When interspecific interactions lead to competitive exclusion, the weaker competitor is forced to retreat to a more restricted niche (its realized niche) than it otherwise would inhabit in the absence of the competition (its fundamental niche). This is what happens between Balanus and Chthamalus

The places where the species can thrive principally because of its characteristics is known as the fundamental niche, on the other hand, the place where the species is living is known as the realized niche. In the intertidal region, the species Balanus and Chthamalus compete for space. In comparison, Balanus is a bigger barnacle that can colonize the place more effectively than Chthamalus.

This competition for space makes Chthamalus displace into higher regions where water is not the same as in lower levels. The fundamental niche of Chthamalus comprises both lower and higher levels, while the realized niche for Chthamalus is the higher levels. In case, if the competition with Balanus was non-existent, the Chthamalus would occupy the lower levels.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.