Diversified cropping strengthens herbivore regulation by providing seasonal resource continuity to predators

Agricultural practices shape arthropod communities in arable fields, consequently influencing their interactions and the resulting ecosystem services, in particular pest regulation. Predatory arthropods play a pivotal role by preying on herbivores, soil fauna, and on other predators. However, the intricate mechanisms through which different agricultural practices shapes the dietary preferences of predators and regulates herbivore populations remain complex and inadequately understood.

We assessed how fertilisation with organic fertiliser and extending crop rotations with perennial ley affected predation pressure across prey taxa. We mapped predator and prey trophic linkages with molecular analysis of carabid predator gut contents and measured densities and taxonomic richness of predators, herbivores and soil fauna in 19 cereal fields during three samplings across the growing season.

We derived two food web structure metrics: prey vulnerability i.e., the average number of predators feeding on a selected prey, and predator trophic redundancy, i.e., dietary overlap. Prey vulnerability was compared among soil fauna, herbivores and other predator species (i.e., interspecific intraguild predation) over the growing season and across treatments. The mechanistic underpinnings of observed shifts in vulnerability of herbivorous prey at different crop stages were identified by using information criteria to select among candidate variables related to the richness, density and interaction structure of the different guilds during both the current and the previous crop stages.

Agricultural diversification via organic fertilisation combined with perennial ley in the crop rotation decreased the vulnerability of both intraguild prey and soil fauna prey and stabilised herbivore vulnerability. Mechanistically, the vulnerability of herbivorous prey at crop ripening emerges from the combination of predator richness and trophic redundancy during this sampling round, rather than from carry over effects from previously in the season.

Application and Synthesis: Our results suggest that locally provided resource continuity through diversified cropping practices bolster biological pest regulation, thus underline the importance of lesser disturbance in arable ecosystems for the provision of ecosystem services. Enhanced predator species richness together with availability of alternative prey through the season  underpins this enhanced pest regulation.

Methods
We assessed regulation of herbivores in two steps. First, we sampled the realised trophic interactions between predators and their prey through metabarcoding of predator gut contents. Second, we characterised predator and prey population densities of soil fauna, herbivore and intraguild prey, sampled with soil extraction, sweep netting and pitfall trapping. With this data, we investigated the vulnerability of soil fauna, herbivores and intraguild prey in time and across diversification treatments, including as predictors predator species richness, population densities and network metrics describing predation redundancy.

We sampled gut contents of carabid predators to identify trophic links between predators and prey below and above ground. To obtain highly resolved data on all consumed prey items in the predators’ diet, gut content samples were analysed through DNA metabarcoding using the primers fwhF2+fwhR2n targeting DNA barcode region in the arthropod mitochondrial cytochrome oxidase subunit I (COI) gene region (Vamos et al., 2017).