Intertidal mesograzers in field microcosms: linking laboratory feeding rates to community dynamics

Mesograzers (herbivores <2.5 cm) are both diverse and abundant, but their relative effects on intertidal communities have rarely been quantified. Here I examine the effects of crustacean and polychaete mesograzers on two intertidal resources, the red alga Odonthalia floccosa Esp. (Falkenb.) and the epiphytic diatom Isthmia nervosa Kütz. The mesograzers were hermit crabs (Pagurus hirsutiusculus (Dana) and P. granosimanus (Benedict)), amphipods (Hyale frequens Stout and H. pugettensis (Dana)), isopod (Idotea wosnesenskii (Brandt)), juvenile kelp crab (Pugettia producta (Randall)), and polychaete worm (Platynereis bicanaliculata (Baird)). Feeding rates on Isthmia, measured in the laboratory for different consumer species and size classes, scaled allometrically with body mass. Consumption rates were 2-23% of body mass daily on a fresh weight basis. However, feeding rates on Odonthalia did not scale, suggesting that size will not always indicate per capita effect. Mesograzer densities were measured on Tatoosh Island, Washington, USA. The mesograzer predicted to have the largest total effect (P. hirsutiusculus), based on densityxfeeding rate, was neither the most abundant nor the most voracious. The validity of these sorts of predictions depends on how well feeding rates measured in the laboratory approximate per capita effects under field conditions. Predictions were compared to observed effects in field microcosms. Given known numbers of mesograzers, predictions were made about the amount of Isthmia biomass that should disappear over 2 weeks from microcosms (9x9x12 cm) anchored in tidepools. Average per capita effects in field microcosms were correlated with laboratory feeding rates, but, for three species with significant feeding on Isthmia, effects were lower than feeding rates predicted. Feeding trials may overestimate community impact because they fail to account for alternative food, search times, resource productivity and stimulation of growth, or interference from other consumers. Nevertheless, densities of mesograzers can reach sufficiently high levels so that even feeble per capita effects combine to alter biomass of epiphytes and perhaps other small algae.