The finding, by researchers at the University of Michigan, Michigan State University and Ohio's Miami University, appears
in the June issue of the journal Global Change Biology.
"When you read about changes in flora and fauna related to climatic warming, most of what you read is either predictive—they're talking about things that are going to happen in the future—or it's restricted to single species living in extreme or remote environments, like polar bears in the Arctic," said lead author Philip Myers, professor of ecology and evolutionary biology at U-M.
"But this study documents things that are happening right now, here at home."
The researchers analyzed distribution and abundance records of opossums and eight species of small forest rodents. In addition to data collected by live-trapping animals over the past 30 years, the researchers relied heavily 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."
The south-to-north expansion
pattern is what you'd expect if climate change is driving the advance, but could there be other explanations, such as forest regeneration or human influence?
"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.
An email interview with Myers elaborated on the study and on what work he believes still needs to be carried out.
”the declining species—woodland deer mice, southern red-backed voles, northern flying squirrels, woodland jumping mice, and least chipmunks—are all northern species.”
Raises this question:
Is there any evidence that these species have moved north as well? Or are they dying out due to habitat change/loss?
"The maps hint at a northward shift for northern flying squirrels and woodland deer mice in the Lower Peninsula, but the change is small and we have so few recent records that I just can’t be sure.
There’s certainly nothing as striking as the northward movement of opossums, white-footed mice, and southern flying squirrels. I’d love to know what’s going on in Ontario at the northern limits of their ranges, but so far no one has looked. And as you probably know, climatic warming isn’t uniform across the globe (a few areas have actually cooled; it’s the overall average that’s up), so whether ranges have changed further north may depend on local patterns.
Another aspect of the answer to this question is that in the Upper Peninsula, northern species have nowhere to go. They very quickly run into Lake Superior, just as populations in the Lower Peninsula run into the Straits of Mackinac."
quoting from your work "What we can say is that the potential is there for serious changes to happen, and it would be really smart of us to figure it out, but that will require a lot of detailed, focused ecological research
What would the detailed, focused research involve?
"This is a much harder question. We need to know a whole lot more about what they eat, how diet changes from year to year, season to season, and place to place; what the breeding patterns are like (when does reproduction begin in the spring, how many young are born per litter, how many litters are born per female per year, when does reproduction end in the fall); more about the survivorship patterns (how many young survive to reproduce, does this vary according to time of year, how long do adults live); details on the microhabitats of each species, including where they forage and nest; information on the social systems (do they live in groups for any part of their life cycles, and if so, how are they groups structured and how do individuals interact).
That’s all at the individual level. Stepping up to look at populations, we need to figure out whether competition among individuals of the same species is important in determining population size. You might think “of course!” but I’m not so sure.
At least the southern species are living in habitats that are probably right at the edge of their physiological tolerance, and it is very possible that they are knocked back so far during the winter that in most years, despite their high reproductive rates during the spring and summer, they never get to the point where individuals actually compete. And if they do compete, what are they competing for? That is, what limits their populations? Food? Nesting places? Something else?
At the level of communities, is competition among species important? Do, for example, increases in the number of white-footed mice lead to decreases in the number of deer mice because white-footed mice are better at getting some critical (and limited) resource? And if so, what is that resource, and how are mice using it?
We need to know more about the importance of predators in determining the numbers of these species in a community (and more about the extent to which the predators rely on each prey species to survive).
And we need to know a whole lot more about how the pieces fit together. A great example of this importance of this is a paper written by Rick Ostfeld and associates in 1996. They work at the Institute of Ecosystem Studies in Millbrook NY. They looked at white-footed mice (same species we have in Michigan), gypsy moths, and Lyme disease.
It’s a very complicated system. Oaks have “mast” cycles, which means that some years they produce tons of acorns and some years they produce few or none. Our oaks in Michigan do this as well. Mice eat acorns, and not surprisingly, in the New York study mouse numbers are linked to the mast cycles. More acorns, more white-footed mice.
Mice also eat young gypsy moth larvae. They can’t eat enough during an outbreak to stop it, but they may consume enough when gypsy moth populations are still low to delay an outbreak, or even prevent it from beginning. So gypsy moth numbers are tied to white-footed mouse numbers are tied to oak masting. Lyme disease figures in because both white-footed mice and deer (which also rely on acorns) are hosts of the deer tick, which transmits the disease to humans.
The larval tick gets the lyme bacterium by feeding on an infected white-footed mouse, then bites a human (rather than a deer, its normal host) and infects that person. The likelihood of ticks being infected depends on the density of white-footed mice, so like gypsy moth outbreaks, Lyme disease prevalence is tied to white-footed mouse numbers and indirectly, oak masting. It’s actually more complicated than this, so I’ll attach a copy of the paper (which is written for a general audience, not that that makes it easy to read).
Finally, some of these species (white-footed mice in particular) have been studied extensively in other parts of their ranges. Why do we need to repeat the work in Michigan? The answer is that ecology, like politics, is fundamentally local.
In the case of white-footed mice, we need to know how they respond to the climatic and biotic conditions they encounter in northern Michigan, which are different from what they encounter in New York or in the mountains in Virginia, where some very excellent work has been done. In fact, we’re finding significant ecological differences among white-footed mouse populations in the Upper and Lower Peninsula! The beauty of the Millbrook study is that it is very focused and very local.
It tells us what kinds of relationships are possible and gives us a starting point, but we can’t assume that we’ll find the same patterns in the northern Great Lakes Region. And in fact, I already know that despite the fact that oaks in Michigan undergo intense masting, in at least the northern part of the state, fluctuations in mouse numbers (which can be very great) appear to be tied to early spring weather conditions and have nothing to do with mast cycles.
This is already too long and complicated, and I could go on. We clearly have much to do! And clearly (to me at least) it’s important that we find answers to some of these questions, which would be important even if climatic warming weren’t an issue."