Fewer predators and more food means more mice
In their natural, rural environment, mouse populations are curtailed by an extensive array of predators and a limited food supply. In an urban setting such as London, such population restrictions are no longer a factor.
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Though mice have many predators in the wild, few of these animals live in the bustling, urban area of London. Common predators like owls, hawks, skunks, and snakes are much more common in the rural areas surrounding the city than in the city itself. The same is true for domestic house pets like cats or dogs which often prey on mice. Fewer people in the heart of London own such pets, due to spatial or financial constraints. Foxes are one of the few predators commonly found in London, but they tend to focus on different prey, specifically rats. This, too, reduces a common threat. Since rats eat mice, any area with fewer rats will have more mice. In addition to harboring fewer predators that could limit mouse populations, London offers vast quantities of food for these pests. Since mice are omnivorous, they are not picky when it comes to their next meal. The massive amounts of garbage the city produces (as well as its questionable sanitation measures) provide a perfect feeding ground for mice. In addition, mice may prey on the many pest insects in the area, from roaches to ants to earwigs. To a mouse, London seems like the ideal place to live and thrive-though human residents may disagree.
Though cities certainly host fewer natural predators than rural areas, mice have their fair share in London, with birds, raccoons, rats, and house pets effectively curbing their population. In addition to these animals, mice face "unnatural" predators-particularly, humans armed with poisons and professional exterminators. It is unfair to argue that it is easier for a mouse to live in the city than in the country since both environments have such drastically different drawbacks and benefits.