Taxonomic and functional assessment of mesopredator diversity across an estuarine habitat mosaic

Abstract

AbstractA long‐standing rule in ecology is that structural complexity increases abundance and diversity of organisms, but this paradigm glosses over potential trait‐specific benefits of habitat structure across different regional species pools. We tested this idea using multiple response variables emphasizing taxonomic and functional diversity in seagrass‐vegetated, edge, and unvegetated habitats across three estuaries in Washington State (USA). We also used these variables in tandem to evaluate functional redundancy as a proxy for ecosystem resistance and resilience. The estuaries spanned a twofold range in richness of mesopredatory fishes and decapods. Increases in per‐sample abundance with habitat structure were confined to three of seven functional groups, specifically those occupying the water column or directly associated with seagrass shoots. Consequently, seagrass reduced mesopredator diversity by reducing evenness. Habitats differed in mesopredator assemblages despite their spatial proximity (~3 m), supported by multivariate analyses performed at functional and taxonomic resolutions, but site differences were less apparent functionally than taxonomically. Functional redundancy did not differ by habitat, but increased with the richness of the regional species pool. Edge habitats were generally intermediate in community structure, per‐sample abundance, and diversity between seagrass and unvegetated habitats. Structural complexity provided a trait‐specific enhancement of abundance, and this pattern applied across species pools. Because seagrass benefits species with certain traits, management focused on the low‐intertidal estuarine habitat mosaic, rather than a particular habitat type, and on places where redundancy is already low, best supports mesopredator diversity and function.