Large herbivores facilitate an insect herbivore by modifying plant community composition in a temperate grassland

Large herbivores often co-occur and share plant resources with herbivorous insects in grassland ecosystems, yet how they interact with each other remains poorly understood. We conducted a series of field experiments to investigate whether and how large domestic herbivores (sheep; Ovis aries) may affect the abundance of a common herbivorous insect (aphid; Hyalopterus pruni) in a temperate grassland of northeast China. Our exclosure experiment showed that three years (2010-2012) of sheep grazing had led to 86% higher aphid abundance compared with ungrazed sites. Mechanistically, this facilitative effect was driven by grazing altering the plant community, rather than by changes in food availability and predator abundance for aphids. Sheep significantly altered plant community by reducing the abundance of unpalatable forbs for the aphids. Our small-scale forb removal experiment revealed an “associational plant defense” by forbs which protect the grass Phragmites australis from being attacke...