Question: Climate change processes could influence the dynamics of bioticinteractions such as plant competition, especially in response to disturbancephenomena such as invasional processes. Are competitive effects of native specieson an invadermediated bywater availability?Location: Glasshouse facility, New SouthWales, Australia.Methods: We constructed competitive hierarchies for a representative suite ofspecies from coastal dune communities that have been invaded by the Asteraceaeshrub, bitou (Chrysanthemoides monilifera subsp. rotundata). We used a comparativephytometer approach, where the invader species was grown with orwithout a suite of native species in glasshouse trials. This was used to constructcompetition hierarchies under two water stress conditions: non-droughted anddroughted. The treatments were designed to simulate current and potentialfuturewater availability respectively.Results: We found that the invader experienced fewer competitive effects fromsome native species under water stress, particularly with regard to belowgroundbiomass effects. Native species were often poor competitors with theinvader, despite their adaptation to periodic water stress in native coastal environments.Of the native species with significant competitive effects on the invader,functionally similar shrub species were the most effective competitors, asexpressed in below-ground biomass. The relative position of species in the hierarchywas consistent across water treatments based on below-ground bitou biomass,but was contingent on water treatment when based on above-groundbitou biomass.Conclusions: The competitive effects of native species on an invader areaffected by water stress. While the direction of response to water stress is species-specific, many species have small competitive effects on the invader underdroughted conditions. This could allow an increase in invader dominance withclimate change.