# Invertebrate Decline Has Minimal Effects on Oak‐Associated Microbiomes

## TL;DR

This study shows that invertebrate decline has minimal effects on oak-associated microbiomes, with more pronounced changes in leaf microbiomes than in roots or soil.

## Contribution

The study experimentally demonstrates the compartment-specific microbial responses to invertebrate decline in an ecotron setup.

## Key findings

- Fungal communities in oak leaves showed increased saprotrophs and pathogens under invertebrate decline.
- Leaf bacteria like Proteobacteria and Actinobacteria increased with reduced invertebrate biomass.
- Belowground microbiomes were minimally affected, but aphid infestation altered microbiota due to reduced predator influence.

## Abstract

Recently, biomass of invertebrates has declined substantially at many locations with the implications of this biodiversity loss for ecosystems yet unknown. Through multitrophic interactions, plant‐ and soil‐associated microbiomes might be altered, causing a cascade of changes on diverse ecosystem processes. We simulated aboveground invertebrate decline in grassland ecosystems with two levels of invertebrate biomass (36% and 100% of current ambient conditions), plus a control with no invertebrates present. Each standardised grassland mesocosm additionally contained one clonal
Quercus robur
L. sapling to investigate the extent of invertebrate decline effects exceeding grasslands. We investigated oak biomass partitioning and mycorrhiza formation, oak leaf transcriptome and microbiome composition of leaves, roots and rhizosphere. While invertebrate decline did not significantly affect oak performance and herbivory‐related gene expression, fungal communities presented an increase of saprotrophs and pathogens, especially in leaves. Among leaf‐inhabiting bacteria, Proteobacteria and Actinobacteria increased under invertebrate decline. The belowground microbiome was only little affected. But, invertebrate decline came along with a reduced influence on predators leading to an elevated aphids infestation that proofed able to alter microbiota. Our findings establish a strong difference between above‐ and belowground, with the impacts of invertebrate decline being more pronounced in the leaf microbiome.

Reduced invertebrate biomass impacts oak‐associated microbial communities: oak compartment specific bacterial and fungal communities showed divergent reactions to insect decline within an experimental set‐up in an ecotron facility, called ‘Insect Armageddon’.