Roadsides, ditches, and railway lines in Norway are awash with colour right now. The lupines are in bloom, and the dense swathes of purple, pink, and white blossoms stacked into perfect pillars brighten the countryside. I love the vibrant colours, but have to stop to remind myself that these are not the friendly wildflowers of my Canadian childhood.
The big-leaved lupine is native to western Canada, but in Norway and the rest of Europe it is an introduced invader. In many European countries, lupines have escaped from household gardens where they were originally planted for their vivid blossoms. Once on the loose, they rapidly colonize disturbed habitats like roadsides where they are particularly well adapted to spread and thrive. What makes them so well adapted to spread? Nutrient poor soils in marginal habitats are less of a barrier to lupines than most native plants because of its two specialized mechanisms for acquiring essential nutrients like nitrogen, phosphorous and potassium. Symbiotic bacteria occur in specialized nodules on lupine roots where they convert atmospheric nitrogen into forms the plant can use. In addition, the roots themselves release massive amounts of carboxylic acid into the soil, facilitating the uptake of phosphorous. Their efficient nutrient scavenging actually allows lupines to enrich the soil they colonize. The plants are biennial, surviving only two years and shedding all of their leaves in winter, introducing new, nutrient rich organic material into the soils, which gradually makes them more amenable for other species to live in too.
However once established the effects of this invader are mixed. Despite enriching soils and making habitats more suitable for other species, lupines do not share well with others. The overall number of plant species in areas invaded by lupines actually decreases as the quick-growing, tall lupines shade later-emerging, shorter native plants and outcompete them. Lupines also produce a wide variety of alkaloid compounds in their leaves that can persist in soils and are thought to negatively impact the germination of seeds from other species of plants. By preventing seeds from germinating around them, the lupines further decrease the biodiversity in areas they invade. And it’s not just other plants that can be negatively impacted by lupine invasions. In Finland, lupines have a bottom-up effect on the ecosystem, decreasing the number and diversity of moths and butterflies occurring in invaded areas too, as they are a poor food source for these insects. Nevertheless, not all species suffer from the presence of these invaders. With their large concentrations of showy flowers, lupines can act as a ‘magnet species’, attracting and sustaining large populations of bumble bees, which can then increase the number of pollinator visits to co-occurring native plants with potential knock-on effects on their reproductive success. Still, as beautiful as they may be, the lupines out my window in Norway are perturbing the ecosystem, and would be better left at home in Canada!