Functional diversity of farmland bees across rural-urban landscapes in a tropical megacity
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.
36 conventional vegetable-producing smallholder farms as study sites, spread along a continuous urbanization gradient.
100m X 2m transect-walk in each farm. Bees were sampled with sweep nets.
12 sampling rounds (once per month).