Farming systems and landscape context: effects on biodiversity and biocontrol

Abstract

The intensification of agricultural practices and the increase of area under agricultural production, which was accompanied by a destruction of perennial habitats, made agriculture to one of the main causers of biodiversity losses. Decreasing species richness may lead to a loss of important ecosystem functions such as biological pest control, which in turn can have negative impacts on agricultural production. Annual crop fields are predominately habitats for organisms, which spend parts of their life cycle in other, perennial habitats. Though annual arable weed populations outlast with their seedbank, they can also benefit from the seed rain from the surrounding landscape. Thus, it is not only important to support the survival conditions within the fields (e.g., by extensive management like organic farming), but also consider the structure of the landscape. In this study, the relative importance of farming system (organic vs. conventional) and landscape complexity on species richness of arable weeds, species richness and activity densities of carabid beetles and ground-dwelling spiders, and for biocontrol of cereal aphids by parasitoid wasps was analysed. In addition, the role of arable weeds in cereal aphid-natural enemy interactions was analysed using a field experiment.The majority of investigations was conducted in altogether 72 winter wheat fields in the vicinity of Göttingen (one organic and one conventional field in 12 landscape sectors per year). Because the analysis of land-use intensity revealed heterogeneous relations with landscape complexity, in the following studies, study sites were selected trying to keep parameters as field size, pesticide use and use of mineral fertilizers as constant as possible. The regional diversity of arable weed species was strongly determined by the heterogeneity among fields. At the field scale, organic farming generated higher weed species diversity, but conventional farming reached similar diversity levels when the surrounding landscape was complex. Species richness and activity density of carabid beetles did not differ between organic and conventional fields, but increased with increasing landscape complexity. Species richness of spiders was also enhanced by landscape complexity, while organic farming supported high activity densities. Complex landscapes were related to high mortality of cereal aphids caused by parasitism, but also to higher aphid colonisation, counterbalancing possible biological control and leading to similar aphid densities across landscapes. Although aphids colonised organic fields in lower densities than conventional fields, aphid densities were similar at the time of wheat milk ripening. Parasitism did not differ between the farming systems. Further, analyses at multiple spatial scales revealed that aphids respond to landscape complexity at larger spatial scales than their parasitoids, reflecting limited dispersal ability of these higher trophic level organisms. With the help of a field experiment, we could show that Metopolophium dirhodum colonised weedless wheat plots in four times higher densities than plots containing arable weeds with 20% vegetation cover. Abundance of weeds, especially with 20% vegetation cover, enhanced aphid predators. Accordingly, densities of the released Sitobion avenae and also of na turally occurring aphids were lowest in this treatment (500% lower than in the control), thereby falling below the threshold level of economic damage.Results show that organic farming as well as complex landscapes may support species richness. However, organic farming unexpectedly did not influence parasitism of cereal aphids, whereas landscape complexity enhanced parasitoid, but also aphid populations. Abundance of arable weeds increased aphid predators and decreased aphid populations. Consequently, to conserve and support species richness and ecological functions in agricultural landscapes, we suggest to enhance the area of organically managed arable land, especially in structurally simple landscapes. Further, near-natural habitats such as grasslands, fallows and hedges should be maintained and renewed, respectively, because landscape complexity was shown to be of major importance for biodiversity and ecosystem functioning.