Deer browsing reduces leaf damage by herbivorous insects through an induced response of the host plant

We aimed to demonstrate an indirect relationship between a mammalian herbivore (sika deer) and herbivorous insects on the induced responses of a shared host plant, Viburnum dilatatum . Field studies were conducted at three sites (i.e. two islands and one mainland) and within a deer exclusion area. One island, Kinkazan (Kz) Island, harbored a high density of deer while the other sites (controls) had no deer or very low densities of deer. The deer exclusion area had been established approximately 10 years earlier on Kz. We collected leaves above the browsing line of the deer and measured leaf hardness and tannin concentration. Leaf damage by insects was used as a measure of insect abundance. Leaves collected at Kz were harder than those from one of the control sites and from inside the deer exclusion area, while no difference was detected among the other controls and inside the exclusion area. In contrast, the tannin concentration of leaves from Kz was lower than in leaves from the control site. Leaf damage by herbivorous insects was lower in Kz than the other study sites. In addition, 'hole'-type leaf damage tended to be higher inside, rather than outside, the exclusion area. These results suggest the possibility that deer browsing increased leaf hardness, which exerted an indirect negative effect on the herbivorous insects utilizing the common host plant. To our knowledge, this is the first study to provide evidence of indirect negative effects between mammalian herbivores and herbivorous insects sharing a host plant.