Partial congruence in habitat patterns for taxonomic and functional diversity of fish assemblages in seagrass ecosystems

Marine conservation aims to maintain ecological functioning through the protection of biodiversity, frequently relying on habitat categories as representative surrogates for biodiversity. Habitat biodiversity surrogates typically represent taxonomic diversity, despite species functional traits being more directly linked to ecosystem functioning. Here, we evaluate habitat biodiversity surrogates based on taxonomic and functional diversity of fish assemblages in seagrass ecosystems of Nova Scotia, Canada. We do so by assessing congruence in habitat differences (i.e., seagrass vs adjacent bare soft sediment) between biodiversity components across varying environmental conditions, and also by identifying relationships of fish biodiversity with environmental, habitat, and landscape variables. We found only partial agreement in habitat differences for each biodiversity type, that is, habitat differences in taxonomic diversity were only sometimes reflected by functional diversity. In fact, taxonomic diversity was consistently higher in seagrass than bare sediment, while this was not always the case for functional diversity. Similar functional diversity between habitat types resulted from redundancy in functional traits and consistent expression of dominant traits. Regression analyses indicated that taxonomic diversity was determined by habitat complexity (canopy height, sediment organic content), wave exposure, and seagrass bed size (R2 ~ 0.60). Functional diversity was determined by habitat complexity (shoot density, canopy height, and sediment organic content) and wave exposure (R2 = 0.475), suggesting the importance of environmental filtering in selecting a certain set of functional traits. Our study suggests that multiple biodiversity components combined with knowledge of the environmental context are necessary for optimal use of habitat biodiversity surrogates in conservation planning.