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


Y. Le Bagousse-Pinguet, Limnological Inst., Univ. of Konstanz, Mainaustrasse 252, DE-78464 Konstanz, Germany. E-mail:


Plant interactions are suggested to shift from competition to facilitation and collapse with increasing grazing pressure. The existence of this full range of plant interactions and the role of underlying mechanisms (i.e. release from competition and protecting effect) in response to herbivory remains poorly documented and mainly described in terrestrial systems. We use a large grazing disturbance gradient (five levels of grazing) to test its effect 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) effects 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. This 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 first experimental evidence of indirect facilitation structuring freshwater ecosystems thereby validating important ecological concepts mainly developed for terrestrial ecosystems.