In Connell's classical competitive exclusion experiment with barnacles, which action was the third step?

• A. Leave Balanus on all rocks
• B. Add more Balanus to half the rocks
• C. Remove Chthamalus from all rocks
• D. Remove Balanus from 1/2 of each transplanted rock

Answer: (D)

The key idea is testing how competition for space shapes where species live, by manipulating who is present and comparing outcomes under the same environmental conditions. In Connell’s barnacle experiment, he created paired microhabitats on transplanted rocks so he could see whether one species prevents the other from persisting.

The third manipulation was to remove Balanus from half of each transplanted rock. This setup directly tests whether Chthamalus can persist when Balanus is absent, isolating the effect of competition from abiotic factors like desiccation. If Chthamalus colonizes the areas where Balanus is removed, it shows that Balanus competes with and excludes Chthamalus in those zones; if not, abiotic conditions are likely the limiting factor. This step is central to demonstrating competitive exclusion in the field.