Snakes and Prairie Restoration at Nachusa Grasslands
As scientists, we know a lot about snakes. We know that snakes evolved from lizards. We know that snakes don’t have eyelids or external ears. We know they can eat things bigger than their own head. Some species, such as the black-tailed rattlesnake, are good mothers, sticking around after birth, and protecting their offspring. Recently, it was discovered that snakes can even have friends! Suffice it to say, snakes are awesome. So awesome, in fact, that some of us prefer them to grasshopper sparrows and fringed gentians. Yes, we herpetologists (scientists who study reptiles and amphibians) are a weird bunch, with our metal probes and pillowcases . . . Anyway, I digress.
While we know a lot about snakes and how cool they are, we still have a lot to learn, particularly when it comes to ecological restoration. Unlike birds and insects, snakes don’t have wings. Snakes are also terrible at crossing roads (probably because they don’t have legs). So, the big question is “if you build it, will they come?” That is, if you go through the hard work of restoring an old corn field back to native tallgrass prairie, will snakes recolonize the site? Well, Dr. Richard King and I tried to tackle this question, which resulted in a publication creatively titled “Responses of Grassland Snakes to Tallgrass Prairie Restoration.” In short, yes, but it’s complicated!
Before we dive into what we found, how exactly do herpetologists actually study snakes? It is not like you can lean against a shady bur oak and listen for the sounds of singing snakes (yes, this is a playful dig at my ornithologically-minded friends). One option is to simply walk around and look for snakes (what we like to euphemistically call a ‘timed meander’). This is, however, not very effective. Think of how many snakes you stumbled across while hiking in a prairie. Maybe a handful at most, right? Less? Certainly not enough to do some fancy statistics. Nor will snakes stumble into a tiny metal box baited with peanut butter (sorry mammologist friends, I had to make it fair to the ornithologists!).
So, what is the intrepid herpetologist to do? Well, we take advantage of a snake’s natural tendency to hide under junk and employ something called ‘artificial cover object surveys.’ In other words, we strategically place things on the ground that snakes like to hide under (in our case plywood boards and rubber mats), and then go back later and check each one. What does a check entail, you may ask? Great question, and the answer is simple: bend down, lift the board, and then try to grab every snake you see! Simple, but not easy; those little buggers are fast and sometimes bitey!
Now the nuts and bolts of our study. To address the question of snakes and habitat restoration, we deployed approximately 240 cover objects across 12 restoration units (2 to 25 years since restoration) at Nachusa Grasslands, a world class prairie restoration in Lee and Ogle counties. Nachusa was established in 1986 by The Nature Conservancy with the purchase of about 400 acres of remnant prairie. Through the hard work of land managers, ecologists and volunteer stewards, the preserve now boasts more than 3,600 acres of native habitat, including 80 prairie restorations. These restorations are a labour of love, requiring literal tons of locally collected native prairie seeds, prescribed fire on a 1- to 3-year rotation, invasive plant control, and even grazing by a reintroduced herd of bison. Illinois (the Prairie State) has lost more than 99 percent of its tallgrass prairies, and Nachusa makes up a large proportion of that remaining 1 percent. Suffice to say, Nachusa is a special place.
Back to snake boards. Each board was checked roughly once a week, between May and October from 2013 to 2016. This resulted in sacrificing our lower backs for science a total of 15,720 times over the four years. Was it worth it? You bet! Overall, we caught 1,028 individual snakes of four focal species: 90 plains garter snakes, 112 eastern fox snakes, 347 Dekay’s brown snakes and 479 common garter snakes. Each snake was given a unique marking so we could identify it if captured again. We also measured and weighed each snake, and found a few other species in small numbers, such as the eastern milk snake.
Right off the bat, we saw that all four species readily colonized the tallgrass prairie restorations at Nachusa, which was great news! We also saw that there was no relationship between restoration age and the abundance or occupancy of plains garter snakes, eastern fox snakes or common garter snakes. That is, newly restored sites were just as likely to have those species as older restorations. However, older sites were much more likely to have Dekay’s brown snakes than younger sites. This is a cool finding, as Dekay’s brown snakes are the smallest of the four species (adults rarely exceed 18 inches), and they also have the smallest home ranges. This suggests that smaller species with limited dispersal capabilities might be slower to colonize restorations. Intuitive for sure, but always great to have data!
Finally, there was a glaring omission from our snake board data: we found zero smooth green snakes! This was really odd, as the species was once common across northern Illinois, and we found them to be relatively common at a nearby Illinois Department of Natural Resources’ wildlife management area (WMA) managed for small game hunting. However, snakes can be really hard to find, so the question became, “Are smooth green snakes truly absent from Nachusa, or did we simply fail to find them?” To address this, we took our data from the WMA and used a statistical technique called logistic regression to calculate something called a ‘detection probability.’ The result? Given our approximately 15,000 cover board checks, we estimated there was a 99.9 percent chance we would have detected smooth green snakes if they were indeed present at Nachusa. Could we have missed them? Certainly, but it is highly unlikely.
So, why are there no smooth green snakes at Nachusa? The most likely explanation is that they simply did not survive in the small remnant prairies at Nachusa prior to restoration. Like Dekay’s brown snakes, smooth green snakes are quite small and are probably not so great at colonizing new areas. In addition, smooth green snakes specialize on eating insects and spiders, and they may be particularly susceptible to insecticide use relative to other species with broader diets. So, if they didn’t survive at Nachusa, crossing miles of roads and corn/bean fields might pose too big of a challenge. Therefore, while a ‘wait and see’ approach might work for other species (such as Dekay’s brown snake), captive breeding and translocation may be necessary to establish populations of smooth green snakes at Nachusa or other restored prairies. This approach has shown great promise in the Chicago suburbs, and I hope one day to see the tail end of a smooth green snake slipping away into a tussock of little bluestem at Nachusa Grasslands.
In conclusion, we found that Nachusa Grasslands, a world-class prairie restoration, boasts plentiful populations of at least four species of grassland snakes, and these snakes are not limited to the remnants, but occur broadly throughout restoration units. Other species also occur, including the eastern hog-nosed snake, eastern milk snake, North American racer and common water snake. However, the smooth green snake, a species that is common in a nearby wildlife management area, appears to be truly absent at Nachusa. I propose they be considered a candidate for assisted translocation or reintroduction, pending further study, of course.
Thanks for reading! Feel free to send me an email for snake identification help from Illinois or anywhere else!
John Vanek, PhD, is an Associate Wildlife Biologist® and a post-doctoral fellow at Northern Illinois University. He is a former advisory board member of Midwest Partners for Amphibian and Reptile Conservation, and is an administrator for the Facebook Snake Identification Group. Find him on Twitter at @wild_ecology.