Trait-based effects of plant invasion on floral resources, hoverflies and bees

1. Plant invasions can lead to homogeneous communities with decreased functional diversity. However, invasive plants, with various morphological and phenological traits, may drive pollinator communities in a less predictable, more complex way. They can promote pollinators compatible with their floral traits while leaving others without foraging resources.

2. Our observational study on 10 invasive herbaceous species applied a trait-based approach to investigate plant invasion-driven changes in floral resources, hoverfly, and bee communities. We sampled invaded and non-invaded (control) sites before and during the flowering of the invasive plants. We analysed the differences in floral traits between invasive and native plants, functional diversity, and trait distributions of flower and pollinator communities between the invaded and control sites.

3. Five invasive plant species differed from natives in floral traits. Plant invasion caused species-specific changes in functional diversity and trait distributions of communities. For instance, invaded sites had a decreased functional diversity of hoverflies before flowering of invasive species, and larger hoverflies during flowering of invasive species compared with control sites. Smaller bees were associated with invasive plants with shallow flowers, while larger and long-tongued bees were associated with two invasive species with restricted floral access.

4. Similar to previous studies, pollinator traits showed mixed or neutral responses to plant invasion. This is probably due to the high integration capability of invasive plants into plant-pollinator systems, or limitations in sampling, trait resolution, and unrevealed environmental factors. We provide recommendations for future studies to better understand the trait-based community composition of flowering plants and pollinators.