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

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 provide fully factorial
field tests of top-down and bottom-up forces in a temperate forest
understory, underscores 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
preassures intensify in our rapidly changing environment.