Numerical responses of different trophic groups of invertebrates to manipulations of plant diversity in grasslands

We studied the effects of plant diversity on abundance of invertebrate herb ivores, parasitoids and predators in two grassland communities tone in Swit zerland and one in Sweden) in which plant species richness and functional d iversity have been experimentally manipulated. Among herbivores, the abunda nce of only the most sessile and specialised groups (leafhoppers and wingle ss aphids) was affected by plant diversity. At both sites, numbers of leafh oppsrs in sweep net samples showed a linear, negative relationship with pla nt species number whereas numbers of wingless aphids in suction samples inc reased with the number of plant functional groups (grasses, legumes, and no n-legume forbs) present in the plot. Activity of carabid beetles and spider s las revealed by pitfall catches) and the total number of predators in pit falls at the Swiss site decreased linearly with increases in the number of plant species and plant functional groups. Abundance of more specialised en emies, hymenopteran parasitoids, was not affected by the manipulations of p lant diversity. Path analysis and analysis of covariance indicated that pla nt diversity effects on invertebrate abundance were mostly indirect and med iated by changes in plant biomass and cover. At both sites, plant species c omposition (i.e. the identity of plant species in a mixture) affected numbe rs of most of the examined groups of invertebrates and was, therefore, a mo re important determinant of invertebrate abundance in grasslands than plant species richness per se or the number of plant functional groups. The pres ence of legumes in a mixture was especially important and led to higher num bers of most invertebrate groups. The similarity of invertebrate responses to plant diversity at the two study sites indicates that general patterns i n abundance of different trophic groups can be detected across plant divers ity gradients under different environmental conditions.