Two barnacles, Balanus and Chthamalus, can both survive on the lower rocks just above the low-tide line on the Scottish coast, but only Balanus actually does so, with Chthamalus adopting a higher zone. Which of the following best accounts for this niche separation?
A) competitive exclusion
B) predation of Chthamalus by Balanus
C) cooperative displacement
D) primary succession
E) mutualism

Two different species that share the same niche can not live in the same place.
If two species have the same requirements, they can not coexist.
Two species can not coexist indefinitely on the bases of the same limited resource.

When two competing species coexist, this is because of niche partitioning or niche differentiation. If there is not any differentiation between them, the dominant species displaces the weak species.

In the exposed example, Chthalamus can live in the upper intertidal zone, where Balanus can not live because they can not tolerate desiccation. Balanus live in the lower intertidal zone, where Chthalamus can not live because of its vulnerability to predation. But these two species compete for the middle intertidal zone, where they can both live.

Balanus excludes Chthalamus because the first species is much bigger and grows faster than the second one. So the middle intertidal zone is only inhabited by Balanus.

The area occupied by Chthalamus is smaller in the presence of Balanus. In the absence of Balanus, Chthalamus can live in a bigger area, occupying the middle intertidal zone.