Invasive plant serves as an important complementary food resource for diverse pollinators in anthropogenic environments at the end of the flowering season

Senecio inaequidens is one of Europe’s fastest plant invaders, which forms large flowering stands in various anthropogenic habitats. This plant is nectar- and pollen-rich and flowers from late May until the beginning of winter, thus providing food sources as a competitive advantage for pollinators that adapt to its presence. However, it also poses a threat in the form of the production of pyrrolizidine alkaloids that are contained in the pollen and nectar. We asked the following questions: 1) Is the model invasive species used as a food resource for pollinators? 2) How do the spatio-temporal patterns of invasive species’ integration into plant-pollinator communities affect this system? 3) Do the floral toxins of the invasive model species cause a negative impact on the fitness of pollinators? We provide the first direct evidence that invasive Senecio inaequidens and native Jacobaea vulgaris serve as complementary food resources for a broad spectrum of flower visitors, including hoverflies, honey bees, bumblebees, and solitary bees. Their abundance and species richness are particularly high if the surrounding anthropogenic landscape provides sufficient nesting resources and other complementary food sources. Other plant species outcompete the two Senecioneae spp. during their flowering period. The pollinators switched to and from Senecioneae spp., corresponding to the beginnings or ends of flowering periods of other abundantly present food resources. Feeding larvae of the polylectic Osmia species with pollen enriched with extracts from Senecioneae spp. inflorescences led to significant decreases in metamorphosis success rates but did not affect body dimensions or symmetry. Therefore, the invasive autumn-flowering plant Senecio inaequidens is an important complementary food resource for diverse pollinators in anthropogenic environments.