On the rocky sea coasts of Scotland, the larger barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus. Which population interaction does this illustrate?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Competition
Both barnacles live in the same intertidal zone and need the same space and resources there. The larger Balanus is the competitively superior species: it takes over the zone and pushes the smaller Chathamalus out. This is evidence for competition in nature, from Connell’s field experiments on the rocky sea coasts of Scotland.
Option (1) Predation — Incorrect. In predation one animal kills and eats the other. Balanus does not eat Chathamalus; it only keeps it out of the zone.
Option (2) Parasitism — Incorrect. A parasite lives on or in its host and feeds on it. The two barnacles live side by side, not one on the other.
Option (3) Commensalism — Incorrect. In commensalism one species gains and the other is unaffected. Here Chathamalus is harmed, since it loses its place in the zone.
Option (4) Competition — Correct. The superior competitor excludes the other species from a shared zone. This is interspecific competition.
NCERT Extract:
Population Interactions – “… Another evidence for the occurrence of competition in nature comes from what is called ‘competitive release’. A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed. Connell’s elegant field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area, and excludes the smaller barnacle Chathamalus from that zone.”