The golden threat— <i>Solidago</i> invasion alters native plant–pollinator interactions by vegetative crowding

Abstract Pollination, a vital biological process, faces potential disruption from invading alien plants. Here, we investigate whether the abundance of an invader enhances or hinders native plants' pollination success. Using a functional trait‐based approach, we examine how floral traits and pollinator behaviour mediate the effects of a major invader of Eurasian wetlands ( Solidago canadensis L.) on native pollination. To study the impacts of our invader, we conducted plant–pollinator surveys along a gradient of invasion for an entire flowering season (720 transects in total), in a protected wetland of the French Alps. We then used structural equation models to (i) determine whether the impacts of S. canadensis on the pollination of native plants depend on their floral traits and (ii) whether these impacts are due to changes in insect behaviour, the insect species pool and/or the plant species pool. We specifically distinguished the effect of vegetative density from that of floral density of S. canadensis. Our findings reveal cumulative effects of the invader on three main components of native plant pollination success: altered flower production, insect visits and insect fidelity. All these effects tend to benefit native plants with flowers similar to those of Solidago and to cost plants with flowers that are dissimilar. Unexpectedly, the vegetative density of Solidago , rather than its floral density, primarily drives these differences in impact between flower types. Synthesis . Our results highlight the importance of 3D vegetation structure in pollinator foraging choices and advocate for its incorporation into the sampling design of pollination studies.