Presence Of Grazers Research Articles

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

ABSTRACTField 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.

The role of environment in control of morphology in Lithopyllum congestum, a Caribbean algal ridge builder : STENECK R. S. and W. H. ADEY, 1976. Bot. mar., 19 (4): 197–215

Experiments were carried out on the southern coast of the Isle of Man, British Isles, in summer 1990. Canopy removal and grazer exclusion treatments were applied to experimental areas on the mid-shore where two crustose coralline algae were growing together: Phymatolithon lenormandii (Aresch.) Adey and P. purpureum (P. and H. Crouan) Woelkerling and Irvine. Samples were taken to verify changes in the biomass of epiphytes on crusts over time, in the presence and absence of canopy and grazers. Two sets of experiments were run in the same area, one using pieces of crust on plates which allowed the standardization of the microhabitat and surface features of crusts, and another with crusts growing naturally on rocks. Epiphytes were largely absent from crusts beneath the Fucus canopy in both sets of experiments, and were reduced by grazing limpets Patella vulgata L. The rough micro-surface texture of Phymatolithon lenormandii enhanced Enteromorpha but not Fucus biomass, while the smooth microscale but protuberant macroscale topography of P. purpureum provided epiphytes with a refuge from grazing. The substratum type did not affect epiphyte loads as much as the presence of an overlying canopy or, to a lesser extent, grazing limpets, but in the absence of these factors significant differences in epiphyte biomass between crusts were found. These are attributable to features of the crusts and various possibilities are discussed.