Interactive effects of herbivory and the level and fluctuations of nutrient availability on dominance of alien plants in synthetic native communities

Numerous studies have tested the fluctuating resource hypothesis and highlighted the role of nutrient availability and fluctuations therein for invasion success of alien plants. Others have tested the enemy-release hypothesis and highlighted the role of herbivores in invasion success. However, how herbivory and the level and fluctuations in nutrient availability interact in driving alien plant invasion into resident communities remains largely unexplored. We grew eight invasive alien species as target species in pot‐mesocosms with five different synthetic native communities in a three-factorial design with two levels of nutrient availability (low vs high), two levels of nutrient fluctuations (constant vs pulsed) and two levels of herbivory (with vs without). Increases in nutrient availability, irrespective of whether it was constant or pulsed, significantly increased the absolute biomass production of the alien and native plants. The presence of herbivores, on the other hand, significantly decreased the biomass production of plants, in particular under high-nutrient availability. The relative biomass production of the alien target plants, on the other hand, decreased in response to an increase in nutrient availability, and increased in response to the presence of herbivores. We also found tentative evidence that herbivory could interact with changes in nutrient availability and nutrient fluctuations to affect the dominance of the alien target species (a marginally significant interaction; 90% CIs: [0.133, 2.745]). Our multispecies experiment indicates that herbivory could mediate the interactive effect of nutrient enrichment and variability in nutrient supply on invasion of alien plants into native communities. Therefore, we recommend that studies testing the fluctuating resources hypothesis should also consider interactive effect of other trophic levels.