Direct and indirect pathways mediating the impacts of non-native rats on cocoa yield in an African island

Abstract

Islands are typically much more sensitive to the impact of invasive non-native species than continental areas, exacerbating conservation and socio-economic challenges. In São Tomé Island, Central-West Africa, non-native rats (Rattus rattus and R. norvegicus) have been blamed losses of up to 40 % of the cocoa yield. To understand the impact of non-native rats on cocoa yield on the island, we examine its drivers using both direct and indirect pathways. We sampled 30 cocoa farm sites, assessing rat abundance using camera-trapping, and estimating both cocoa yield and the percentage of rat-damaged cocoa by counting the number of intact and damaged pods in 25 × 25 m-plots. We also estimated the abundance of potential rat predators and characterized local habitat structure, landscape, and cocoa management. We obtained 466 records of non-native rats (15.5 ± 17.1 records per site), which damaged 18.81 % ± 21.02 % of the cocoa pods, reducing the yield to 430.4 ± 253.4 pods per site. Using path analysis, we detected that cocoa yield is directly boosted by mean cocoa trees diameter but diminished by rat damage. Such damage was positively associated with rat abundance, which in turn was negatively affected by the frequency of weeding. By demonstrating that cocoa yield can be enhanced indirectly by increasing the frequency of weeding, and thus suppressing rat abundance, our results indicate that socio-economic challenges caused by invasive species can be reduced by adjusting farm management practices. Identifying such practices has the potential to benefit both biodiversity and livelihoods on tropical agrosystems, and especially on islands.