FACTORS CONTROLLING THE INTERTIDAL ZONATION OF IRIDAEA FLACCIDA (RHODOPHYTA)1

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Abstract
Field experiments were used to examine the effects of biological interactions on the abundance of Iridaea flaccida (S. & G.) Silva within and below its normal range in the intertidal zone of central California. A combination of competitor removal, grazer exclusion and transplant manipulations showed that within the I. flaccida zone, this alga can be the first macroscopic plant to become reestablished after clearing and that this can occur in less than two months. Grazing by molluscs within this zone retards, but does not prevent, re‐establishment. Although absent prior to the experiments, both I. flaccida and I. cordata (Turn.) Bory settled, grew rapidly and reproduced in cleared areas below the I. flaccida zone, regardless of the presence of grazers. These plants did not establish themselves if entire thalli or holdfasts of other algae normally occupying the lower zone were present. The results support the few other in situ experimental investigations of algal zonation by showing that, when bounded by other plants, the lower limits of intertidal algae are directly determined by competition. They also suggest that I. flaccida and I. cordata may be conspecific.