<b>Top-down control of rocky intertidal “trees”: the sea palm-limpet interaction</b>

These forests occupy space in the upper intertidal on (a) horizontal or slightly sloping mussel-dominated areas and (b) steeply sloping rocky areas. Steep-wall <i>Postelsia</i> forests shade a species-rich understory.

Observations revealed that the small (1-3 cm length) limpet <i>Lottia pelta </i>fed heavily on the much larger (50-70 cm tall) <i>Postelsia</i>. Limpet grazing effects on the sea palm community were tested in 1 m² plots in the center of the <i>Postelsia </i>bed, in half of which we manually removed limpets. Contrary to our hypothesis that grazing would have minimal effects, limpets reduced the density, percent cover, and size of <i>Postelsia, </i>maintained higher diversity,<i> </i>and had mostly positive indirect effects on the understory.