All animals need food, water and shelter to survive and reproduce. If any of these are in short supply, animal numbers decline proportionately. On the other hand, when these requirements are met, the animal population will reproduce and increase in numbers, remaining stable until the supply of one requirement becomes limiting. In situations where wildlife species become troublesome or compete with man's economic or health interests, the animals are called pests.
Many wildlife species coexist with humans very successfully.
In fact, some, like the common rat and mouse, have become more or less dependent on people. Most wildlife species have the potential of becoming pests. Whether or not a species becomes a pest can be directly correlated to the degree at which that animal can be tolerated. Wildlife species are usually only pests in certain situations, such as when their numbers become excessive in a particular area. Human change in the environment will often result in increased numbers of a species.