The word "niche" is associated with some empty space. And if you hear that a certain kind of took its place, you might think that before that it was empty. Actually it's not so easy. Ecological niche of a particular type occurs when this becomes adaptive traits (adaptations, allowing him to live in a certain place). Outside of the species ecological niche can not exist. And if one area is inhabited by two close friends, not to destroy each other as a result of natural selection, they need to buy accessories that will make them geographically in different habitats. Thus, they will occupy different ecological niches.
In nature there are no two identical ecological niches, as there cannot be two similar species.
But sometimes ecological niche can be relatively free. So, in Australia at the beginning of this century was inhabited by marsupials wolf. He occupied a niche of a large predator. But marsupial wolf was exterminated, and the niche left empty. Currently in Australia there is a possibility of formation of the ecological niche of a large predator.