Parasite-grass-forb interactions and rock-paper- scissor dynamics: predicting the effects of the parasitic plant Rhinanthus minor on host plant communities

P> Parasitic plants affect the growth, reproduction and metabolism of their hosts and may also influence the outcome of competitive interactions between host species and, consequently, the structure of entire host communities. We investigate the effect of the root hemiparasitic plant Rhinanthus minor on plant community dynamics using a spatial theoretical model. The model is parameterized with data from pairwise interaction experiments under two nutrient levels between the hemiparasite and three grass species (Cynosurus cristatus, Festuca rubra and Phleum bertolonii) and three forb species (Leucanthemum vulgare, Plantago lanceolata and Ranunculus acris). Relative interaction coefficients were intransitive, with the dynamics of the system conforming to a rock-paper-scissors game. Stable deterministic dynamics emerge from parameters obtained under low-nutrient conditions. Under high-nutrient conditions, the dynamics are unstable, but are stabilized in spatially explicit models. The outcomes are sensitive to initial spatial pattern and frequency. Synthesis. This study supports the idea that hemiparasite populations may form 'shifting clouds' in natural populations and explains seemingly unpredictable shifts in host community structure following introduction of hemiparasites. Management of plant communities using hemiparasites needs to take these complex dynamics into account.