In ecology, neutralism describes the relationship between two organisms that do not affect each other. It contrasts with relationships like predatory and mutualistic, in which one or both of the species benefit from their interactions. As all life is interconnected to some degree, true neutralism is essentially impossible. In practice, the term is used to describe the relationship between two species that rarely have any substantive interactions.
Examples of neutralism include a wide variety of different creatures. Tarantulas, for example, have no tangible effect on cactuses. Likewise, white-tailed deer do not affect snakes, nor do the snakes affect the deer. Parasites also provide a number of examples of neutralism, even if they do concretely affect the species that they parasitize. For example, the fleas that inhabit squirrels may negatively affect the squirrels, but they do not affect the lives of frogs at all.
Neutralism is a different type of relationship than commensal relationships, in which one of the species benefits while not harming the other. An example of a commensal relationship is found between pigeons and humans. Pigeons rely on human cities and towns to provide them with food in the form of uneaten scraps, while the humans are not negatively or positively affected to any great degree.