Final answer:
Barnacles cover rock surfaces while mussels are in crevices due to intertidal limiting factors such as competition, predation, and environmental conditions. Nutrient availability, like plankton, is critical for sustaining their populations in the intertidal zone.
Step-by-step explanation:
The observation of barnacles dominating the rock surfaces and mussels being restricted to crevices can be understood by evaluating the limiting factors present in the intertidal zone. Barnacles secrete a cement-like substance allowing them to firmly attach to open, exposed surfaces, which helps them to filter feed efficiently in this constantly changing environment. In contrast, mussels are often found in the somewhat more protected crevices, possibly due to a combination of biological and environmental factors such as competition for space, predation, and physical conditions (e.g. wave exposure, desiccation). Given that both mussels and barnacles are filter feeders, the availability of nutrients, primarily plankton, plays a critical role in establishing the maximum sustainable population.
Therefore, factors such as the distribution of phytoplankton and the rate of water flow, which influence the delivery of nutrients, are crucial for maintaining dense populations in the intertidal zone.