Effects of exclusion of above- and belowground insects along with molluscs in a biodiversity experiment

Background/Question/Methods

In a number of manipulative studies, invertebrates have been shown to change plant community structure and ecosystem functioning. More recently, the role played by herbivores and other invertebrate feeding guilds including belowground detritivores has also been investigated in the context of biodiversity-ecosystem functioning research. However, no studies have investigated the impacts of these groups of invertebrates along an experimentally produced plant diversity gradient.

We used the Jena grassland biodiversity experiment, to test whether plant species richness and plant functional group richness modify the effects of molluscs, aboveground and belowground insects on plant community biomass, plant community species composition and the phylogenetic signal in the plant community. We reduced the density of these invertebrate groups alone and partly in combination on different subplots using pesticides, for periods of between five and seven years. We ask the following questions: (1) Does exclusion of invertebrates lead to an increase in plant community biomass that decreases at higher levels of plant species richness? (2) Are there consistent diversity-dependent shifts in plant community composition when invertebrates are reduced? (3) Do the changes in plant community composition reveal a phylogenetic signal that points to a community of specialized plant antagonists and conserved plant defences?

Results/Conclusions

Invertebrate effects on plant community biomass and phylogenetic structure were generally small, variable in time and largely independent of plant biodiversity. However, we found significant shifts in plant species composition due to the reduced density of the invertebrate groups, and these effects were partly affected by the initial diversity of the plant community. Insects feeding belowground generally had larger effects on plant community variables than both aboveground feeding insects and molluscs. The response of individual plant species to reduced invertebrate density dependent on the composition of the constituent plant community. Resulting idiosyncratic responses of individual plant species were an important reason for the few consistent responses at the community level. Our results show that even low levels of herbivory and simplification of invertebrate communities can lead to persistent changes in plant community structure, yet understanding these responses will require studying the plant-animal interactions at the plant species level.