However, in doing so, the researchers also raised intriguing new questions about how those animals might have died in the first place.

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A gradual dying out of large herbivores about 15,000 years ago, lead to the disappearance of certain plant populations, a new study says.

The findings contradict pervious ideas that climate change could have killed off these plant communities.

Here, mastodons graze on black ash trees in a Pleistocene swamp. He likes to figure out how — and why — plant communities change over time.

"For my entire career, I've been keenly interested in the problem of 'no-analogue' plant communities, which are communities that existed in the past but are no longer found today," said Williams, a professor of geography at the University of Wisconsin at Madison who is an expert on ancient climates and ecosystems.

They found that the death of these creatures apparently prompted a proliferation of broadleaved trees, and ultimately, the accumulation of woody debris that contributed to a dramatic increase in wildfires.

They also determined that the animals' decline probably was gradual, meaning they didn't die from some sudden event.Taken together, the evidence appeared to eliminate some popular theories about what caused their mass extinction, including the impact of a meteor or comet, a "blitzkrieg" of human hunting, or a loss of habitat due to climate changes."They are often composed of still-alive species, but in combinations not found at present, and would look very strange to a modern ecologist." For example, high abundances of needle-leaved trees, such as spruce and larch, and deciduous trees (those that lose their leaves, such as ash and hornbeam) have been found in fossil records in the upper Midwest dating from the end of the last ice age.But today, these trees live in very different geographic areas."So my question is simple: Why did these communities form in the past, and why are they no longer around today? Recently, these interests took on a new dimension for him.In November, Williams and his colleagues, including graduate student Jacquelyn Gill, released research on how the extinction of ancient large plant-eating animals, such as mammoths and mastodons, affected ecosystems when the enormous mammals began their decline in North America about 15,000 years ago.