Reptiles at Risk

Our First Plains Garter!

Posted by Jeff Hathaway on August 3, 2006 at 6:53 PM

While at Turtle Mountain, we saw our first wild Plains Gartersnake (Thamnophis radix). Sadly, it was dead on the side of the highway (http://www.flickr.com/photos/scisnake/206045563/). Yet another reptile that didn't make it across the road!

Road mortality is actually a big problem for some reptile populations. Unlike raccoons, skunks, deer, and other mammals which reproduce quickly and mature at a young age, reptiles often cannot keep up with the numbers that are killed by vehicles. Especially turtles, which take a long time (15+ years for some species) to reach sexual maturity. In some cases, they do lay a fairly large number of eggs, but most of these, and most of the baby turtles, get eaten by predators.

Without getting too detailed on the population ecology math, if one female turtle normally lives for 80 years, and lays eggs for perhaps 60 of those years, theoretically only 1 or 2 of those eggs would survive to become an adult turtle in a stable population (the exact number would depend upon the ratio of males to females in the population). Our turtle species have been around for a long time, possibly millions of years. While they haven't lived in some parts of Canada for that long, it is fair to say that since the glaciers retreated, they likely established fairly stable populations in most areas of their range by the time European settlers arrived in North America.

Many species have lost tremendous amounts of habitat in the few hundred years since, though they continue to persist in areas where there is suitable habitat. However, within the last half century, we have constructed networks of roadways that often surround such areas of habitat. This is certainly true in southern Ontario, Manitoba, and Saskatchewan, where few areas can be found that don't have roads nearby. Try this for an exercise- find a map of your area, and see if you can find a spot where you could be more than 1 km from a road in any direction! If anyone in the southern parts of Canada can find such a spot, please email us!

This means that turtles often have to cross roads when they move around on land; frequently this is to lay their eggs. Some percentage of these turtles get hit by cars or trucks, and the turtles often don't survive such an encounter!

Back to that math- normally, at least for the species studied so far, adult turtles simply aren't very likely to die. Virtually nothing eats them. Odds are, in any given population of turtles, only a few die each year. Let's say, there are 100 turtles in a wetland- likely only 2-3% would die in a year (this is actually higher than some studies show), so let's say 3 turtles would die yearly. This is normal, and theoretically if the population is stable, it should be fine. Some fairly complicated computer modelling shows that it only takes a small increase (<5%) in the number of turtles that die each year to result in the population starting to decline. Since you can't kill part of a turtle, let's say that only one turtle in that wetland is killed each year by a car. If 3 normally die, and extra one is a 33% increase! That's a lot more than 5%! This means that the population will decline, and since it is unlikely that the number of roads and cars will go down, the decline is likely to continue for a long time, until there aren't any turtles left to get. This could take a long time, of course, to get them all, but a slow decline to zero still means they're being wiped out! This is actually happening right now to many of our turtle populations!

Even more insidious, we tend to run over the nesting females, so we selectively remove them from the population. Recent studies in the United States have shown that turtle populations near roads have significantly more males and fewer females than populations away from roads. So, it could be that eventually all the females will be gone, but people will still see quite a few male turtles out basking on logs. Once those males eventually start dying off from old age, there won't be any left, but it will be much too late by then to do anything about it. We need to start now!

What can we do? Good question. So far, there is no easy answer. Education can help, but it cannot solve the problem. One thing that could help- tunnels or ecopassages and barriers to keep turtles off of the road, in areas where there are significant turtle populations! Expensive, but hopefully worth it!