Well, when my girls were little, they asked me to come out to the barn to see something. Rats were tearing into the horse feed and eating it.
My oldest one reminded me "you said if you don't have rats, you don't need cats - and we have rats!"
So, I let them each get a cat, as long as it was an OUTSIDE cat.
Sure enough, over the next few months, I'd occasionally hear some noise on the back porch, and there would be one of the cats with a half-dead rat, mouse, snake, or other vermin. The cat wouldn't kill it until I got to see it, and the cat would kinda bat at the vermin as if to say, "see there? I'm on the job!"
It wasn't long before we seemed to be rid of rats and snakes.