In an ideal ecosystem, each species has its own niche – a different “job description”: what it does, what it eats, where it sleeps, and more.
But world is often not an ideal place. In many instances, two species may live in the same spot, yet overlap in some of their roles or needs. They may both compete for the same tree-holes or caves for dwelling, or they may eat the same food. It is not necessary for the two species to be aggressive toward each other, but it is likely that one of the species will be more efficient in gaining the resource than the other.