Nutrient addition, but not vertebrate predator exclusion, shapes arthropod communities and herbivory in a temperate forest

Abstract Plants support diverse arthropod communities, and arthropod herbivores respond differently to plant traits, nutritional content and defences, which influence their host plants' selection, survival and performance (bottom‐up control). At the same time, arthropod herbivores are affected by their interactions with predators (top‐down control). Investigating how these forces interact and how they are affected by nutrient availability is crucial for understanding the mechanisms driving herbivore populations and their impact on ecosystems. We investigated how predator exclusion and fertilisation affect arthropod densities, sizes and herbivory on two temperate forest tree species. Using a factorial design, we compared fertilised and unfertilised trees with and without the exclusion of flying vertebrate predators in the forest understory during September 2020 and 2021. We collected and identified arthropods into feeding guilds. Fertilisation, but not predator exclusion, increased herbivory damage on trees as well as the size of predatory arthropods on the fertilised trees. Nutrient addition and predator exclusion had no significant effect on arthropod density. These patterns may indicate that the additional nutrients could have attracted herbivores, which in turn attracted their predators and may have enhanced their activity, thus potentially offsetting detectable changes in herbivore density. These results suggest that nutrient enrichment and predator exclusion interact, with nutrient addition affecting plant growth and herbivory damage primarily, but also increasing the size of predatory arthropods, but not the size of all arthropods. Effects of predator exclusion were less pronounced, potentially due to larger predatory arthropods compensating for the absence of flying vertebrate predators. Our study provides fully factorial field tests of top‐down and bottom‐up forces in a temperate forest understory, underscoring the critical need to evaluate how diverse ecological interactions mediate the synergistic effects of nutrient pollution and the ongoing decline of insectivorous vertebrate predators on arthropod communities and herbivory damage, particularly as these pressures intensify in our rapidly changing environment.