UC San Diego

Non-native plant species can disrupt plant–pollinator interactions by altering the foraging behavior of pollinators and by contributing to the transfer of interspecific pollen to native plant species. In this study I investigate these potential effects in fennel (Foeniculum vulgare), which has become common throughout coastal California since its introduction from Europe in the 1800s. Fieldwork for this study took place on Santa Cruz Island (Santa Barbara County, CA) where fennel has become locally abundant following the removal of non-native sheep and pigs. I conducted a controlled and replicated fennel-flower removal experiment to test three hypotheses: (i) fennel shares pollinators with native plants, (ii) fennel flower removal alters visitation by insect pollinators to flowers of native plants, and (iii) fennel flower removal decreases the transfer of fennel pollen to flowers of native plants. The native plant considered in this study was the Santa Cruz Island buckwheat (Eriogonum arborescens), which is endemic to Anacapa, Santa Cruz and Santa Rosa Islands. Overlap in the insects visiting both plant species was relatively low, but buckwheat and fennel did share certain pollinators (e.g., Augochlorella pomoniella, Colletes sp., Hylaeus sp. and V. pennsylvanica). Native green sweat bees preferentially visited buckwheat, whereas yellowjacket wasps (Vespula pensylvanica) preferentially visited fennel. After the experimental removal of fennel flowers, insect visitation to buckwheat flowers was significantly greater in the control group than in the fennel flower removal group, but fennel pollen transfer was unaffected by the removal of fennel flowers. These results indicate that fennel acts as a magnet species for certain pollinating insects and that this attraction spills over to the native Santa Cruz Island buckwheat. These results also indicate that insects readily transfer fennel pollen beyond the 13 x 13 m area of our study plots. Future research could focus on the extent to which fennel pollen deposition on native plant stigmas hinders plant reproduction.