Root hemiparasites and plant diversity in experimental grassland communities

1 We studied the relationship between the diversity of grassland communitie s and the effects of the generalist hemiparasitic plant Rhinanthus alectoro lophus. We compared resistance against biomass loss as a consequence of inf ection, performance of the parasite and resistance of the parasitized commu nities to invasion by other plant species. Seeds of the parasite were sown into experimental plots containing 1, 2, 4, 8 or 32 plant species belonging to one or more of three functional groups (grasses, legumes and non-legumi nous herbs). 2 We predicted that infection will reduce host biomass, total community bio mass and resistance to invasion, particularly in host communities with low diversity, but that the performance of the parasite will be at its lower le vel in such communities. 3 The presence of the parasite caused an overall reduction in host biomass per plot, which was mainly due to a strong reduction in the biomass of gras ses. as predicted, the effect was smaller in communities with greater funct ional diversity. However, total community biomass (including the parasite b iomass) was increased by more than a third in infected communities of one o r two host species, while the parasite had no effect on total biomass of sp ecies-rich communities. 4 Germination of the parasite was hardly influenced by the diversity of its host community, but early survival decreased with increasing number of fun ctional groups and was lower in plots with legumes than without. However, o ur hypothesis that the performance of the surviving parasites would benefit from a high functional diversity of hosts was supported. Parasite biomass per individual and per m(2) increased with the number of functional groups in the host community, as did reproductive potential. 5 Death of the parasite led to a higher proportion of bare ground in commun ities when the previously infected communities had low functional diversity , thus enabling subsequent colonization by weeds.