Herbivory and the Seasonal Abundance of Algae on a High Intertidal Rocky Shore

This paper examines the contribution of herbivory to seasonal variations in algal coverage at the uppermost intertidal levels of a rocky shore on the outer coast of Oregon, USA. In this habitat, simple, fast—growing algae are abundant during wetter, less sun—exposed, and cooler weather of the winter months, but disappear in the drier, sunnier, and warmer conditions of the summer. Acmaeid limpets, the predominant herbivores in the habitat, are present throughout the year. Previous studies of the high intertidal zone have suggested that physical stresses alone prevent the survival of algae through the summer months, and that grazing affects algal abundance when physical conditions are more benign. To test these hypotheses, cageless methods were used to exclude the limpets from plots on the uppermost intertidal rocks. The experiments were repeated at 3—4 mo intervals over a period of several years. This partial reduction in herbivory resulted in the development and persistence of dense covers of algae through the summer months, as well as increasing algal abundance during the other times of year. Apparently the primary cause of the seasonal fluctuations in algal abundance is variation in rates of algal production rather than change in absolute rates of algal loss to herbivory or physical stresses. Evidence from this study and the literature indicates that the limpets in this habitat are probably consuming more, not less, algae in the winter months. However, higher rates of algal production in winter more than compensate for the increased herbivory, and the algae become more abundant. During the summer, drier conditions cause rates of production by the algae to fall below rates of consumption by limpets, and the limpets remove most of the standing crop of algae. Thus, increased grazing pressure coincides with increased physical stress. To be a perennial or summer annual in this habitat, an alga not only must be resistant to desiccation, insolation, and high temperatures, but also must be inaccessible to herbivores or be resistant to herbivory. In the uppermost intertidal zone, the abundance of algae in winter evidently depends on the pattern of changes in primary production over the entire year. Scarcity of forage in the summer months reduces populations of the resident limpets, and the favorable conditions for primary production in winter months allow the transient species of algae to establish rapidly, grow, and reproduce before the populations of limpets recover. If even the most favorable physical conditions for algal growth remained constant through the year, the transient algae would probably disappear. It is speculated that under present conditions the establishment of perennial algae in this habitat is limited by the intensity of herbivory in the summer and by competition with transient algae in the winter. If this is correct, less fluctuating physical conditions, even if severe, would change the species composition of this high intertidal community by favoring the establishment of perennial algae.