An assessment of two taxonomic distinctness indices for detecting seaweed assemblage responses to environmental stress

We tested the efficacy of two biodiversity indices, average taxonomic distinctness and variation in average taxonomic distinctness, for indicating environmental stress in seaweed assemblages from the Bay of Fundy, New Brunswick, Canada. These indices, which measure the average number of taxonomic levels between species in a sample, offer a potential panacea for biomonitoring because their calculation requires only a species list and a regional taxonomic hierarchy, they offer a statistical framework for testing whether assemblages deviate from an expected taxonomic breadth, and previous studies involving animal assemblages have demonstrated an independence from sampling effort. However, our results were not consistent with previously published studies or with our perception of site conditions. Specifically, putatively impacted sites scored above-average taxonomic distinctness values, while sites otherwise regarded as healthy were indicated as environmentally degraded. We also demonstrate that average taxonomic distinctness values can be negatively correlated with species richness, Shannon diversity and with functional diversity. Further, increasing the breadth of the regional species list to which specific sites were compared resulted in a more conservative test of impact. We recommend that a qualitative understanding of how specific biotic assemblages respond to stress is a necessary prerequisite to use the taxonomic distinctness indices for environmental stress assessments.