Artichoke thistle Cynara cardunculus L. (Asteraceae) pollination: Flower density effects and impacts on native species

This research investigated hypotheses regarding the pollination biology of Artichoke thistle (Cynara cardunculus), a non-native, invasive weed of central and southern California. Recent studies demonstrated negative impacts of non-native species on the pollination and reproductive success of co-flowering native species. I investigated the influence of C. cardunculus floral density on pollinator abundance, visits per flower head, and seed set and in addition, examined whether it affects pollinator visits to co-flowering native species. I found no significant difference in pollinator abundance or visits per flower head between dense and sparse thistle plots. These results suggest that even small populations of C. cardunculus may be 'hot spots' of floral resource for pollinators and could be important potential sources of population expansion. A flower removal experiment determined there were no significant differences in pollinator abundance or visits per flower per hour to native plants in the presence or absence of C. cardunculus in 2002. Dry conditions in 2002 influenced both the presence and number of pollinators in this study.