Threats and benefits of invasive alien plant species on pollinators

Invasive alien plant species are usually disliked due to their high pressure on native communities. However, their ecological effects on pollinators are complex: some species provide abundant floral resources, boosting the number of pollinators, while they often disrupt plant-pollinator interactions by outcompeting native plants. Our direct knowledge is mainly based on single-species studies, while understanding the mechanism of these complex ecological interactions needs multi-species field-based approaches. It is also imperative to clarify the pros and cons of invasive plants and drivers of invasion from the perspective of pollinators. We conducted a standard protocol-driven regional study in Central and Eastern Europe, comparing 6-7 invaded and non-invaded sites of 12 herbaceous invasive plant species. We sampled floral resources, bees, and hoverflies before and during the flowering of the invasive plants. We analysed the effects of plant invasion at the invasive plant species level and in combined analyses, and tested whether the life span (perennial vs. annual) and flowering time (early-, middle-, and late-flowering) of invasive plants affect the abundance, species richness, diversity and species composition of native plants and pollinators. The combined analyses showed lower abundance and species richness of flowering plants and pollinators before, and higher abundance of both during the flowering of invasive plants in invaded sites. However, invasive plants had significant species-specific effects. Perennial invasive plants had a stronger negative impact on floral resources and pollinators already before their flowering compared to annuals. Flowering time of invasive plants affected the pollinator guilds differently. We suggest that in certain critical time periods of the year, invasive plants might provide the dominant foraging resources for pollinators in an invaded ecosystem. But, they also often cause significant losses in native floral resources over the year. Instead of simple eradication, careful preparation and consideration might be needed during removal of invasive plants.