Human-mediated community assembly reshapes urban phytochemical diversity

Kilian Perrelet , Gregory Röder, Sebastian Ruile, Sergio Rasmann, Marco Moretti

Human activities increasingly determine which plant species coexist, particularly in cities, yet whether this reshapes the chemical dimension of biodiversity remains largely unknown. Here, we investigated how human-mediated plant community assembly influences phytochemical diversity by integrating community composition, functional traits, phylogenetic relationships, leaf metabolomes, and floral volatile profiles across 60 urban green spaces spanning a wide gradient of human influence in Zurich, Switzerland. Human influence was associated with greater phytochemical richness, an effect mediated primarily through increases in plant functional diversity. Human influence was also associated with coordinated shifts in functional, phylogenetic, and phytochemical composition, generating distinct chemical assemblages across urban green space types. Yet phytochemical differentiation was especially sensitive to fine-scale species turnover: chemically distinct communities emerged even when species replacement produced relatively little divergence in broader functional or phylogenetic structure. These patterns were strikingly consistent between leaf metabolites and floral volatiles despite their contrasting ecological functions and evolutionary constraints. Our findings reveal phytochemical diversity as an emergent property of community assembly and show that human decisions can reorganize the chemical environments through which ecological interactions occur.