The effects of plant diversity and insect herbivory on performance of individual plant species in experimental grassland

1 There is increasing evidence that components of biodiversity affect processes at the ecosystem level; yet, the effects of biodiversity on the performance of individual organisms or particular trophic interactions are largely unexplored. 2 We transplanted 10 individuals of Rumex acetosa into 82 experimental grassland plots differing in plant species and functional group richness. Half of the plants received an insecticide treatment to manipulate insect herbivory. 3 We measured the amount of herbivory, plant size, survival and reproductive parameters in 2003 and 2004. 4 Insect herbivores removed on average 4.3% (2003) and 5.1% (2004) of leaf area in unsprayed plants. Spraying significantly reduced damage levels on average by approximately 50%. Herbivory significantly decreased plant weight, leaf size and number, and inflorescence length and size. 5 Plant height and inflorescence size of R. acetosa significantly decreased with an increase in species diversity. Mortality was slightly higher in the species-poor mixtures. Plant functional group diversity had little effect on plant performance. The presence of legumes generally increased, while the presence of grasses generally decreased, morphological parameters and fitness in R. acetosa. 6 Overall, the presence of particular plant functional groups was more important than functional group or species richness per se, and insect herbivores had additive effects of the same magnitude as the presence of particular plant functional groups. 7 Insect herbivory and plant functional identity, rather than species richness, determine the performance of individual plant species in temperate grasslands.