Answer:
Shallow water is the barnacles´ realized niche.
Step-by-step explanation:
The case of Balanus and Chthalamus is an example of competitive exclusion, which refers to the exclusion of the inferior competitor -Chthalamus- by the superior competitor -Balanus- when there is not habitat differentiation, and both species can not share the same niche. In this case, the dominant species occupies part of the fundamental niche of the inferior competitor -medium to low tides-.
The fundamental niche refers only to physic conditions in which a species can live and survive in the absence of any interaction with other species. In the case of Chthalamus, the fundamental niches are the upper intertidal zones, the middle intertidal zone, and the lower intertidal zone.
The realized niche refers to the restricted conditions in which a species can live and survive as a result of environment physic characteristics and the interaction with other species. Chthalamus ´ realized niche is the shallow waters. It can not live in the middle intertidal zone because it competes with Balanus and this last species is the dominant one so it occupies that area. Chthalamus can not live either in deep water because it is predated by other species.