Niche specialization and community niche space increase with species richness in filter-feeder assemblages

Ecological theory posits that higher species richness should be associated with greater exploitation of resources and niche packing resulting from either increasing species niche overlap or specialization of species' niches. Research evaluating niche theory in animals tends to focus on organisms among functional feeding guilds, while resource partitioning might be more critical within functional groups. Freshwater mussels (Family: Unionidae) are a diverse and imperiled group of animals that are ideal models to test niche occupancy due to their functional similarity as filter-feeders and their occurrence in spatially and temporally stable multispecies aggregations. We evaluated the relationship between species richness and the trophic niche area for 25 mussel species occurring in 22 aggregations in the southeastern United States using stable isotope analysis (d(13)C and d(15)N) of soft tissue (n = 1057). Mean species standard ellipse area decreased with species richness, whereas ellipse overlap was not related to richness, indicating increased niche specialization may be the primary mechanism allowing coexistence in species-rich communities. Total community isotopic area increased with richness, suggesting species-rich communities also use a broader range of resources and may not be species-saturated. Overall, our data support the niche-packing hypothesis by illustrating the importance of niche partitioning within a species-rich guild of aquatic animals.