant-like mastodon. [svgallery name=”mastadon”] As Australian paleontologist Christopher Johnson wrote in Science this week, all 10 species of mammals weighing more than a ton had gone extinct in North America by 10,000 years ago. Why? Many theories are proposed for the sudden disappearance: An impact of a comet or asteroid around 12,900 years ago. Rapid ecological changes that accompanied the warming. Widespread wildfires. And hunting – the “overkill” hypothesis. Although similar disappearances roughly coincided with the arrival of people in Europe, Eurasia and Australia, and hunger is certainly the ultimate motivation, did people actually lay waste to entire groups of large mammals? The debate may seem academic, and it has been one of the most brutal and tenacious debates in academia.

Reading the dung calendar

Now we get some solid evidence that the extinction of the mastodon and other large herbivores closely followed the arrival of humans in North America, and that it preceded a pervasive change in type and prevalence of trees. The new evidence, contained in research by Jacquelyn Gill and Jack Williams of the University of Wisconsin-Madison, and colleagues, was published in Science this week, and although it does not prove the overkill hypothesis, it does usher a new type of evidence into the debate: spores of fungi that grow in herbivore dung. Between 14,800 and approximately 13,700 years ago, fungal spores of the genus Sporormiella declined by up to 98 percent in sediments found in lakes in Indiana and New York State. For decades, students of ancient ecology have been poking through pollen in sediments to see what plants were alive when the sediment was deposited. Pollen are durable structures, but it turns out that Sporormeilla spores are equally tough, and if you have the patience (Why Filers immediately excuse ourselves at this point!) counting spores provides a good gauge of the number of herbivores. Because the same sample also contains pollen and charcoal, it’s also possible to document the co-existing plant community, and get an idea of the extent of wildfires. Fungi are a new addition to the paleoecologist’s toolkit, says Gill, first author of the paper, and a graduate student in Williams’s lab. “Only recently have fungal spores been getting any attention; we used to basically ignore them if we counted them at all, but now we realize they are a good source of information about early conditions.” Being skeptics, we asked whether the decline could simply represent a change in conditions that was less conducive to preservation, but Gill says not. “If so, you would expect other proxies to show similar transitions. Since the same sediment that contains the spores also contains pollen, we’d expect to see pollen disappear, but we don’t.”

The dating game

Having a firm date for the decline of mastodons and other large herbivores is mainly helpful for eliminating some possible explanations, says Gill. The decline started almost 2,000 years before the putative impact of a comet or asteroid. And a change in climate apparently did not cause a broad habitat loss, Gill adds. “The extinction started before the habitat changed; the vegetation is relatively stable until after the extinctions began. We do have evidence of warming taking place, but if climate change is causing the extinctions, it’s not through a loss of food.” A major ecological change did follow the elimination of large mammals, however, as documented by pollen representing a new assembly of trees, including ash and ironwood, which had probably been held in check by hungry herbivores, growing along with less nutritious conifers like spruce and larch. Once the grazers left, these trees began to dominate the landscape — and then became fuel for wildfires that burdened younger sediment with charcoal. Although the sexy “overhunting” hypothesis is sure to get a boost from the Science paper, Gill says one study hardly proves the case. And as Johnson notes in his commentary in Science, the Clovis people who spread across much of North America arrived more than 1,000 years after the decline began. Evidence for earlier North American populations is sketchy and scarce, but it is arising, Johnson added. A second focus of the Gill paper may be equally important: the effect, rather than the cause, of the extinctions. “What happens when half of the species larger than a German shepherd go extinct in North America?” Gill asks. “Elephants eat 300 pounds of food a day, and when animals like the mastodon are rapidly taken out, you would think the landscape would notice, but that has been absent from the discussion. People were underestimating the power of these fungal spores to tell about the local presence of animals and vegetation.”