Plant diversity enhances the density and diversity of aboveground arthropods and the ecosystem processes they govern

Background/Question/Methods

Plant species loss affects the abundance and diversity of above- and belowground arthropods, which have significant impacts on ecosystems. However, only few studies have so far linked plant diversity effects on arthropods to processes affected by these organisms within the ecosystem. Also the mechanisms underlying changes in the arthropod community and arthropod-related ecosystem processes remain unclear.

Here, we assessed how plant diversity affect the abundance and diversity of herbivores and decomposers, using an experimental plant diversity gradient ranging from monocultures to 60 species mixtures within a grassland biodiversity experiment (The Jena Experiment, Germany). Eight years after establishment of the experimental plots, arthropods were collected across an entire growing season using pitfall traps (decomposers) and suction samples (herbivores). Arthropod herbivory was measured in spring and summer, and decomposition rate of plant litter was quantified during a nine week litterbag experiment. We used structural equation modeling to investigate direct and indirect effects of plant diversity on the arthropod community and on herbivory and decomposition.

Results/Conclusions

We identified direct positive effects of plant diversity on abundance and diversity of herbivore insects, leading to higher herbivory rate. Moreover, herbivory was increased by an indirect effect of plant diversity, mediated by higher aboveground productivity at higher plant diversity levels, leading to higher herbivore abundance and diversity. Similarly, decomposition increased with increasing plant diversity, through both direct and indirect effects. The indirect positive effect was mediated by higher plant aboveground biomass, leading to higher abundance and diversity of decomposing arthropods. Higher diversity of decomposers in turn enhanced decomposition. Our results show that plant diversity not only affects the composition of the arthropod community, but also ecosystem processes governed by them. This is important for the current discussion on the importance of biodiversity for ecosystem services where the relationship between increased biodiversity and increased function is often assumed rather than proven, in particular for groups other than plants.