Data from: Pollinator interactions with yellow starthistle (Centaurea solstitialis) across urban, agricultural, and natural landscapes

Pollinator-plant relationships are found to be particularly vulnerable to land use change. Yet despite extensive research in agricultural and natural systems, less attention has focused on these interactions in neighboring urban areas and its impact on pollination services. We investigated pollinator-plant interactions in a peri-urban landscape on the outskirts of the San Francisco Bay Area, California, where urban, agricultural, and natural land use types interface. We made standardized observations of floral visitation and measured seed set of yellow starthistle (Centaurea solstitialis), a common grassland invasive, to test the hypotheses that increasing urbanization decreases 1) rates of bee visitation, 2) viable seed set, and 3) the efficiency of pollination (relationship between bee visitation and seed set). We unexpectedly found that bee visitation was highest in urban and agricultural land use contexts, but in contrast, seed set rates in these human-altered landscapes were lower than in natural sites. An explanation for the discrepancy between floral visitation and seed set is that higher plant diversity in urban and agricultural areas, as a result of more introduced species, decreases pollinator efficiency. If these patterns are consistent across other plant species, the novel plant communities created in these managed landscapes and the generalist bee species that are favored by human-altered environments will reduce pollination services.传粉者-植物（Pollinator-plant）关系被证实尤其易受土地利用变化的影响。尽管目前针对农业与自然生态系统已开展了大量研究，但针对邻近城市区域的这类互作关系及其对传粉服务的影响，相关关注仍较为匮乏。本研究于美国加利福尼亚州旧金山湾区近郊的城郊景观中开展，该区域兼具城市、农业与自然土地利用类型。我们通过标准化的访花观察，以及对常见入侵草地植物黄星蓟（Centaurea solstitialis）的结实量进行测定，以此检验以下三项假说：随着城市化程度提升，1）蜜蜂访花频率会降低，2）可育结实量会下降，3）传粉效率（即蜜蜂访花与结实量之间的相关性）会降低。我们意外发现，城市与农业土地利用场景中的蜜蜂访花频率最高，但与之相反，这些受人类活动干扰的景观中的结实率却低于自然生境。造成访花频率与结实率之间这种不一致现象的原因可能是：由于引入物种更多，城市与农业区域的植物多样性更高，从而降低了传粉者的传粉效率。若这些模式在其他植物物种中也成立，那么这些受管理景观中形成的新型植物群落，以及受人类活动干扰环境青睐的广食性蜜蜂物种，将会削弱区域传粉服务能力。