Urbanization is a major driver of biodiversity change but how it interacts with spatial and temporal gradients to influence the dynamics of plant-pollinator networks is poorly understood, especially in tropical urbanization hotspots. Here, we analysed the drivers of environmental, spatial and temporal turnover of plant-pollinator interactions (interaction β-diversity) along an urbanization gradient in Bengaluru, a South Indian megacity. The compositional turnover of plant-pollinator interactions differed more between seasons and with local urbanization intensity than with spatial distance, suggesting that seasonality and environmental filtering were more important than dispersal limitation for explaining plant-pollinator interaction β-diversity. Furthermore, urbanization amplified the seasonal dynamics of plant-pollinator interactions, with stronger temporal turnover in urban compared to rural sites, driven by greater turnover of native non-crop plant species (not managed by people). Our study demonstrates that environmental, spatial and temporal gradients interact to shape the dynamics of plant-pollinator networks and urbanization can strongly amplify these dynamics.

Urbanization alters the spatiotemporal dynamics of plant-pollinator networks in a tropical megacity.

How urbanization affects crop pollination has scarcely been studied, especially in the tropics. Here, we focus on the richness and abundance of wild bees and their pollination services to 30 small-scale fields of Lablab purpureus, a globally wide-spread grain legume, in the Indian megacity Bangalore. Farms were selected along a gradient of urbanization, measured as percentage of impervious surface (grey area) at the landscape scale, ranging from 0% to 30%. We found that the abundance of lablab-visiting wild bees increased with increasing grey proportion on the landscape scale and that, in particular, ground-nesting and large-bodied bees benefitted, as well as Xylocopa bees. The higher availability of forage and open soils in low-density urban areas appeared to enhance bee populations in Bangalore. When pollinating insects were experimentally excluded, lablab plants produced 36% less and 31% lighter fruits. Yet, we did not detect any changes of pollination outcomes along the urbanization gradient, as lablab seem to receive stable pollination services. Finally, we found that the local bee richness was negatively affected by the number of on-field pesticide applications, resulting in 35% fewer species after 3 application rounds. In summary, we conclude that low density urbanization can be beneficial for wild bees in lablab farms, but intensive pesticide use could counteract this positive effect. Large and ground-nesting farmland bees benefited most from urbanization, but more studies on different crops in tropical cities are urgently needed. 

Wild bees benefit from low urbanization levels and suffer from pesticides in a tropical megacity

Abstract Expanding cities increasingly encroach fertile farmlands, questioning the viability of maintaining agriculture within and around them. Yet, our knowledge on how urbanization influences pollinator communities and the provision of pollination services to crops is limited, especially for the urbanization hotspots of the Global South. Mango Mangifera indica is one of the most important fruit crops in tropical countries. To analyse the dependency of mango on its main insect pollinators, and the direct and indirect effects of urbanization and insecticides on pollinator abundance and mango yield, we conducted a pollinator exclusion experiment and sampled flower visitors on 16 mango farms spread across rural–urban landscapes in Bengaluru, a South Indian megacity. We found that allowing flowers access to ants and flying visitors (bees, hoverflies, nonsyrphid flies), dramatically increased mango yield by 350%, highlighting the importance of wild insects for mango pollination. We detected a trend between wild bee abundance and the final fruit set, with an increase of 20% when the number of bees increased from 25 to 125. Urbanization did not directly affect pollinator abundance or mango yield. However, the amount of insecticide applications had strong negative effects on wild bee abundance at low and intermediate levels of urbanization, while it had no effect in highly urbanized areas, presumably because of higher availability of flowering resources. Moreover, the amount of insecticides decreased the weight of harvested mango fruits by almost 30%. This may indicate trade‐offs between conventional pest control and enhanced crop yields through pollination by wild insects in rural areas. Synthesis and applications : Our results indicate that mango production can be maintained at a profitable level in urbanized landscapes with insect pollinators more than tripling final yield. However, increasing use of insecticides, besides raising farmers' expenses, can have negative effects on wild insect pollinators and mango yield, especially in rural areas. To safeguard crucial pollination services, it is therefore critical to conserve and promote wild insect pollinators by minimizing the negative effects of insecticide applications in these areas. 

Direct and indirect effects of urbanization, pesticides and wild insect pollinators on mango yield

Wild Bees Benefit from Low Urbanization Levels and Suffer from Pesticides in a Tropical Megacity

Agricultural intensification effects on spatial growth variability of staple crops in south India

Urbanization is a major threat to biodiversity and food security, as expanding cities, especially in the Global South, increasingly compete with natural and agricultural lands. However, the impact of urban expansion on agricultural biodiversity in tropical regions is overlooked. Here we assessed how urbanization affects the functional response of farmland bees, the most important pollinators for crop production. We sampled bees across three seasons in 36 conventional vegetable-producing farms spread along an urbanization gradient in Bengaluru, an Indian megacity. We investigated how landscape and local environmental drivers affected different functional traits (sociality, nesting behaviour, body size and specialization) and functional diversity (functional dispersion) of bee communities. We found that the functional responses to urbanization were trait specific with more positive than negative effects of grey area (sealed surfaces and buildings) on species richness, functional diversity and abundance of most functional groups. As expected, larger, solitary, cavity-nesting, and surprisingly, specialist bees benefitted from urbanization. In contrast to temperate cities, the abundance of ground-nesters increased in urban areas, presumably because larger patches of bare soil were still available besides roads and buildings. However, overall bee abundance and the abundance of social bees (85% of all bees) decreased with urbanization, threatening crop pollination. Crop diversity promoted taxonomic and functional diversity of bee communities. Locally, flower resources promoted the abundance of all functional groups, and natural vegetation could maintain diverse pollinator communities throughout the year, especially during the non-cropping season. However, exotic plants decreased functional diversity and bee specialization. To safeguard bees and their pollination services in urban farms, we recommend (1) to preserve semi-natural vegetation (hedges) around cropping fields to provide nesting opportunities for above-ground nesters, (2) to promote farm-level crop diversification of beneficial crops (e.g., pulses, vegetables and spices), (3) to maintain native natural vegetation along field-margins, (4) to control and remove invasive exotic plants that disrupt native plant-pollinator interactions. Overall, our results suggest that urban agriculture can maintain functionally diverse bee communities and, if managed in a sustainable manner, can be used to develop win-win solutions for biodiversity conservation of pollinators and food security in and around cities.

Functional diversity of farmland bees across rural-urban landscapes in a tropical megacity.

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Functional diversity of farmland bees across rural-urban landscapes in a tropical megacity.