Why Is There A Few Areas Of The United States That Have No Indigenous Poisonous Snakes?

Question

I realize Cottonmouths Copperheads and Coral Snakes are found only in the Southeast.Rattlesnakes are supposed to range into Canada.But a few isolated areas of the country have no rattlers.I grew up in an area of Northern Illinois that had none and the Puget Sound area of Washington has none.How is this possible?

Answer 1

I believe it all has to do with climate and habitat, as well as availability of suitable prey. It's something that I'm going to look more into.