Top–down control of rocky intertidal ‘trees' by a small herbivore: the limpet–sea palm interaction

Community theory proposes that consumer control strength varies with environmental stress, with unimodal effects on diversity. Diversity is high with intermediate consumer pressure when consumers favor the dominant competitor(s). At low or high stress, diversity will be low due to resulting high or low consumer pressure. We tested these ideas in a rocky intertidal, kelp‐dominated community. The annual kelp Postelsia palmaeformis is abundant on wave‐exposed, rocky headlands along the northeastern Pacific coast. Winter storms drive high levels of disturbance while in summer wave action and disturbance are reduced. Postelsia forests occur in the upper intertidal on (a) gently sloping mussel‐dominated areas and (b) steeply sloping rocky areas. Steep‐wall Postelsia shade a species‐rich understory. The small (1–3 cm length) limpet Lottia pelta feeds on the much larger (50–70 cm tall) Postelsia raising questions about herbivory effects on community structure. Limpet grazing effects were tested in 0.7 × 0.7 m plots in the center of the Postelsia bed, in half of which limpets were manually removed. We expected that grazing would have minimal effects due to foraging inhibition by high wave stress, but found that limpets reduced the density, percent cover, and size of Postelsia and had mostly positive indirect effects on the understory. These effects abated in winter as the kelp senesced and was torn from the rock. Next‐generation sporophytes arise from overwintering microscopic juvenile stages in late winter. Diversity peaked in winter when kelp were absent and declined in summer when kelp were abundant and competed with understory taxa. Overall limpets maintained higher algal diversity in winter by reducing negative effects of sea palm competition on understory taxa, but this effect weakened in summer when kelp were large and dense. Hence, grazer effects were relatively subtle, suggesting that the limpet was a strong interactor but not a keystone consumer.