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

Abstract

Urbanization poses 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 assess 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 behavior, 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 gray 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 benefited 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 beside 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 promotes taxonomic and functional diversity of bee communities. Locally, flower resources promote the abundance of all functional groups, and natural vegetation can maintain diverse pollinator communities throughout the year, especially during the noncropping season. However, exotic plants decrease functional diversity and bee specialization. To safeguard bees and their pollination services in urban farms, we recommend (1) preserving seminatural vegetation (hedges) around cropping fields to provide nesting opportunities for aboveground nesters, (2) promoting farm‐level crop diversification of beneficial crops (e.g., pulses, vegetables, and spices), (3) maintaining native natural vegetation along field margins, and (4) controlling and removing 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, be used to develop win–win solutions for biodiversity conservation of pollinators and food security in and around cities.

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

“…Additionally, pollinators require diverse nesting habitats, which can be scarce in urbanized landscapes. For instance, Marcacci et al, (2022) found that A. florea, the main mango pollinator in our study, declined with urbanization because of the lack of suitable nesting sites such as hedges and bushy vegetation. We thus encourage the creation of patches of native wild flowering plants as well as the preservation of seminatural vegetation within mango farms to provide floral resources and nesting sites, which will eventually benefit pollinators and mango pollination.…”

“…We then calculated the proportion of grey area within a 2000 m radius around the centroid of each mango farm. A radius of 2000 m was found to be the best spatial scale to explain the abundance of pollinators in our study region (Marcacci et al, 2022).…”

“…Data available via the Dryad Digital Repository https://doi. org/10.5061/dryad.rbnzs 7hhc (Marcacci et al, 2023).…”

“…widespread use of pesticides), landscape homogenization, loss of flowering resources and nesting sites to pathogens and invasive plant species (Goulson et al, 2015; Potts et al, 2010), and many of these factors are associated with urbanization (Wagner et al, 2021). For example, urbanized landscapes are highly fragmented and pollinators have to travel long distances to find the floral resources they need, while natural habitats are converted into sealed surfaces (Burdine & McCluney, 2019; Marcacci et al, 2022; Wenzel et al, 2020).…”

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

“…A pixel-wise image classification was done using a deep learning model, that is a multilayer perceptron network (Marcacci et al, 2021). We then calculated the proportion of grey area in the study sites' surroundings within a 500 m radius, corresponding to the spatial scale at which most bee species experience the landscape in our study region (Marcacci, Grass, et al, 2022).…”