Visitors to a tide pool often see large bare patches in the rocks that are surrounded by large beds of mussels and barnacles. There are several reasons this pattern may occur.
The first possible reason is wave action. Mussels and Barnacles will live in very wave exposed areas. They prefer these locations as they provide more food. Large wave events however have the power to dislodge mussels and barnacles. Large waves in the right conditions will rip the mussels and barnacles off the rocks.
Another reason is due to impacts of objects. In the Pacific Northwest, large trees and logs are swept out to sea during stronger store events. These logs will frequently crash into the tide pool areas and dislodge vast areas of mussels and barnacles.
Limpets can also cause large bare areas in mussel beds. Limpets will graze on rocks eating the algae on the surface. As these limpets move around grazing exposed rock, they will also prevent juvenile mussels and barnacles from settling by dislodging the juveniles. These bare areas are clear of most all other forms of live. Owl limpets in particular will farm these open areas for a period of time.
The most common reason for bare spots in rocks is predation by sea stars. Sea stars will devour large amounts of mussels and barnacles in a short amount of time.