Ecological, social and economic determinants in cocoa production systems in southern Cameroon

Abstract

More diversified and species rich systems relate to functional consequences, which in turn will influence ecosystem processes, reducing outbreaks. This has been shown for landscape management practices and for intercropping systems. However, the role of biodiversity in biological control still remains uncertain. I studied the relationship between biodiversity and biological control of cocoa pests and diseases, the social implications and the associated cost-benefit ratios under different management regimes in 17 traditional cocoa forest gardens (TFGs) in southern Cameroon. I hypothesized that increasing biodiversity, measured as plant and ant species richness, will enhance biological control of the main cocoa pests and diseases and will thus contribute to increase in farmer incomes and sustainability of TFGs. The results of this study aim to understand the ecological, social and economic determinants in cocoa production systems in southern Cameroon. In a survey of 400 cocoa farmers, they described a complex situation ranging from biological, institutional to socio-economic production constraints affecting their cocoa-based livelihoods. The survey also revealed that with rising land values, women have created new access and new rights to land. The floristic surveys showed that land-use management as practiced in traditional cocoa forest gardens in southern Cameroon, following a gradient of intensification from extensive cocoa forest gardens with high floristic diversity to intensive ones, strongly impacts plant diversity, above ground plant biomass and to some extend carbon storage, respectively, with possible negative consequences on biodiversity. Significant differences were found associated with the different managements with regard to species richness and abundance of ants. Ecosystem functioning showed a significant responses to shade cover reductions in TFGs for herbivory. Biotic interactions were studied by looking at the population dynamics of mirid bugs: Salhbergella singularis (SS), the interactions between SS and black pod disease caused by Phytophthora megakarya (BPD), predation by ants and interactions between ants and BPD. Based on my surveys, I have developed and recommend a sequential sampling plan based on counts of mirid bugs. I observed that the interaction between the mirid bugs and black pod disease was found to be mutually exclusive in space and time independent of management regimes. These findings release SS from the popular notion of being a vector of BPD. Our results also support the insurance hypothesis and suggest an important role for ants in controlling pest and disease outbreaks in TFGs. Moreover, I provide evidence for the first time that ants, although generally regarded as beneficial, do contribute to the incidence of black pod disease given a low diversity of ant species on cocoa trees. However, if ant communities become more diverse and niche structure more restricted on these trees, incidences of BPD significantly decrease. My findings result in a set of non-linear relationships between yield and ant species richness in TFGs. I provide a direct measurable assessment of the biodiversity-net income relationships in cocoa. My model describes a concave relationship between biodiversity and profitability and highlights the importance of the Management Index (MI) in certification procedures. Economic incentives are required to prevent further intensification of cacao production systems beyond ecologically acceptable shade cover percentages. Finally, my study identifies potential determinants that researchers and policy makers need to focus their attention to, to facilitate the development of coherent innovations in cocoa production.