THE RELATIONSHIP BETWEEN SHADE AND MOLLUSCAN GRAZING IN STRUCTURING COMMUNITIES ON A MODERATELY-EXPOSED TROPICAL ROCKY SHORE

The relationship between physical factors (principally temperature) and biological factors (herbivory) in structuring mid and low shore communities were investigated on a moderately-exposed tropical rocky shore. Physical stress, as measured by rock surface temperature and evaporation of distilled water, was greatest higher on the shore as a result of increased exposure time. Exclusion of herbivores (by caging) and alteration of physical factors (by shading) caused little visible change in community structure in the mid shore. There was also no difference in community structure between open rock areas and control treatments for experimental manipulations at either shore level. Chlorophyll levels increased in rock chips from treatments at both shore levels that excluded herbivores, but still allowed access to sunlight, indicating a developing biofilm. In the low shore, however, treatments that provided refuge from herbivores and direct sunlight were colonized by barnacles (Balanus amphitrite) and an encrusting alga, Hildenbrandia. There appeared to be an interaction between these factors in structuring the community; removal of herbivores allowed the settlement of barnacles which was greatest in shaded treatments. On Hong Kong shores during the summer months biological interactions are important low on the shore in combination with physical factors. In the mid shore, however, physical factors appear the major structuring agent. Community structure on Hong Kong shores is dictated by seasonal variation in physical factors which mediate the effect of biological factors' such as herbivory.