Does temporal variability in floral resources at the landscape scale impact wild bee diversity and watermelon pollination?

The stability of flower resources in agricultural landscapes affects wild bee communities. Mass flowering crops provide monospecific resource pulses for pollinators in rural landscapes, while semi-natural habitats provide biodiverse plant communities and more continuous flower resources. In this study, we hypothesized that organically-managed watermelon fields and those surrounded by landscapes with high cover of semi-natural habitats would: i) provide abundant and stable flower resources, ii) host abundant, diverse and stable wild bee communities, iii) offer a high and stable pollination service for watermelon production. We studied wild bee communities and estimated pollination in 59 organic and conventional watermelon fields of various sizes in north central California at three periods during the watermelon flowering season. Semi-natural habitats and flower resources and their temporal variability were mapped to investigate their effects on the abundance, diversity, and stability of wild bee communities and an estimation of watermelon pollination based on flower visitation. Wild bee abundance and richness were significantly higher in small organic watermelon fields. A high proportion of semi-natural habitats and land cover diversity were both positively linked to wild bee functional diversity. In contrast, the proportion of land cover offering flower resources and its variability were negatively linked to wild bee abundance, presumably due to a dilution effect of mass flowering crops. The stability of watermelon flower pollination was higher in landscapes providing high flower abundance in early spring (March). Our study highlights the importance of early spring as a crucial time window for wild pollinators and stability of watermelon pollination.