Niche Partitioning and Barnacle Distribution

Fundamental and Realized Niches

• Most species thrive within a physiologically optimal range of conditions known as the fundamental niche.
• However, when competitors occupy a portion of the fundamental niche, an organism's distribution range may be restricted to the realized niche, where the competitor is absent.
• Researchers often determine an organism's fundamental niche by temporarily removing a competing species and observing the effects on the remaining species.

Connell's Experiments with Barnacle Species

• Chthamalus stellatus and Semibalanus balanoides (formerly known as Balanus balanoides) are two barnacle species dominating the Scottish coast.
• Each species has a well-defined realized niche in the intertidal zone:
  • Chthamalus is found in the upper intertidal zone.
  • Semibalanus is restricted to the lower intertidal zone.
• Joseph Connell aimed to determine the potential distribution range of adult Chthamalus in the absence of competition with Semibalanus.
• Experiment Setup:
  • Connell collected rocks with young and mature Chthamalus from the upper intertidal zone.
  • He moved these rocks to the Semibalanus zone and secured them.
  • Semibalanus was allowed to colonize the rocks.
  • After colonization, Connell removed all Semibalanus organisms from one side of each rock using a needle.
  • The rocks were then returned to the lower intertidal zone.
• Results:
  • Chthamalus mortality was high on rocks with Semibalanus.
  • Chthamalus survived on Semibalanus-free rocks.
• Connell also monitored natural patches of both barnacle species where they co-occurred at the upper margin of Semibalanus's distribution.
• During a period of unusually low tides and warm weather, desiccation became a threat.
  • Young Semibalanus suffered a 92% mortality rate, and adults a 51% mortality rate.
  • Young Chthamalus experienced a 62% mortality rate, while adults had only a 2% mortality rate.
• Semibalanus is less resistant to desiccation than Chthamalus and cannot survive in the upper intertidal zone.
• Chthamalus is more resistant to desiccation and can be found higher in the intertidal zone.
• The lower limit of Chthamalus is established by competition with Semibalanus, while the upper limit is controlled by desiccation.
• The fundamental niche of Chthamalus extends throughout the intertidal zone, but its realized niche is restricted to the upper zone.

Experiment Summary

• Hypothesis: Chthamalus stellatus is competitively excluded from the lower intertidal zone by Semibalanus balanoides.
• Location: Intertidal zone of the rocky shores of the Scottish coast.
• Steps:
  1. Transfer rocks with young and mature Chthamalus from the upper to the lower intertidal zone and secure them with screws.
  2. Allow Semibalanus to colonize the rocks.
  3. Remove Semibalanus from half of each rock, leaving the other half undisturbed. Return the rocks to the lower intertidal zone.
  4. Monitor the survival of Chthamalus on both sides of the rocks.
• Expected Outcome: Chthamalus grows on the side where Semibalanus has been removed, indicating that Semibalanus excludes Chthamalus from certain habitats.
• Data:
  • Percentage of Chthamalus mortality over one year.
    • Rock Number 13b:
      • * Semibalanus removed:
        * Young barnacles: 35
        * Mature barnacles: 0
      • * Semibalanus not removed:
        * Young barnacles: 90
        * Mature barnacles: 31
    • Rock Number 12a:
      • * Semibalanus removed:
        * Young barnacles: 44
        * Mature barnacles: 37
      • * Semibalanus not removed:
        * Young barnacles: 95
        * Mature barnacles: 71
    • Rock Number 14a:
      • * Semibalanus removed:
        * Young barnacles: 40
        * Mature barnacles: 36
      • * Semibalanus not removed:
        * Young barnacles: 86
        * Mature barnacles: 75

Conclusion

• The data indicates that Chthamalus is not found on the lower rock face due to competition with Semibalanus.
• Other studies suggest that Chthamalus occupies the upper rock face because it is more resistant to desiccation.