Because of their close association with humans, Norway rats in Illinois probably rarely directly impact native species. Instead, they mainly conflict with people. The Norway rat feeds on human waste in urban areas, while in rural landscapes its diet consists mainly of grain seeds (i.e., corn and wheat). On islands around the globe, Norway rats and black rats (Rattus rattus) pose a critical threat to native birds, particularly ground-nesting shorebirds; as such, on islands, eradication of rats is imperative for conservation efforts.
Norway rats may be an important food source for snakes, hawks, and weasels. However, away from islands, we know little about their effects on ecosystem function. It is well known that Norway rats may significantly impact public health. Norway rats carry many pathogens, including the Seoul virus, a form of hantavirus, and hepatitis E, and bacteria like Leptospira, Bartonella, and Rickettsia. Furthermore, rats carry endo- and ectoparasites like fleas, lice, protozoans, and trematode worms. In the 1300s, rats were vital to spreading the bacterium Yersinia pestis, which lives inside the fleas carried by rats; Yersinia caused the bubonic plague, the most fatal pandemic known to humanity.
Like all rodents, Norway rats have a propensity for gnawing. This behavior can lead to significant damage to human structures, as Norway rats will chew on insulation, wires, and other building materials. Consumption and contamination of livestock feed is a problem in rural areas, including livestock operations, and rats also may undermine the integrity of structures under which they create extensive networks of burrows.
In urban areas, such as Chicago, residents experience a suite of negative emotions about rats, including fear and disgust. People may avoid going outside to avoid rat encounters and must spend money to control rat populations in their homes.