Plant diversity changes can impact the abundance,
diversity, and functioning of species at higher trophic
levels. We used an experimental gradient in grassland
plant diversity ranging from 1 to 16 plant species to study
multitrophic interactions among plants, cavity-nesting bees
and wasps, and their natural enemies, and analysed brood
cell density, insect diversity (species richness), and bee and
wasp community similarity over two consecutive years.
The bee and wasp communities were more similar among
the high (16 species) diversity plots than among plots of the
lower diversity levels (up to 8 species), and a more similar
community of bees and wasps resulted in a more similar
community of their parasitoids. Plant diversity, which was
closely related to flower diversity, positively and indirectly
affected bee diversity and the diversity of their parasitoids
via increasing brood cell density of bees. Increasing plant
diversity directly led to higher wasp diversity. Parasitism
rates of bees and wasps (hosts) were not affected by plant
diversity, but increased with the diversity of their respective
parasitoids. Decreases in parasitism rates of bees arose
from increasing brood cell density of bees (hosts), whereas
decreasing parasitism rates of wasps arose from increasing
wasp diversity (hosts). In conclusion, decreases in plant
diversity propagated through different trophic levels: from
plants to insect hosts to their parasitoids, decreasing density
and diversity. The positive relationship between plant
diversity and the community similarity of higher trophic
levels indicates a community-stabilising effect of high
plant diversity.