UC Davis

Globalization and industrial agriculture have led to a severe homogenization of ecological communities. European honey bees are now present on every continent expect Antarctica and are rapidly replacing native bees as plant visitors. Maintaining global crop productivity requires that we support honey bees, but honey bees may be unsuitable replacements for native pollinators, and agricultural intensification should not come at the expense of plant and animal biodiversity. This dissertation examines how increasing honey bee abundance impacts native bees and their interactions with plants. In Chapter 1, I investigate the impacts of increasing honey bee abundance on native bee visitation patterns, native bee pollen diets, and nectar and pollen resource availability in two California landscapes: wildflower plantings in the Central Valley and montane meadows in the Sierra. I find that, in both ecosystems, honey bee competition increases niche overlap between honey bees and native bees, leading to important shifts in the network of interactions between plants and pollinators. In the Sierra, native bees re-shuffle their interactions to escape competition, but honey bee abundance decreases pollen and nectar availability in both systems. This suggests that, although native bees can adapt to competition, increasing honey bee abundance may reduce native bee pollen and nectar collection with negative repercussions for native bee populations when floral resources are limiting. Increasing honey bee abundance and associated shifts in plant-pollinator interaction patterns may also have important functional consequences for plants. However, assessing the overall impact of honey bee introductions on pollination is complicated because abundant honey bees can influence pollination directly, through their own floral visits, but also indirectly, by competitively influencing visits from other pollinators. In Chapter 2, I disentangle the direct and indirect impacts of increasing honey bee abundance on the pollination of an ecologically important wildflower, Camassia quamash. I find compelling evidence that honey bee introductions indirectly decrease pollination by competitively excluding visits from more effective native bees, suggesting that hive introductions in sensitive ecosystems should be approached with extreme caution. Although I focus on just one plant species in Chapter 2, I expect that replacing native bees with honey bees might decrease pollination for other plant species, particularly when honey bees are ineffective pollinators. In Chapter 3 (originally published in the American Journal of Botany), I assess the pollination effectiveness of honey bees compared to other floral visitors using a hierarchical meta-analysis and find that honey bees are less effective than the average bee and rarely the most effective pollinator of plants globally. As such, honey bees may be imperfect substitutes for the loss of wild pollinators. Taken together, my dissertation demonstrates that increasing honey bee abundance may erode longstanding plant-pollinator mutualisms with negative consequences for plant reproduction and native bee floral resource collection. However, negative impacts for native bees will depend on whether their population growth is limited by floral resource availability and negative impacts for plant species will depend on the relative effectiveness of honey bees as pollinators. Improving our understanding of when, where, and how honey bee competition negatively impacts native bee and plant populations will be crucial for calibrating the balance between biodiversity conservation and agricultural production.