Feeding preferences of periwinkles among four species ofFucus

A widely accepted view of intertidal community organizatiton in the NW Atlantic proposes that fucoid vegetation is maintained by the actions of predators which remove species competitively superior toFucus species. Herbivory is an important component of these predatory interactions, but has been studied largely with reference to the interaction betweenF. vesiculosus andLittorina littorea. There are many species of fucoids and herbivorous invertebrates on the shores of the NW Atlantic and this paper reports field and laboratory experiments performed in 1987 (in or near Halifax, Canada) on the effects of grazing by three species ofLittorina on adults and juveniles of four species ofFucus. In laboratory experiments, portions ofFucus species were presented singly (no choice) or together (multiple choice) to single species ofLittorina. AdultF. distichus was grazed to only a small extent.F. evanescens andF. vesiculosus adult tissues were heavily grazed in most laboratory experiments.F. spiralis adults were heavily grazed in no choice experiments, but were grazed only slightly in multiple choice tests. When adult fucoid tissues were exposed to a mixed suite of grazers in the field, onlyF. vesiculosus was grazed heavily. Tests done on adult tissues showed clearly thatFucus species are highly variable in their vunerability to grazers. Moreover, we found variability among periwinkle species in grazing rates on adultFucus. We also found a great disparity in the feeding rates of different periwinkle species on juveniles ofFucus. Therefore, it is not possible to extrapolate from a single periwinkle species/single fucoid species interaction when attempting to identify vegetation structuring processes.