"Clearly there's a lot more forest now than in the late 1800s and early 1900s, when logging and fires almost completely destroyed the forests of the northern Great Lakes region," Myers said. "But that doesn't work as an explanation for the patterns we see, because the species that are moving in and becoming more common are actually ones that do very well when forests are cut over." What's more, the change is happening even in the uncut forest of the Huron Mountain Club.
Similarly, increases in human population and the changes in land use that go along with them can't completely explain the changing mammal distribution and abundance patterns, Myers said. For one thing, the mammal changes are not restricted to habitats that have been disturbed by human habitation. For another, they're seen both in the Lower Peninsula, where the human population has increased over the past 50 years, and in the Upper Peninsula, where the trend has been in the opposite direction.
That leaves warming climate as the likely cause. But has such warming actually occurred in Michigan? To investigate, the researchers downloaded maximum and minimum daily temperatures from the National Climate Data Center for 16 weather stations in the Upper Peninsula, where changes in the small forest rodent community have been especially pronounced. They then calculated monthly averages for minimum and maximum daily temperatures for each year between 1970 and 2007 for each station and for the region as a whole.
Across all 16 sites, average annual minimum daily temperatures increased significantly over the 37-year period. Average annual maximum daily temperatures also rose, although not as dramatically.
The research team's results and conclusions dovetail with those of other groups that have found northward expansions of particular species in Wisconsin and Ontario and a shift from lower to hi
|Contact: Nancy Ross-Flanigan|
University of Michigan