A critical reassessment of the novel weapons hypothesis and allelopathy as an adaptive strategy that facilitates plant invasion.

Plants have evolved an assortment of chemical adaptations that integrate environmental cues with developmental processes to regulate growth and reproduction. A subset of these phytochemicals may be considered allelopathic adaptations if they enhance fitness by suppressing competition for limiting resources. Despite compelling critiques by John Harper almost a half century ago, research on allelopathy nonetheless experienced a revival at the turn of the 21st century that gave rise to a 'novel weapons hypothesis' (NWH) to explain plant invasions. We briefly review the storied history of allelopathy and apply a systematic literature review to scrutinize the NWH from an eco-evolutionary perspective. Although it remains a highly influential metaphor, we find confused definitions and insufficient empirical evidence to support the NWH. Channeling Harper's original critiques, we propose that research on the NWH - and allelopathy more generally - would benefit from field experiments that causally link plant chemistry and competitive interactions to resource availability, fitness components, and population growth. We introduce a set of postulates to help address confusion over definitions and guide empirical tests of the causal processes linking plant chemistry to competitive outcomes, adaptive evolution, and the structure of plant communities.