Agricultural biodiversity and associated services across rural-urban landscapes

Urbanization is a primary threat to biodiversity and the functioning of both natural and agro-ecosystems. Especially in the Global South, expanding cities increasingly encroach fertile agricultural lands, questioning the viability of maintaining agricultural activities in urbanized landscapes. Yet, the growing interest in urban agriculture may offer an opportunity to improve the sustainability of cities. Indeed, urban agriculture often consists in small-scale wildlife-friendly farming, and thus can provide resources and habitats for various organisms, while locally producing food within and around cities. However, the effects of urbanization on agricultural biodiversity and the delivery of ecosystem services upon which urban smallholders depend such as biological pest control and crop pollination are little understood, especially in global urbanization hot spots from the tropics. This thesis is part of a larger interdisciplinary and collaborative research project between Germany and India that aims to investigate how urbanization affects urban agriculture. This project takes place in Bengaluru, an emerging megacity in South India, which exemplifies key characteristics of urbanization and serves as an in situ laboratory to assess their effects on agro-ecological and socio-economic attributes of urban agriculture. Within the project, this thesis focuses on agricultural biodiversity and associated services across rural-urban landscapes. The thesis is divided into four chapters. The first three chapters investigate how urbanization drives the spatial and temporal dynamics of service-providing organisms, namely birds and bees. Going one-step further, the fourth chapter, as a case study, looks at how urbanization affects the delivery of pollination service to mango, one of the most important tropical fruit crops. Chapter 1 reports on the drivers of taxonomic and functional beta-diversity of farmland birds along an urbanization gradient. This chapter demonstrates that urbanization acts as an environmental filter and homogenizes farmland bird communities, and discusses the potential implications for ecosystem services. Chapter 2 focuses on the functional responses of farmland bees to urbanization, and how urban farm characteristics can modulate these responses. It emphasizes how some functional groups of bees thrive in urban landscapes while others suffer, and how results from studies conducted in tropical countries do not necessarily align with that of studies from temperate regions. It also highlights how certain urban agricultural practices, such as crop diversification or flowering field margins, can promote bee communities. Chapter 3 provides novel insights on an understudied topic, namely the effects of urbanization on spatial and temporal dynamics of plant-pollinator interaction networks. The results show that urbanization alters the spatiotemporal dynamics of plant-pollinator networks, by amplifying the seasonal turnover of their interactions. This study demonstrates that environmental, spatial, and temporal gradients interact to shape the dynamics of plant-pollinator networks. Finally, Chapter 4 serves as an example on how the direct and indirect effects of urbanization and farm management influence the delivery of ecosystem services, using mango production (Mangifera indica) as a case study. The results indicate that mango production can be maintained at a profitable level in urbanized landscapes with insect pollinators more than tripling final yield. However, insecticides applications had negative effects on insect pollinators, in turn reducing mango yield. This suggests a trade-off between conventional pest control and mango pollination. In conclusion, this thesis displays the variable effects of urbanization on the diversity of different animal groups and related ecosystem services. Whereas farmland bird communities homogenize with growing urban areas, the response of farmland bees varies largely among taxonomic and functional groups. This thesis also emphasizes when the findings align or differ between studies conducted in tropical vs temperate regions, and highlights that generalizations drawn from studies conducted in one region do not necessarily apply to the other. Further, our result suggests that urban farming can be maintained at a profitable level, at least in low intensity level urbanized landscapes. Finally, this work suggests that small-scale urban agriculture, if managed in a sustainable way, might be a promising strategy to reconcile biodiversity conservation and food production within and around cities, thus increasing their sustainability.

2024-07-25