COS 65-3 - Quantifying ecosystem services with a foraging biology perspective: floral resources mediate pollinator behavior and the resulting pollination service cascade

Abstract: Ecosystem services are essential for human well-being, especially in urban areas where an estimated 60% of the world’s population will live by 2030 and where an increasing proportion of food production (currently ~15-20%) takes place. Pollinators mediate critical plant reproductive services in urban agroecosystems, but as mobile foragers, they are also sensitive to local floral resources, which could mediate their service provision. Past work suggests that pollinators can respond to local floral resources by increasing (concentration effect) or decreasing (dilution effect) per-plant visitation in high floral resource settings. Despite these potential contrasting patterns, it is unknown how the pollination service cascade, from pollen deposition, to fruit set, to fruit volume, is impacted by interactions between pollinator communities and local floral resources. Across 24 urban gardens in central California, we investigated potential interactions between the pollinator community (abundance and diversity) and local floral abundance, as well as the impact of non-floral habitat management (garden size, mulch cover, herbaceous plant richness, and number of trees and shrubs), on three response variables in the pollination service cascade: 1) pollen deposition, 2) flower-to-fruit phenological turnover, and 3) fruit volume, for the widely cultivated garden tomato (Solanum lycopersicum).

We found that pollinator abundance and diversity significantly interacted with local floral abundance to positively mediate plant reproduction early in the pollination service cascade, but had more limited impacts on downstream pollination response variables. Specifically, both pollinator diversity and abundance significantly interacted with local floral resource levels to positively impact tomato pollen deposition, indicating a forager concentration effect. Pollinator diversity also significantly interacted with floral resources to positively impact flower-to-fruit phenological turnover. Finally, the number of trees and shrubs within a garden had a negative effect on the volume of tomato fruit produced, revealing a key trade-off between food production and tree density within urban garden systems. The observed forager concentration effects indicate that increasing overall floral abundance within urban garden systems does not reduce focal crop pollination, but in fact enhances it. This demonstrates that crop production benefits from maintaining high density floral cover within urban gardens. Overall, our results show that gardeners can manage local flowering plant communities to enhance pollinator visitation and resulting pollination service within urban agroecosystems.