Too many of the top level predators are gone and we have a proliferation of bunnies and some transplanted prairie dogs that have reproduced like bunnies. The coyotes are not enough to keep them down. The owls get some of the bunnies, but not the prairie dogs.

Lee AKA Fireraven9
"Constant Vigilance!" Mad Eye Moody

In Toby Hemenways book, "Gaia's Garden", there is a story of how a family fixed up their place. Cattails proliferated for a few years which they liked to eat. Then one season no fresh shoots. They found that muskrats had come in and were eating the fresh shoots. Bummer. But instead of the typical knee jerk reaction to immediately get out the guns, traps, whatever, they let nature take its course and in a couple of years they started seeing cattail shoots again. They discovered that otters had found the muskrats and a balance had once again been restored.
You might, if you haven't already, check to see what has caused this increase in rats. Is there an unnatural food source, like an open feed bin? Or is it a cyclical thing that nature has heaped on you? Encourage the ermines.
Mike

So, how does one allow things to work out so that things balance when some of the residents have been removed or displaced? Ah, to have black foot ferrets (they fit in the niche of the otter and ermine as predators here) or maybe more opportunistic birds.

Lee AKA Fireraven9
The snow on your eyelids that curtsy with age
A freezing rare stare on tyranny's wings
The bitterness, hardness, warmth of your skin
Is diseased with familiar caresses (Winter Song - Nico/Cale)