Chthamalus can live in both deep and shallow zones (its fundamental niche). Semibalanus occurs in the lower intertidal zone, where conditions are more constant.
Semibalanus is the better competitor and can exclude Chthamalus from areas where both are able to live. Semibalanus forces Chthamalus out of the part of its fundamental niche that overlaps the realized niche of Semibalanus (see Fig 25.2).
The entire niche that a species is capable of using, based on its physiological requirements and resource needs, is called the fundamental niche. The actual niche the species occupies is called its realized niche. Because of interspecific interactions, the realized niche of a species may be considerably smaller than its fundamental niche.
Chthamalus is able to survive because it has a refuge: it is more tolerant of desiccation than Semibalanus and can live further up the shore.
To leran more, see Interactions among competing species shape ecological niches from Ilandart.org and Competition and niches from Glyndwr.ac.uk.