I do wonder if the kill method does eventually work if done consistently. I'm part of a sailing club, and marina's always have rat problems. Last year a member took it upon himself to try to get rid of them. I forget how many he got over the course of the year, over 100 i think. And we're seeing noticeably less rats. Yea rats move in to occupy the space, but then the surrounding areas start getting a little depleted, albeit to a lesser extent, and then rats further out move in to fill in those surrounding areas, etc etc. But it did seem to create a bubble of less rats.
Although, to bring it all full circle, we also had a stray cat show up. We took her in and got her fixed and shots, and she pretty much lives there now (no predators, no traffic, so hush ;P ). I'm sure she's helping with the rat problem too, but it does seem like being vigilant with the kill method (either via traps or addition of predators) does dwindle the population.