Persistence of a rare aquatic species through gradients of disturbance and sediment richness.

This study examines the capacity of establishment of a rare aquatic macrophyte, Luronium natans, within plant communities and habitat types in which it does not occur spontaneously. The species, generally Limited to disturbed or nutrient-poor habitats, was transplanted into a series of sites situated along natural gradients of disturbance (flush-floods and intermittent sediment exposure) and sediment nutrient-richness. The transplanted colonies were given a competition-free establishment period. Colony dynamics of Luronium as well as size structure of the recolonizing macrophyte communities were monitored over three growing seasons. At the end of this period, transplanted colonies still persisted in five out of 12 transplantation sites. Apparently 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 values within the recolonizing community. The study supports previous presumptions that long-term persistence of Luronium depends on processes limiting community biomass through occurrence of disturbance. But it also reveals the existence of suitable, yet not occupied habitats in which the species can persist as successfully as species from the local species pool. This finding modulates the presumption that Luronium's rarity is mainly caused by a weak competitive ability in the established phase. It thus rises questions about the species' performance at other stages of its life cycle and on its dispersability