Differential resource use in filter-feeding marine invertebrates

Coexistence theory predicts that, in general, increases in the number of limiting resources shared among competitors should facilitate coexistence. Heterotrophic sessile marine invertebrate communities are extremely diverse but traditionally, space was viewed as the sole limiting resource. Recently planktonic food was recognized as an additional limiting resource, but the degree to which planktonic food acts as a single resource or is utilized differentially remains unclear. In other words, whether planktonic food represents a single resource niche or multiple resource niches has not been established. We estimated the rate at which 11 species of marine invertebrates consumed three phytoplankton species, each different in shape and size. Rates of consumption varied by a 240-fold difference among the species considered and, while there was overlap in the consumer diets, we found evidence for differential resource usage (i.e. consumption rates of phytoplankton differed among consumers). No consumer ingested all phytoplankton species at equivalent rates, instead most species tended to consume one of the species much more than others. Our results suggest that utilization of the phytoplankton niche by filter feeders is more subdivided than previously thought, and resource specialization may facilitate coexistence in this system. Our results provide a putative mechanism for why diversity affects community function and invasion in a classic system for studying competition.