Climate Change Forces Michigan Mammals Northward

The finding that historically southern species now are replacing the declining northern species by scientists at the University of Michigan, Michigan State University and Ohio's Miami University, appears in the June issue of the journal "Global Change Biology."

"This study documents things that are happening right now, here at home," lead author Philip Myers said today.

The professor of ecology and evolutionary biology at University of Michigan said, "We're talking about the commonest mammals there, mammals that have considerable ecological impact."

"They disperse seeds, they eat seeds, they eat the insects that kill trees, they disperse the fungus that grows in tree roots that is necessary for trees to grow, and they're the prey base for a huge number of carnivorous birds, mammals and snakes," Myers said.

"But we don't know enough about their natural history to know whether replacing a northern species with a southern equivalent is going to pass unnoticed or is going to be catastrophic," he said. "It could work either way."

In the study, Myers and coworkers analyzed distribution and abundance records of opossums and eight species of small forest rodents - white-footed mice, woodland deer mice, southern red-backed voles, woodland jumping mice, eastern chipmunks, least chipmunks, southern flying squirrels, northern flying squirrels, common opossums.

In addition to data collected by live-trapping animals over the past 30 years, the researchers relied on specimens and notes in research museums including the U-M Museum of Zoology and the Michigan State University Museum.

"Museum collections have been underutilized in studying the effects of climate change," Myers said. "We're fortunate in Michigan to have an amazing resource in the U-M Museum of Zoology collection, which contains great records of thousands of Michigan species from hundreds of locations, sampled over the past 100 years."

Long-term comparisons also were possible at the Huron Mountain Club, an 18,000-acre tract of pristine forest in Michigan's Upper Peninsula owned by a private association. A 6,400 acre research area where scientists can carry out field work is part of the the property.

The non-profit Huron Mountain Wildlife Foundation has funded three animal surveys there: the first between 1939 and 1942, the second in 1972-1973 and the most recent in 2004-2005.

An eastern chipmunk in Michigan, one of the increasing species. (Photo by Tony Tanoury)

Combining trapping data from Huron Mountain Club and other locations with museum material and road kill reports, the researchers ended up with 14,614 records for the nine mammal species in the study. When those records were analyzed, they painted a clear picture of mammals moving north.

Of the nine mammal species studied, four have established strongholds or increased in abundance, while five have declined.

The increasing species - white-footed mice, southern flying squirrels, eastern chipmunks and common opossums - all are southern species.

The declining species - woodland deer mice, southern red-backed voles, northern flying squirrels, woodland jumping mice, and least chipmunks - are all northern species.

Myers tries to deduce why these animal abundance patterns are changing as they are.

"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."

But the shift in mammal populations is happening even in the uncut forest of the Huron Mountain Club, nor is it restricted to habitats that have been disturbed by human presence.

The shift is being seen both in the Lower Peninsula, where the human population has increased over the past 50 years, and in the Upper Peninsula, where out-migration is the trend.

That leaves warming climate as the likely cause.

The team analyzed 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 observed. They found that the average annual minimum daily temperatures "increased significantly over the 37-year period."

The research team's results and conclusions harmonize with those of other groups that have found northward expansions of particular species in Wisconsin and Ontario and a shift from lower to higher elevations in the Yosemite National Park.

The research was supported by the Huron Mountain Wildlife Foundation, the Michigan Department of Natural Resources, Miami University, Michigan State University, the U-M Undergraduate Research Opportunities Program, Rackham Graduate School and the U-M Biological Station.

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