Bottom-up and top-down effects combine to drive predator-prey interactions in a forest biodiversity experiment.

The bottom-up effect of producers and the top-down effect of predators are well-known factors shaping community assembly and ecosystem functioning through trophic interactions. Communities differing in their functional composition may induce ecological effects with varying directions and intensities, but previous studies in highly diverse ecosystems have struggled with reliably quantifying these interactions at the community level. We used spider gut-content metabarcoding in a subtropical tree diversity experiment to examine the impact of multiple diversity components of both trees and spiders on prey diversity and the network structure of predator-prey interactions. Our findings reveal that prey richness and spider-prey network structure are simultaneously driven by the bottom-up effects of tree communities and the top-down effects of the spider communities. When categorized by hunting modes, the drivers of prey richness and network structure differed between spider guilds. Large phylogenetic and functional differences within web-building spider communities promoted coexistence, leading to increases in the utilized prey richness, generality, niche overlap and prey vulnerability. For hunting spiders, the effects of vertical tree structure complexity indicated restricted mobility but facilitated coexistence through increased shelter availability, and a concomitant reduction of prey richness and dietary breadth. Our study underscores the significance of integrating multiple diversity components and considering functional trait composition across trophic levels when analysing the ecological effects of generalist predators. Our findings enable a better understanding of how predator-prey interaction patterns may be altered under current environmental changes that result in biodiversity loss.