The specificity of induced chemical defence of two oak species affects differently arthropod herbivores and arthropod and bird predation

Plant-animal interactions and the plant chemical defence systems are a keystone of ecology and of particular interest because they fundamentally shape ecosystem functioning. Despite that, the factors and mechanisms driving the interactions between insectivorous predators and plants, via herbivore-induced plant volatiles—HIPVs remains to be fully understood. Insectivorous birds and arthropods can use chemical cues provided by herbivore-damaged plants, yet the specific chemical defensive traits that are involved in the interactions with predators remain unknown for many plant species. In our experiments, we compared plant volatiles of English oak (Quercus robur) and Sessile oak (Quercus petraea) saplings treated with methyl jasmonate (MeJA). We studied how the MeJA-induced plant volatiles differ between these two closely related plant species, and how it influences higher trophic taxa (e.g., predation, associated arthropod communities). Using plasticine caterpillars, we assessed attractiveness of MeJA-treated oaks (vs. control untreated oaks) for predators. Overall, in both plant species, attack attempts were significantly higher on MeJA-treated saplings compared to control saplings. Birds were responsible for the majority of attack attempts, followed by ants and other arthropods. The mean total amount of volatiles emitted by MeJA-induced saplings differed among the experimental oak species over time and repeated applications of MeJA. MeJA application had no significant effect on the abundance of arthropods. However, the mean body size of individuals of different feeding guilds was affected by MeJA treatment both negatively and positively, depending on the specific feeding guild. Overall, our study concludes that (1) the application of MeJA led to increased attractiveness of oak saplings for insectivorous predators, including invertebrates and birds; (2) MeJA-treated oak saplings emitted eight specific compounds that were not detected in control saplings and two additional compounds that were emitted at 100-fold higher levels in MeJA-treated saplings compared to control saplings; and (3) the predation rate on artificial caterpillars did not significantly differ between oak species.