Pollinator Interactions with Yellow Starthistle (Centaurea solstitialis) across Urban, Agricultural, and Natural Landscapes

Urban Farm DB summary

This study investigated pollinator-plant interactions in a periurban landscape on the outskirts of the San Francisco Bay Area, California, where urban, agricultural, and natural land uses interface. Using standardized observations of floral visitation and measurement of seed set in yellow starthistle (Centaurea solstitialis), a common grassland invasive, it tested the hypothesis that increasing urbanization would decrease bee visitation rates, viable seed set, and pollination efficiency (the relationship between bee visitation and seed set). Contrary to expectation, bee visitation was found to be highest in urban and agricultural land use contexts. However, seed set rates in these human-altered landscapes were lower than in natural sites. The study attributes this mismatch between visitation and seed set to higher plant diversity in urban and agricultural areas — driven by more introduced species — which reduces pollinator efficiency. If consistent across other plant species, the novel plant communities and generalist bee species favored by human-altered landscapes would reduce pollination services.