Garter snakes are known as dietary generalists. They aren't constrictors, or truly venomous, so they are usually incapable of overpowering prey such as adult rodents. The usual fare can include amphibians, annelids (worms, leeches, etc), fish, defenseless young rodents, and possibly soft-bodied insect larvae. However, multiple studies have shown that the Butler's garter snake is an earthworm specialist (nobody likes them, everybody hates them, guess they'll go eat worms...no? I tried). This is interesting, when you consider the fact that many of the earthworms found within the range of the Butler's garter snake are invasive, non-native species such as the nightcrawlers. Nightcrawlers enjoy a cosmopolitan distribution largely because of humans. So this begs the question - what did the Butler's garter snake eat before many of the non-native worms showed up? Did they subsist on other species of worms? Did they adapt to eating the invasive worms, and if so, how did that affect their numbers? We may never know, but what we do know is that food is the least of their problems, particularly in Wisconsin.
In my opinion, the single most interesting aspect of the Butler's garter snake is its range. In the east, it is only found in parts of Ohio, Indiana, Michigan, as well as extreme southern Ontario. Then, of course, is the isolated, disjunct population in southeastern Wisconsin. The Wisconsin population is separated from the much larger eastern swath of range by several hundred miles. Why such a big gap? The Butler's garter snake is found in wet meadows, prairies, savanna, and other types of habitat (usually in close proximity to water). There are areas between the two ranges that offer that habitat. So why aren't the Butler's garter snakes found in those areas? Certainly, the species once had a contiguous range at one point in the past, so what caused the rift?
Several theories have been postulated that offer suggestions as to how this happened. My favorite is one presented by Davis (1932). He looked at the ranges of both the plains and the Butler's garters and noticed something interesting. The range of the bigger, stronger plains garter snake extends from the west toward Wisconsin, Illinois, and a tiny bit of Indiana. Bordering the range of the plains garter snake is that of the Butler's - a relatively tiny patch in Wisconsin and then again in and east of eastern Indiana. When you look at a map of the two species' range, it does seem as though the plains garter snake has slowly been encroaching eastward into the range of the smaller, less-specialized Butler's - and outcompeting it for food and other resources. It seems to make perfect sense. It is very likely that the Butler's garter snake once enjoyed a much larger range - one that reached from Wisconsin all through northern Illinois and then eastward toward and into Indiana and beyond. This could be a case of naturally-occurring displacement that took possibly thousands of years to complete. In fact, the process, if valid, is probably still happening.
In Wisconsin, the Butler's garter snake is found in parts of only eight counties in the extreme southeastern portion of the state. They were at one point known from an isolated population in Racine County (Pope, 1944), but today, their southernmost range in the state is believed to include parts of Jefferson, Waukesha, and Milwaukee Counties (Wisconsin DNR). They have not been found south of the state line (Pope, 1944), and multiple exhaustive surveys in the northern Illinois counties have not produced any (though that does not absolutely rule them out, it's still very unlikely they're here) even though appropriate habitat exists, particularly in southern McHenry County (T Anton, 2015, pers. comm. 19 July). Due to their small range in Wisconsin, the species was listed as threatened in 1997. However, for reasons that frustrate me to no end, the state delisted the species on January 1st, 2014, to allow developers to build on land occupied by the snakes. As a result of the delisting, the species' habitat is even more fragmented, and ideal habitat that once supported the species as well as other wildlife is and will be forever altered/destroyed.
In order to assess the species in the wild, on July 19th, I drove up to a site in Milwaukee County - in fact, within the city of Milwaukee itself - and met with Wisconsinites Mike Day and Sam Fellows. Sam lives in Beertown and offered to show me a site in the city where the Butler's garters are common. The site is a small wooded park along a river. Much of the park is used for recreation/sports, and the natural areas that are left are fairly degraded (as to be expected with a park entirely within a big city). We hit an east-facing, grassy slope leading to the river, and found about fifteen of the snakes under cover boards Sam laid out as part of a research project with the Wisconsin DNR.
Here's a good-sized adult Butler's garter snake. Note the placement of the lateral stripe on parts of rows 2 and 4 and all of row 3.
Sam (on left) and Mike posing and photographing a Butler's garter while I get a voucher shot in-hand with my cell phone.
Also found during our search was this midland brown snake (Storeria dekayi).
Sam couldn't stay long, so we all decided to leave together. On our way back to the parking lot, I stepped over a soggy old discarded mattress. As I did so, Mike shouted, "Snake!". I didn't see this big female Eastern garter snake (Thamnophis sirtalis) basking in the morning sun. She was in blue, a few days shy from a good shed. Contrast this animal with the Butler's shown above.
Sam knew about a ruby-throated hummingbird (Archilochus colubris) nest and took us to it. I'm not a skilled birder, so I have a ton of respect for anyone who can spot this diminutive little feature atop a small tree branch. We waited about ten minutes, hoping that a bird would return. Eventually we saw the nest's tenant, flying around the perimeter of the nest, hopping around every which way, until I guess it determined that we were not a threat. To give you an idea of the size of the bird, the nest was about the size of a plum.
Mike and I decided to head to another site known for its Butler's garters. We hiked all around and searched high and low to no avail. I noticed some railroad tracks along the perimeter of the preserve, and we decided to head that way in hopes of flipping some old junk.
We weren't having much luck. There was a lot of old discarded pieces of plastic, metal, and wood, particularly old railroad ties, but no herps. That is, until after about an hour and a half of flipping, when I turned over a big piece of a railroad tie, and this beautiful milk snake (Lampropeltis triangulum) plopped out from a hollow in the wood. Check out the bulge in this one! Whatever this animal ate, it had to have been a struggle, both to kill and swallow. From the only population of milks in the county.
Nearby, I found this American toad (Anaxyrus americanus) under an old tie. It was the only toad we saw.
On our way out, I saw this amazing sight. It's an old tire maybe 50 feet up a tree near a bridge. I'm thinking either the tree began growing in the tire while the tire was on the ground and got pushed up over the years, or the tire was placed through a young sapling by someone years ago. Either way, this is a really interesting illustration depicting the tenacity of trees. I've seen objects such as fence posts swallowed by trees, but this is on a whole other level.
