Cacao pod yield as a function of pollination, plantation management and landscape context in Western Ghana

Thesis

Theobroma cacao (cacao) is an important commodity that supports the livelihood of Ghanaian smallholder farmers and makes a major contribution to the gross domestic product of Ghana. Unfortunately, cacao production impacts the environment through unsustainable expansion of farmlands into forest land with potential consequences for biodiversity conservation. The underlying motivation for expanding farmland is to increase cacao production. This thesis argues that increasing on-farm productivity by narrowing the existing cacao yield gap can reduce the environmental impact of cacao production in Ghana. The thesis supports this claim in three papers. Paper one identifies an existing yield gap in cacao and explores the potential avenues to effectively narrow the yield gap. The biotic, abiotic and management factors that impact cacao yield are identified, and it is suggested that the current yield gap is more related to tree physiology and pollination limitation. The second paper of the thesis expands on paper one to test whether physiological factors or pollination limitation has the greater impact on cacao yield in the study area. Using empirical data from eight plantations, paper two shows evidence of pollination limitation in the study area. Open pollination is estimated to achieve 43% yield, indicating a 56% pollination limited yield gap at peak growing season in the study area. The paper also identifies the abiotic and landscape factors that drive pollination limitation in the study area. To better understand the pollination biology of cacao, with the goal to address open pollination deficit, paper three evaluates the diversity and abundance of cacao flower visitors. Twelve taxonomic groups of flower visitors are identified. The factors that affect the biodiversity of flower visitors are assessed, and the functional relationships between the flower visiting arthropods are analysed. Thrips and Dipterans are identified as the most abundant cacao flower visitors, most functionally implicated in the arthropod trophic network. The mechanism of cacao pollination by thrips is described for the first time. The thesis concludes by addressing the implication of the study with recommendations to sustain the future of the cocoa sector of Ghana, and the global value-chain.

Authors: Atta-Boateng, A

• DOI: 10.5287/ora-pmmeydym0
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: microarthropod network; multitrophic assemblage; pollination limitation; complementary symbiosis; pollinator networks
• Subjects: Pollination biology; Ecology; Plant ecophysiology; Biodiversity; Agricultural ecology