The intermediate disturbance hypothesis underlies community structure in rocky intertidal forests.

The intermediate disturbance hypothesis (IDH) is a foundational ecological concept explaining how diversity responds to opposing forces of disturbance and competition in a successional context. Wave-exposed rocky shores typically experience high levels of hydrodynamically-driven disturbances as important determinants of community structure. Research in such environments is challenging so knowledge of the underlying community dynamics is limited. On Northeast Pacific rocky shores, the annual tree-like kelp Postelsia palmaeformis is abundant. On relatively accessible platforms, Postelsia interacts with competitively dominant mussels and has a patchy distribution. Postelsia also occurs above the mussel zone on steep, marginally accessible walls. We tested the role of disturbance and competition on wall communities using a two-year (1991–92) “press” experiment. Treatments mimicked severe disturbance, two levels of intermediate disturbance, and no disturbance. Cessation of experiment maintenance allowed determination of short-term (12-month) and long-term (5 to 10 year) recovery rates. Results were consistent with the IDH, with diversity varying unimodally with disturbance but changes were modest. Experimental results mimicked natural changes, with lower summer diversity driven by Postelsia’s competitive dominance and higher diversity in winter due to disturbance and loss of Postelsia. Post-experiment recovery was punctuated by the 1997–98 El Niño, after which recovery took five years.