Conserving invertebrate biodiversity in agricultural and urban landscapes

Abstract

Agricultural intensification and urbanisation are two of the main drivers contributing to the global decline in biodiversity and ecosystem functioning. Increasing food demands make agricultural intensification inevitable, while urbanisation offers social and economic opportunities that drive people out of rural areas and into cities. The four chapters of this thesis explore these two human-modified landscapes, investigating their impacts on biodiversity and ecosystem functioning. The findings of this thesis offer practical solutions to enhance biodiversity and the ecosystem services in agriculture and urban areas without compromising their core functions.

The first chapter estimates the effects of crop and landscape heterogeneity on agrobiodiversity through a global meta-analysis, covering 6397 fields across 122 studies conducted in Asia, Europe, North and South America. Results consistently reveal positive effects of crop and landscape heterogeneity for plant, invertebrate, vertebrate, pollinator, and predator biodiversity. Importantly, this study shows that the positive effects of these heterogeneity components on invertebrate and vertebrate biodiversity are consistent across both tropical/sub-tropical and temperate agroecosystems, in annual and perennial cropping systems, and at small to large spatial scales. The study indicates the importance of increasing crop and landscape heterogeneity to restore biodiversity within agricultural landscapes, rather than concentrating solely on non-crop elements.

The second chapter evaluates the potential of designing crop fields to suppress crop pests using 52 Southern Chinese farmlands. The study focuses on the abundance of the butterfly, Pieris canidia, a major pest of cruciferous (Brassicaceae) vegetables in Asia. The study findings suggest that increasing crop heterogeneity can effectively suppress P. canidia abundance, offering a promising and cost-effective approach for farmers seeking pest management solutions.

The third chapter estimates the effectiveness of dragonflies and damselflies in controlling mosquito populations. A comprehensive meta-analysis of 31 studies was conducted across 14 countries in Asia, Africa, and North and South America. Results show that dragonfly and damselfly naiads can significantly reduce the larval populations of Aedes, Anopheles, and Culex mosquitoes. These species are commonly found in urban landscapes and serve as vectors for serious diseases such as Chikungunya, Dengue, and Malaria, among others. The study highlights the potential of dragonflies and damselflies as eco-friendly and cost-effective agents for controlling mosquito-borne diseases, particularly in urban landscapes.

The fourth chapter evaluates the ecological suitability of urban road verges for biodiversity conservation, employing causal inference methods to analyse butterfly communities along main roads across the city-state of Singapore. Results indicate that structural complexity and nectar-floral diversity within road verges, along with nearby green spaces, positively influence butterfly diversity. Conversely, traffic density negatively impacts butterfly diversity, while road verge size and plant richness indirectly affects butterfly diversity through increasing nectar-floral diversity. The study findings indicate that small structural adjustments in road verge design and adjacent green spaces can enhance pollinator resources and support insect pollinators in highly urbanised landscapes.