Organic farming promotes arthropod predators, but this depends on neighbouring patches of natural vegetation

Alistair D. Galloway^a,b, Colleen L. Seymour^b,c, etl.,

Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa.

Abstract

Current global declines in insect populations emphasise the need for biodiversity appropriate farming. Organic farming can promote in-field biodiversity, yet this may depend on surrounding vegetation, which acts as a source habitat for arthropod predators that promote pest suppression. Here we assess the role that natural vegetation plays in relation to farm management type (conventional versus organic farming) for arthropod predator diversity in a perennial cropping system (citrus orchards). We also determine how environmental variables, specifically plant species richness, flower abundance, cover crop height, litter depth, as well as grass, herbaceous, woody plant and rock cover, influence arthropod predators across the orchard – natural vegetation landscape mosaic. Arthropod species richness and abundance were significantly higher on organic farms, but species composition varied with biotope (natural vegetation, orchards with associated natural vegetation and those without associated natural vegetation). There was an interesting interaction between farming type and surrounding biotope. Organic farms had greater arthropod predator species richness and abundance than conventional farms in the absence of natural vegetation, but there was no difference between organic and conventional orchards in the presence of natural vegetation. Thus, patches of remnant natural vegetation improve the biodiversity of arthropod predators within conventional orchards. The environmental variables sampled here were all significant drivers for some focal taxa in most biotopes, emphasising the need to conserve heterogeneity in agricultural systems. The value of organic farming depends on landscape context, particularly proximity of natural vegetation. It is therefore recommended that to improve predator species richness and abundance, farmers should aim to diversify both local- and landscape-level environmental heterogeneity.