Take Me to the Limit: What Restricts Species’ Ranges?

A cold opossum. Photo by USFWS.

Recently, I was running, lost in my thoughts, and—WHOOPS—almost tripped over a shivering opossum crossing the bike path! After we both recovered our wits, I jogged in place and watched it waddle away, naked tail dragging through the snow. I rubbed my gloved—and still cold—hands together and wondered, why the heck do opossums live in Wisconsin?

When I got home, some Googling revealed an interesting fact: Wisconsin is at the limit of the opossum’s geographic range. In turn, this got me wondering—what governs the limits of a species’ range?

Ecologists typically classify range-limiting factors as either abiotic or biotic. Abiotic factors do not involve living organisms; climate is the quintessential example. Biotic factors are interactions with other organisms. A classic example is competition between organisms, which is a direct biotic interaction. However, biotic interactions can also be indirect, such as when one species improves or degrades habitat for another. Abiotic and biotic factors usually work in concert to limit an organism’s range.

The opossum I saw behind Olbrich Gardens bespeaks both. Opossums, with their naked tails and ears, have a difficult time surviving cold environments. And yet, opossums live in snowy Wisconsin! However, this is a relatively new phenomenon—opossums did not occur in Wisconsin until the 1850’s, when their range expanded northward. The opossum’s conquest of Wisconsin has been aided and abetted by another organism, namely Homo sapiens. Humans provide extra resources (like trash), which help opossums survive Wisconsin’s cold winters. A biotic interaction has helped opossums overcome an abiotic limitation.

Regardless of the exact cause, opossums reach the northern limit of their range in Wisconsin. Several other species reach range limits in the state, a fact that can come in handy while classifying Snapshot Wisconsin photos. Look a photo’s metadata—what county was it taken in? In some cases, this can narrow down identification possibilities. For example, any rabbit-looking creature in Waueksha County is likely an eastern cottontail, since snowshoe hares do not occur in southern Wisconsin. A good source for species range maps is NatureServe Explorer.

For more information about opossums, see this recent Snapshot Wisconsin blog post by Emily Buege.

For more information about the opossum’s range expansion northward, I recommend reading Walsh and Tucker (2017).