The biggest change they saw in the fossil deposits was the manner in which different small-animal species were spread across the landscape. "In the Pleistocene, there were about as many gophers as there were voles as there were deer mice," Hadly said. "But as you move into the warming event, there is a really rapid reduction in how evenly these animals are distributed."
Some species became extremely rare, others quite common. And the species that became king of the landscape by virtue of its very commonness was the deer mouse.
"That is a pretty big, somewhat startling result," she said, noting that deer mice are so common in western landscapes that most people assume they have virtually always been so. "What these data tell us is that in the Pleistocene they were not dominant at all."
Prior to this study, Hadly said most researchers would not have expected species that survived the warming to show any effects. After all, they survived. "What we are saying is there was a big effect," she said. And as some species such as deer mice flourished, many other species declined.
"Local declines of species are the precursor of local extinction," said Rodolfo Dirzo, a biology professor at Stanford who was not involved in the study. Local population declines also imply disruption of the local ecosystem even without extinctions, he said.
"Small mammals are so common, we often take them for granted," Blois said. "But they play important roles within ecosystems, in soil aeration and seed dispersal, for example, and as prey for larger animals." And d
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