Local management and landscape context effects on bee pollination, ant seed predation, and yield in Indonesian homegardens

Tropical countries contribute substantially to global agricultural production, but the majority of farmers are small-scale subsistence farmers. Their comparably low agricultural productivity is coupled with major yield gaps. Agricultural expansion and the intensive use of agro-chemicals are the major cause for the destruction of tropical habitats and biodiversity loss and pose a threat to ecosystem services. However, many tropical small-holder farmers rely on ecosystem services like insect pollination or natural pest control, which may play an indispensable role in closing yield gaps. In addition, losses from pollination deficits or pest pressure are usually mitigated by the use of managed pollinators or pesticides that in turn can be harmful to ecosystem services provided by wild insects. We aim to shed light on the relative importance of pollination services, common management practices and their interaction. We evaluated the benefits from pollination services for cucumber (Cucumis sativus) production and how wild bees (the main cucumber pollinators in the region) are influenced by management on four different spatial scales. We furthermore assessed income loss due to dis-services from seed predating ants in order to suggest management measures that may reduce yield gaps. Our work comprises experimental field studies on small-scale vegetable production in homegardens in tropical central Sulawesi, Indonesia and a review on pollination services to highlight existing information and to close information gaps on pollinations services for crops. In the first study (chapter 2) we tested how different management practices (insect pollination, weed control, fertilization and herbivore control) affect cucumber fruit set and yield and how these variables influence each other. We found that insect pollination, fertilization and weed control increased crop fruit set and yield in an additive way. However, fertilization and weed control alone could not compensate for pollination loss, which was the most important driver and accounted for 75 % of the yield. We found an interaction between the weed control and pollination treatment in which weed control strongly influenced insect-pollinated plants but not wind and self-pollinated plants. This indicates that weed control contributes to closing yield gaps, but only in addition to insect pollination. In contrast, insecticides to control herbivores did not influence yield. We recommend shifting the focus from common management practices towards more sustainable management to enhance pollination services and stress the importance for policy driven regulations of reduced and better targeted pesticide application in tropical agroecosystems. In the second study (chapter 3), we evaluated variables from three spatial scales to better understand bee communities indispensable for cucumber production. We further assessed if the response to these scale predictors depends on pollinator body size and sociality. Yield increased significantly with increasing number of flower-visiting bee individuals (mainly composed of wild solitary bees which translates into a net income decline of 47% if half of the bees would be lost. For optimized bee management, farmers need to consider four spatial scales. On the garden scale (1), the homegarden-wide percentage of flower cover predicted pollinator attraction best, if, on the adjacent-habitat scale (2), a higher percentage of homegardens (at least 20%, best 50% in a 200m radius) surrounded the study garden. In addition, the landscape scale (3), distance to the rainforest (up to 2.2 km) had also a significant effect on total bee flower visitor. This effect was dominated by small bees which increased closer to the forest. We conclude that farmers need to adjust bee management accordingly to reduce major yield gaps. High percentages of crops and non-crop plants flowering inside the homegarden can attract pollinators from adjacent source habitats that are mainly homegardens as well. In the third study (chapter 4), we aimed to assess the effect of seed predation by ants on sown seeds of four crop species (C. sativus, Daucus carota, Capsicum frutescens and Solanum melongena) and the resulting impact on the net income of small-scale farmers. Furthermore, we tested if ant seed predation differs with or without insecticide and herbicide applications. We found that ant seed predation was high for all crops (42.0 %, 49.4 %, 48.0 % and 50.6 % for C. sativus, D. carota, C. frutescens and S. melongena, respectively), potentially reducing farmers’ net income by half. Application of insecticides and herbicides did not influence ant seed predation or total ant abundance, but influenced ant species-specific abundances positively or negatively. Despite ant species-specific responses to insecticide and herbicide applications, we found consistently high seed predation rates across all gardens. We conclude that high seed predation is caused by high overall ant abundance mediated through functional redundancy of ant species. We recommend more environmentally friendly and sustainable practices such as overseeding or seedling production in nurseries to reduce chemical pest control. In the fourth study (chapter 5), we reviewed pollination services and their importance to crops focusing on another understudied geographical region - the Neotropics - to get an overview of the main pollinator taxa and the dependence on pollination services of crops. In addition, we summarized pollination research methodologies and discuss pollination relevant farm and landscape management, as well as socio-economic drivers affecting pollination services. We have shown that pollination services by wild pollinators are important for crop production. However, knowledge gaps exist in terms of the quantity, quality and stability of crop production provided by animal pollinators. It is also critical to understand how multiple socioeconomic drivers influence the selection of particular management systems and, thus, the environmental services delivered. In conclusion, pollination services are of major importance for closing yield gaps in tropical small-scale agriculture, such as homegardens. Although common management practices such as the use of agro-chemicals influence yield, they do not compensate yield gaps due to pollinator loss or ant seed predation. The enhancement of pollination services should be considered as well as the reduction of pesticide use in the majority of tropical agroecosystems. Farmers should adapt environmentally friendly and more sustainable practices adjusted to bee management considering four spatial scales to reduce major yield gaps