Larval predation by epifauna on temperate reefs: scale, power and the scarcity of measurable effects

Benthic suspension feeders often occur at high densities on temperate reefs and other hard bottoms. Because many such organisms have prodigious feeding rates in the laboratory, it has often been predicted that established epifauna would deplete larval populations, thereby influencing density, spatial distribution and species composition at settlement. Most field manipulations of filter-feeder density have not controlled for factors other than larval predation that could produce the same pattern. In controlled experiments, larval predation effects are seldom observed. Intermittent feeding and small radii of predator influence present many more opportunities for settlement than has generally been assumed. Two important factors often foil attempts to measure predation effects: low statistical power due to small effect sizes, and choice of spatial scales larger than these where predation effects occur.