Mildred from New Mexico writes with a question I’ve received on several occasions:

I was told that the most effective way to combat gophers is with snakes. Is it possible to introduce garter snakes into my back yard? Have you ever heard of this solution to the gopher problem and do you have an opinion on this matter? Is there a way to “direct” the snakes to the gophers? Where does one obtain a garter snake?

Mildred goes on to say that she isn’t sure that New Mexico’s climate can support garter snakes, but in fact there are eight species of garter snake in that state — New Mexico is actually tied with California for the most garter snake species in the U.S. Now that doesn’t mean that every spot in New Mexico is ideal for garter snakes, nor that Mildred’s back yard is suitable garter snake habitat.

And garter snakes are not the best predators on gophers. While several garter snake species can and do hunt rodents, gophers are more than a little on the large side. A lot of people use “garter snake” to refer to harmless snakes in general, but the common striped snakes we know and love won’t do any good against gophers. What Mildred really wants is a bullsnake or a gopher snake, which specifically target pocket gophers. Bullsnakes and gopher snakes are essentially the same snake: the bullsnake is the large subspecies of gopher snake found east of the Rockies. They’re found all over New Mexico.

Now having said that, a lot of people have heard that snakes are really good at controlling rodents, and several of them have asked me whether they should get some snakes to control rodents on their property, and where they can get some.

The problem is that, generally speaking, if your property has a healthy rodent population, it should already have a healthy snake population feeding on them. If it doesn’t, there’s probably a reason for it.

A good food supply is absolutely necessary to maintain a local snake population, but it’s not the only requirement. In colder climates, further north or up in the mountains (and New Mexico is kind of mountainous, I’m told), snakes need somewhere to hibernate: if they can’t get below the frost line, they won’t survive the winter. Snakes need appropriate habitat as much as they need a good food supply.

Snakes also don’t necessarily do well around people. You might want to have snakes in your back yard, but your neighbour might not, and his opinion might be backed up by very sharp garden tools. In cities and towns there are plenty of ways for a snake to get killed — usually by getting run over. Even in the country, being near a busy highway can cut into a local snake population: snakes basking on the road (especially on warm pavement on a cool night) or simply crossing it can end up road kill pretty fast, especially when drivers swerve to hit them rather than avoid them. My point is that even if there’s lots of food for a snake, there might be lots of hazards too.

So my first question is usually: where are the local snakes? If there aren’t any, what happened to them?

Sometimes there are snakes, but my correspondent wants even more of them. Generally speaking, the population has probably already grown to what the local habitat can sustain. Adding more snakes won’t necessarily help: most of them will either end up as food for predators such as hawks or as road kill. Introduce too many and you’ll end up with too many snakes chasing too few rodents. Once the rodents are gone, you’ll end up with a lot of hungry snakes.

Sometimes they want to introduce snakes that aren’t native to the area, which is just about the worst possible scenario. If they can’t adapt to the local conditions, they’ll die, whether from starvation or freezing — and that’s inhumane. If they can adapt, there’s a risk that they’ll crowd out local species, which upsets the local ecosystem. Even worse, they might cross-breed with the local snakes (a Pacific gopher snake brought to the Midwest might interbreed with the local bullsnakes, for example), which messes with the local gene pool. Introducing foreign snakes won’t do the local snakes any good.

So my usual advice is to focus on encouraging local snakes (by, for example, building a hibernation den on your property) than on importing them.