Niches are the functional position of a species in its ecosystem, how it responds to the distribution of its resources, and how it, in turn, alters those resources for other species.
Fundamental Niche is the total tolerance range of a species.
Realized Niche is the niche for which an organism is best adapted. This is narrower than the organisms fundamental niche due to competition for resources and other limiting factors.
It is possible (and not at all rare) for two or more species’ niches to overlap
However, a overlap in two or more niches’ of species fundamental niches results in competition causing species to adapt to a narrower, realized niche over time.