Final answer:
Two species cannot coexist in the same community if they have exactly the same niche due to the competitive exclusion principle, which leads to competition for resources and the possible local extinction of one species.
Step-by-step explanation:
The competitive exclusion principle states that no two species can occupy the exact same niche in the same habitat for a prolonged period of time. This principle explains that two species cannot coexist if they compete for the same resources without one undergoing some sort of niche differentiation or becoming locally extinct. When two species have the same niche, and live in the same habitat, they would compete fiercely for the same resources, which might lead to one species outcompeting the other, resulting in the decline or local extinction of the less competitive species.
Species A and B cannot coexist in the same community if they have exactly the same niche because of this principle. They would need to partition their niche in some way—either by utilizing different resources, or by utilizing the same resources at different times or in different ways. If they don't, one species will become predominant while the other diminishes.