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The intensification of agricultural production in recent decades is widely recognised to have contributed substantially to global declines in biodiversity and associated ecosystem services, such as natural pest control and pollination. Agroforestry systems, where trees and/or shrubs are integrated into agricultural fields or landscapes, have the potential to increase biodiversity and associated ecosystem services. This thesis therefore aims to evaluate how agroforestry systems affect invertebrate pests, their natural enemies, and pollinators, in addition to productivity and farm income, relative to arable monocultures. A review and meta-analysis of the literature revealed that arthropod pests are significantly suppressed, and natural enemies significantly enhanced, in agroforestry systems relative to arable monocultures. However, the results were equivocal with high heterogeneity. Empirical data collected from three agroforestry sites with paired arable controls confirmed higher levels of plant and invertebrate biodiversity in agroforestry systems, and also revealed that the agroforestry systems led to a change in plant and invertebrate communities. These changes could be explained in terms of life-history traits, for example, plant communities in agroforestry systems were more perennial while invertebrates were less likely to be winged. Functional trait diversity of natural enemies was significantly higher in the agroforestry systems, indicating a higher level of biological control. Furthermore, species-level pollinator data from the same sites revealed that additional bee species in agroforestry contributed to functional trait diversity through niche complementarity. To further explore causes of heterogeneity, understorey management was manipulated at one agroforestry site, and was found to significantly affect natural enemy abundance and diversity, aphid suppression, and pollinator visitation. Although arable yields were up to 11% lower in agroforestry than arable systems, financial modelling predicted that agroforestry systems were capable of increasing farm income after at least seven years. Agroforestry systems therefore represent a viable option to restore farmland biodiversity and improve agricultural sustainability.

Agroforestry systems, which incorporate trees into agricultural land, could contribute to sustainable agricultural intensification as they have been shown to increase land productivity, biodiversity and some regulating ecosystem services. However, the effect of temperate agroforestry systems on pest control and pollination services has not been comprehensively reviewed, despite the importance of these services for sustainable intensification. We review and analyse the available evidence for silvoarable agroforestry systems, following which we propose a predictive framework for future research to explain the observed variation in results, based on ecological theory and evidence from analogous systems. Of the 12 studies included in our meta-analysis of natural enemies and pests, the observed increases in natural enemy abundance (+24%) and decreases in arthropod herbivore/pest abundance (-25%) in silvoarable systems were both significant, but molluscan pests were more abundant in silvoarable systems in the two available studies. Only three studies reported effects on pollinators, but all found higher abundance in silvoarable compared with arable systems. Measures of pest control or pollination service are scarce, but suggest stronger effect sizes. Our framework seeks to establish hypotheses for future research through an interpretation of our findings in the context of the wider literature, including landscape characteristics, silvoarable system design and management, system maturity, trophic interactions and experimental design. Our findings suggest that silvoarable systems can contribute to sustainable intensification by enhancing beneficial invertebrates and suppressing arthropod pests compared with arable, but future research should include measures of pest control and pollination and implications for productivity and economic value.

Silvoarable systems have the potential to be an effective and productive form of sustainable agriculture, in part due to the enhancement of biodiversity and associated ecosystem services. However, currently there is limited understanding of how higher biodiversity in silvoarable systems promotes ecosystem services, such as pest regulation, pollination and nutrient cycling (for example, see Peng et al. 1993; Thevathasan and Gordon 2004; Varah et al. 2013), versus ecosystem disservices, such as encouraging certain pests and weeds (Griffiths et al. 1998; Burgess et al. 2003), and, furthermore, how this cost-benefit ratio might change with how the system is designed, managed and matures over time (but see Burgess et al 2003; Stamps et al. 2009).\ud This paper reports on preliminary results of a cost effectiveness analysis based on the FarmSAFE model (Graves et al. 2011; 2016), as part of a PhD investigating how management of silvoarable influences biodiversity-derived ecosystem services, and their economic implications. Our study is focussed on silvoarable systems in the UK that combine top-fruit production with arable alley-cropping, which are emerging as a promising design with limited shade effects (Smith et al. 2016). We compare our findings to a monocropped arable system, with and without purported associated biodiversity benefits (Varah et al. 2013, 2015).