The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus

On a Scottish shore, two barnacle species occupy separate bands: one high, one lower. Rock areas were mapped and individually marked young barnacles followed. On some areas the lower-shore competitor was physically removed; on matched control areas it was left in place. Survival and growth of the high-shore species were then recorded across the whole tidal range.

Studied in

Chthamalus stellatus, Semibalanus balanoides

Sample size

marked shore rock areas with and without competitor removal

Where the competitor was removed, the high-shore species survived and grew well below its usual limit. Where it was left, individuals of the high-shore species were crushed, undercut or smothered as the competitor grew. The upper limit of the lower-shore species was set by desiccation, not by competition.

• One shore, two species, and a system where the competitors are sessile and the interaction is physically visible.
• Removal changes more than competition, though the direct crushing and undercutting were observed rather than inferred.

• Taking one barnacle off the rock

  Two barnacle species occupy separate bands on a shore, one above the other. Is the boundary set by the sea, or by the other barnacle?

• Ecological niche

  Where a species lives has boundaries of two kinds: what it can physically stand, and what another organism will let it have. You cannot tell them apart by looking — you have to remove the neighbour.