You will get rats if you have food.
Case point - I do Rat controll specifically for a farmer.
Farmers make sugar beat and root veg storage clamps ( what they pile all the food in ) out of straw bales. Why? its cheap, its easy, it can be done anywhere, it provides a good clamp, doesn't require any paper work, if you need a bigger clamp, add more bales!
Anyway, straw is a nice easy thing for rats to nest in, and nice and warm in winter, and safe from predators. The food piled up in it is a full of sugar, and close by, so we have the perfect situation for any kind of vermin. Winter strikes, Rats come in off the land and group together in warm places - normal rat behavior, survival mechanism for vermin. Because of the perfect environment this created they stayed and bred. All year, for about two years they lived off the rotting sugarbeat at the back of the clamp. Then the clamp was cleared out, but the rotting food was left. I got a call and in that first week we shot 384 all during the day, on the first day we shot 146. By the time i switched to lamping we had well over 500 out of that clamp. When need made me create a night vision rig i added another 100 to that and also found other nests near by. Rats are vermin and will opportunisticallyr eat and create living quarters near food. The only thing i can see that limits their size is their access to food and the size of their runs, ( which in the case above made a small rabbit warren ).The foods gone and this year their are no rats in that area ( more down to the lack of food than anything else. )