Release from competition and protection determine the outcome of plant interactions along a grazing gradient

Abstract

Plant interactions are suggested to shift from competition to facilitation and collapse with increasing grazing pressure.Th e existence of this full range of plant interactions and the role of underlying mechanisms (i.e. release from competition and protecting eff ect) in response to herbivory remains poorly documented and mainly described in terrestrial systems. We use a large grazing disturbance gradient (fi ve levels of grazing) to test its eff ect on the outcome of plant interactions and underlying mechanisms in freshwater ecosystems. In a mesocosm experiment, we manipulated the presence of neighbouring plants to test their negative (competition) or protective (facilitation) eff ects on target plants along the grazing pressure gradient. We predicted that plant interactions 1) shift from competition to indirect facilitation with increased grazing pressure, 2) indirect facilitation collapses at high levels of grazing, 3) release from competition mainly drives the outcome in lowly grazed conditions and, 4) decreased protection occurs in highly grazed conditions responsible for the collapse of facilitation. Th is study shows the occurrence of the full range of outcomes in plant interactions under a wide spectrum of grazing pressure and indicates how the complex combination of underlying mechanisms shapes variations in plant interactions. We show that both, the release from competition and the increased protection by neighbouring plants drove the shift from competition to indirect facilitation.Declined protection by neighbouring plants resulted in a collapse of indirect facilitation for survival under intense herbivory. Our study provides the fi rst experimental evidence of indirect facilitation structuring freshwater ecosystems thereby validating important ecological concepts mainly developed for terrestrial ecosystems.