Do exotic plants and flower colour facilitate bumblebee invasion? Insights from citizen science data*

Invasive species are one of the most important drivers of biodiversity loss worldwide. As exotic species invade new ecosystems, they establish ecological interactions with the existing species, which may facilitate their spread. Flower colour is an important trait that influences pollination and pollinator preferences. To determine the role of these variables on the bumblebee preferences, we compared the floral associations of two exotic bumblebee species: Bombus terrestris and B. ruderatus (introduced two decades ago for crop pollination) and the native B. dahlbomii with native and exotic plants in Chile. We analysed 3,019 citizen science records in order to understand bumblebee associations with native and exotic plant species regarding flower colour. We found that the exotic bumblebees were mainly recorded visiting flowers of exotic plants (86% for B. terrestris and 93% for B. ruderatus), while the native B. dahlbomii visited exotic and native plant species in a similar proportion. Also, the exotic bumblebees visited purple and pink flowers more often, while the native bumblebee species frequently visited white, pink, and purple flowers. Although the three species share trichromatic vision, they seem to have different flower colour preferences. The association of B. terrestris and B. ruderatus with exotic non-crop species (i. e., exotic ornamental plants) may be acting as steppingstones from croplands to other habitats, facilitating the bee invasion process. Despite its inherent limitations, citizen science data provide large amounts of valuable information that is difficult to be obtained otherwise, which can be used to improve our understanding of invasion processes, as is the case of exotic Bombus species in southern South America.