Wide climatic niche and adaptive phenology contribute to the spread of the invasive plant Himalayan balsam (Impatiens glandulifera Royle) in Europe

Invasive species are one of the greatest threats to biodiversity. However, our understanding of how these species persist and spread in novel environments remains limited. Specifically, the relative importance of species interactions versus environmental conditions and the role of rapid evolutionary adaptation are not fully understood. Here, we investigate the impact of these factors on the distribution of the invasive Himalayan balsam ( Impatiens glandulifera ). We examined whether the climatic niche of the species is pre-adapted to the environmental conditions in the introduced range through niche modeling. Field surveys were conducted to assess the importance of herbivory and competition, and greenhouse treatments were used to investigate local adaptation. We found that the species has not yet fully occupied the suitable climatic space in its introduced range in Europe. Our results suggest that the species may have experienced enemy release while also facing increased biotic pressure at the northern range edge. We identified adaptive differentiation in flowering time, which enhances reproductive success when plants grow in climates similar to their origin. Our results indicate that Himalayan balsam has rapidly adapted to differences in growing season length in its introduced range, with trait plasticity providing an adaptive advantage. Together, these findings suggest that the species may continue to spread across its introduced range in Europe.