Chipmunks Galore!

Beautiful, if a bit chilly, today, and these guys are finally out:

Eastern Chipmunk (Tamias striatus),
Van Patten Woods FP, Lake Co, IL 10/20/2013

This is, of course, an Eastern Chipmunk (Tamias striatus). Here's another chipmunk:

Least Chipmunk (Tamias minimus), Devil's Tower National Monument,
Crook Co, WY  5/29/2013

This one is a Least Chipmunk (Tamias minimus). They don't live around here, but they are found across the northern edge of the US and south through the Rockies.

And there you have the entire cast of eastern chipmunks. The same can't be said for out west - although any one location may only have a couple of species, overall the US west of the Great Plains has 20 species! Many of them are very similar in appearance and have very limited ranges.

A similar pattern can be seen with tree squirrels -- 3 in the East, 6 in the West -- and Ground Squirrels, with 4 species on the Great Plains and 24 West of them. Mice (8 East, 19 West) and voles (7 East, 14 West) show the pattern as well. This suggests that western rodents tend to speciate much more readily than eastern ones. Why?

The obvious part of the answer is habitat -- not only is the West broken up by mountains, the areas between those mountains tend to be much less hospitable to strangers. Deserts are tough places for visitors to survive, and many of their regular residents don't do well away from them. So we get small populations in places they can't really leave.

We don't see the same pattern with larger animals. While there are 4 species of deer commonly found in the West and only 2 in the East, Elk (Cervus canadensis) used to range much more widely, and is being reintroduced to a handful of eastern states. With the possible exception of the Grizzly Bear (Ursus arctos), all of the west's large carnivores have historical ranges that include the eastern US. With rabbits, we see a much more equitable pattern -- 6 in the East and 8 in the West.

So, size could well be part of the explanation. Except for these guys:

Black-and-White Warbler (Mniotilta varia),
Waukegan Beach, Lake Co, IL 5/20/2012

This is a Black-and-White Warbler (Mniotilta varia). They're considerably smaller than most rodents, with the largest one weighing in at less than an ounce. And yet, we see 38 species in the East and only 19 in the West. What gives?

Well, part of the issue is that by flying, they utilize their habitat in more diverse ways -- one county in Maine could conceivably have 23 nesting species. But the other part of the story is that by flying, they can overcome the obstacles that rodents and other small mammals are faced with when trying to disperse across inhospitable habitat. Since isolation is typically an important part of speciation, populations of good dispersers tend not to speciate as frequently. This explains what we see in larger mammals as well, with deer and carnivores able to wander more widely, and thus to cross desert basins on a regular basis.

With all the focus on genes and development these days, it's easy sometimes to forget that evolution involves a species' response to the environment around them, and something as seemingly trivial as how far a critter can walk can have big implications for the future.

Bowers, Nora, Rick Bowers, & Kenn Kaufmann. 2004. Kaufman Field Guide to Mammals of North America. Houghton Mifflin Company, NY.

Sibley, David A. 2000. The Sibley Guide to Birds. Alfred A. Knopf, Inc. NY