Who pollinates exotic plants? A global assessment across native and exotic ranges

The spread of exotic plant species is a global issue with substantial ecological and economic costs. One factor that can contribute to their capacity to establish and spread beyond their native range is their ability to attract novel pollinators (i.e., pollination flexibility). Humans have contributed to the spread of plant species across different continents, where they may encounter completely different pollinator families or even orders. While interest in exotic plants has grown considerably in recent years, we lack a global assessment of the common pollinator taxa that visit exotic plants, as well as comparisons of pollinator similarity across native and exotic ranges. This systematic review explores the diversity of floral visitors observed at exotic plants worldwide while also assessing whether these plants tend to attract native or exotic animals. We further quantified the taxonomic similarity of floral visitors in the native versus exotic range. Additionally, we tested the hypothesis that floral visitor similarity is negatively correlated with geographic distance. The data compiled in this review show that exotic plants exhibit pollination flexibility at multiple levels. Most floral visitors were bees, but at least 964 animal species (94 families, 12 orders) were found to visit the 201 exotic plant species (59 families, 29 orders) compiled in this review. Such visitors are typically generalists that forage from diverse plant species. Moreover, exotic plants attract both native and exotic floral visitors, however, the proportion of native visits was found to vary by geographic region (highest in Europe, moderate in North America, and lowest in Oceania). Exotic plants generally attract taxonomically similar floral visitors throughout their global distribution, but can attract novel visitors in regions far from their native range, as predicted. This review synthesizes the currently available data and improves our understanding of plant-pollinator interactions at exotic plants, which is important for restricting the spread of invasive plant species and mitigating their negative effects on native biodiversity worldwide.