Tree diversity, tree growth, and microclimate independently structure Lepidoptera herbivore community stability

Insect herbivores are integral to the functioning of forest ecosystems. However, increasing herbivore outbreaks highlight the need to understand the factors driving the spatial and temporal stability of herbivore communities. While the longer term consequences of climatic fluctuations are well established in this context, the role of local-scale interactions between herbivores, their host communities, and local microclimates in influencing herbivore stability remains unclear. In this study, we investigated the relative importance of host tree species richness, functional diversity, trait composition, tree growth dynamics, and climate in driving herbivore spatiotemporal stability and the resulting patterns in abundance and diversity. We focused on Lepidoptera caterpillars as very diverse and functionally highly relevant herbivores in forest ecosystems. Tree species richness promoted mean caterpillar abundance, species richness, and phylogenetic diversity by positively affecting their temporal and spatial stability. These effects were mostly direct but counteracted by largely independent and overall negative effects of tree functional diversity, tree growth stability, and microclimate temperature stability. The strength and direction of these effects varied across seasons, reflecting shifts in environmental conditions and herbivore species turnover. The effects of tree diversity on caterpillar communities were related to compositional changes through distinct pathways by reducing taxonomic beta diversity and thus enhancing species richness stability and by increasing phylogenetic beta diversity which may promote asynchrony among distantly related species. Crucially, our findings suggest that tree diversity buffers herbivore communities against climate fluctuations by enhancing their spatiotemporal stability. In consequence, ongoing biodiversity loss may lead to greater fluctuations in herbivore populations and an increased risk of outbreaks. Our study provides novel insights into the mechanisms underlying bottom-up regulation of herbivores, emphasizing the critical role of tree diversity in maintaining stable herbivore communities in a changing climate.