Phylogenetic, allometric, and functional group structure in chemical sensitivity across aquatic taxa

Chemical risk assessment across diverse communities remains challenging because most chemical-species pairs are empirically untested. Identifying phylogenetic, allometric, and functional group structure in chemical sensitivity may support cross-species extrapolation and community risk assessment, yet how these factors explain sensitivity across diverse taxa and chemicals remains poorly understood. We examined how phylogenetic relatedness, body mass, and functional group shape relative sensitivity across 580 aquatic species and 1103 chemicals spanning six chemical-use groups. We quantified phylogenetic signal in species relative sensitivity and fitted Bayesian phylogenetic hierarchical models to test global allometry, functional group-specific baseline sensitivity, and functional group-specific allometry. Phylogenetic signal was strong in invertebrates but weak in algae and fish. Models allowing both baselines and slopes to vary among functional groups received the strongest predictive performance across all taxonomic and chemical-use group combinations. However, most standardised slopes were weak, with moderate allometric effects concentrated in a few functional groups. Fixed effects explained only a modest proportion of variation, whereas chemical identity and phylogenetic structure captured substantial additional variation, with phylogeny dominating. These findings reveal limited support for general allometric rules and provide a basis for integrating functional group and phylogenetic information into cross-species extrapolation and higher-tier chemical risk assessment.