Priority effects in herbivore communities vary in effect on plant development and reproduction in four Brassicaceae plant species

Insect herbivores can directly affect plant reproduction by feeding on reproductive tissues, or indirectly by feeding on vegetative tissues for which plants are unable to compensate. Additionally, early arriving herbivores may have cascading effects on plant fitness by altering the richness and abundance of the later arriving community. Studies are divided on whether herbivory early in the development of plants can impact plant fitness and whether these effects are predominantly mediated through changes in plant development or subsequent herbivory. Obtaining insight into the generality and consistency of mediated effects on plant reproduction induced by early-season herbivory requires a comparative approach across plant species and environmental conditions. By excluding the herbivore community in an exclosure experiment and by manipulating early-season herbivory in a common garden experiment replicated across four Brassicaceae species and two years, we tested whether early-season herbivory could affect plant development, reproduction, and the herbivore communities associated with individual plants. In addition, we tested a causal hypothesis to disentangle the direct effect of herbivory on plant reproduction, and their indirect effect through changes in the development of plants. Early-season herbivory affected plant development and reproduction, but effects were highly dependent on the plant species, the inducing herbivore species, and the biotic and abiotic environment. The exploratory path analysis indicated that plant reproduction was best predicted by variation in plant development, explaining up to 90.88% of the total effect on seed production. Even though the richness and abundance of the subsequent herbivore community were conditionally affected by the initial colonising herbivore, herbivore pressure is predicted to have only minor effects on reproduction. Importantly, the effects of herbivore pressure on seed set were not direct but were mediated by changes in plant development. Synthesis: Early-season herbivory has the potential to affect plant reproduction through changes in the development of plants and, less strongly, through effects mediated by the plant-associated herbivore community. However, as plants are often able to compensate and attenuate the effects induced by herbivory, the detection, sign, and strength of effects are highly dependent on the plant species and environment.