Influence of Biotic Factors in an Upper Intertidal Community: Dipteran Larvae Grazing on Algae

Abstract

Field experiments were used to determine the effects of herbivorous fly larvae on ephemeral algae at several rocky beaches in central California. The larvae of marine Diptera form dense populations during seasonal blooms of ephemeral algae in certain higher areas of rocky shores. In three field experiments, changes in the percent cover and species composition of algae on natural and artificial substrates which larvae were allowed to colonize were compared with the changes in algal cover on otherwise similar substrates from which larvae were removed. A system of barriers constructed of copper paint, plastic, iron, and other materials was used to exclude grazing crabs and limpets, which otherwise might have confounded effects produced by the smaller Diptera larvae. In two unreplicated experiments, larval removals caused increases in algal abundance. Statistical analysis of a third, replicated, experiment showed that in certain circumstances the larvae changed the species composition of algal blooms. This effect apparently occurred when grazing hastened species replacements during algal succession. Grazing dipteran larvae are apparently part of a complex association of herbivores that influence algal succession and partially determine the abundances of some high intertidal algae.