Excerpt from Jim Conrad's
SPECKLED RACERS, AGAIN
Since being here I've seen surprisingly few snakes -- only four or five. Moreover, they've all been the same species, and this species is one we ran into up in Querétaro. It's the Speckled Racer, DRYMOBIUS MARGARITIFERUS, a Querétaro picture shown above. You can read about my Querétaro experience http://www.backyardnature.net/n/07/070526.htm.
I'm getting the impression that Speckled Racers are one of Mexico's most common snakes. Around here I can understand why they're so common. In Spanish often they're called "Raneras," or "Froggers." Our many weedy-margined irrigation canals support plenty of frogs for this frog-eating snake.
That doesn't explain why I've not seen other snake species, however. I'm thinking that this area's heavy use of agricultural chemicals keeps down predators such as snakes and hawks (very few hawks seen, too), so maybe the real question is how do these Speckled Racers survive in such a chemical soup when other species don't?