Plant-mediated effects of soil invertebrates and summer drought on above-ground multitrophic interactions

1. We conducted a large-scale field study to determine how the interactive effects of earthworms (Aporrectodea caliginosa) and summer drought affected plant communities containing barley (Hordeum vulgare), shepherd’s purse (Capsella bursa-pastoris) and common groundsel (Senecio vulgaris), and how such effects then influenced populations of the aphid Rhopalosiphum padi and its parasitoid, Aphidius ervi. 2. Total biomass of plant communities decreased by around 25% with summer drought, but was increased by the presence of earthworms by over 11%. The effects of drought and earthworms on H. vulgare differed in monocultures and mixed plant communities. In monocultures, earthworms had the biggest impact, increasing plant biomass, but drought had the biggest effect in multi-species communities, causing a decrease in biomass. 3. Drought had an extremely negative impact on S. vulgaris shoot and root biomass, but this was ameliorated in the roots when earthworms were present. Capsella bursa-pastoris was not significantly affected by drought or earthworms. Drought caused a significant increase in shoot nitrogen concentrations in all plants, including H. vulgare (from19.2 to 23.8 mg g)1). 4. In total, 35 234 aphids were recorded, of which 3936 were parasitized. Drought conditions reduced aphid abundance by over 50%. The interaction was moderated by earthworms, which caused further declines in R. padi populations under drought conditions, most notably in monocultures. 5. The plant-mediated effects of drought and earthworms (negative and positive, respectively) on R. padi had cascading effects on the parasitoid, Aphidius ervi, which declined in abundance with reduced numbers of R. padi. In addition, drought had a negative impact on A. ervi abundance beyond its impacts on aphid density, suggesting reduced prey quality as well as quantity. 6. Synthesis. This study demonstrated the effect of predicted climate change (i.e. reduced summer rainfall) on plant-mediated interactions between earthworms and above-ground multitrophic groups. These effects were seen to differ between monocultures and multi-species plant communities, suggesting that changes in above-ground–below-ground linkages in response to drought may influence plant communities in the future.