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Experimental Removal of an Introduced Pollinator Reduces Reproductive Success of California Native Clustered Tarweed


Honey bees (Apis mellifera), introduced worldwide by humans, are often a numerically dominant pollinator in non-managed ecosystems, but surprisingly few experimental studies have examined the effect of honey bee visitation on wild plant reproduction. I experimentally removed honey bees from plots of clustered tarweed (Deinandra fasciculata: Madiinae), a native annual forb, to measure the contribution of Apis visitation to tarweed seed set. While removal of Apis did reduce seed set, their much higher rate of visitation suggests that honey bees contribute modestly to seed set compared to non-Apis pollinators. Visits by non-Apis visitors significantly increased when Apis was removed, indicating possible competition betwee Apis and other insects. In ecosystems where honey bees become numerically dominant, they can contribute a significant proportion of visits to native plants. Apis removal may negatively affect plant reproduction, especially during years in which native pollinators are relatively uncommon because of lack of floral resources, but it may also release native pollinators from competitive displacement.

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