Question

The diagram below shows the distribution of two barnacle species, Chthamalus and Balanus on a rocky sea shore.

When Balanus is experimentally removed, Chthamalus expands its range in lower intertidal zone.

(a) Identify and define the ecological phenomenon demonstrated by this observation.     [1]

OR

(a) State the principle that explains the elimination of one species.     [1]

(b) Two different species can compete for the same resource. Give another example of it.     [1]

(c) How do species avoid competition in nature? Explain with an example.     [2]

Answer 1

(a) Competitive exclusion (observed as competitive release).

Definition: when one species (Balanus) outcompetes and excludes another (Chthamalus) from part of its habitat; removing the dominant species allows the inferior competitor to expand into that area.

OR

(a) Competitive Exclusion (Gause) principle: Two species competing for the same limiting resources cannot stably coexist; the better competitor will exclude the other.

(b) Two herbivorous fish species feed on the same phytoplankton they compete for the same food resource.

(c) By niche partitioning/resource partitioning, species use different parts of a resource or use it at different times (e.g., two coexisting species may feed on the same food but at different depths/times or occupy different microhabitats). This separation of niches reduces direct competition and allows coexistence.