Mussel bed disturbance processes: intrinsic mechanisms

Established disturbance theory aimed at understanding community structure maintains that externally imposed disturbances occur randomly over the landscape and interrupt competitive exclusion, thereby increasing community diversity. In contrast to this idea, our detailed landscape surveys show that disturbances happen most frequently in sub regions of the bed that support the greatest productivity and hence thickest cover of Mytilus californianus. Crowding reduces the attachment strength of mussels, making them more susceptible to dislodgment, and also lowers the reproductive condition of those imbedded individuals. A layer of empty shells and debris develops above the rock surface, barring direct rock attachment and promoting lateral attachments in the surface layers. This structurally unstable aggregation with thickened layers incurs more frequent propagating gaps than thinner layers. Mussel beds are comprised of different disturbance regimes, arrayed predictably over the intertidal landscape, and composed of competitive dominants that show self-limitation in thickened regions while maintaining subordinate species.