Persistence of a rare aquatic species along gradients of disturbance and sediment richness

This study examines the capacity of establishment of a rare aquatic macroph yte, Luronium natans, within plant communities and habitat types in which i t does not occur spontaneously. The species, generally Limited to disturbed or nutrient-poor habitats, was transplanted into a series of sites situate d along natural gradients of disturbance (flush-floods and intermittent sed iment exposure) and sediment nutrient-richness. The transplanted colonies w ere given a competition-free establishment period. Colony dynamics of Luron ium as well as size structure of the recolonizing macrophyte communities we re monitored over three growing seasons. At the end of this period, transpl anted colonies still persisted in five out of 12 transplantation sites. App arently successful integration into the community occurred at both ends of the nutrient gradient, in periodically disturbed habitats. At intermediate to high nutrient richness Luronium maintained one of the highest cover valu es within the recolonizing community. The study supports previous presumptions that long-term persistence of Luro nium depends on processes limiting community biomass through occurrence of disturbance. But it also reveals the existence of suitable, yet not occupie d habitats in which the species can persist as successfully as species from the local species pool. This finding modulates the presumption that Luroni um's rarity is mainly caused by a weak competitive ability in the establish ed phase. It thus rises questions about the species' performance at other s tages of its life cycle and on its dispersability.