Which barnacle species would occupy the left half of the rock at the end of summer if niche differentiation occurred between Chthamalus and Balanus?

• A. Chthamalus
• B. Balanus
• C. Both
• D. None

Answer: (B)

Niche differentiation allows species to partition space by different tolerances to environmental conditions and by competitive ability. In the intertidal, one species tends to dominate the lower, wetter zone while the other can persist higher up where conditions are harsher.

Balanus is a strong competitor for space in the lower intertidal, where conditions stay moist most of the time. It rapidly covers available surfaces there, but it cannot tolerate long exposure to air as well as Chthamalus. Chthamalus, with greater desiccation and heat tolerance, can survive higher on the rock where Balanus cannot persist. So, when they differentiate their niches, Balanus would occupy the lower, left portion of the rock, and Chthamalus would occupy the upper portion. End of summer conditions reinforce this partition, making Balanus the best fit for the left half.