Rare endemic plant species face elevated extinction risk, in part due to narrow niches and often low reproductive output. Invasive species are one of the leading drivers of habitat loss and degradation for these rare plants. In addition, species invasions may alter community dynamics and affect critical interactions such as rare endemic plant-pollinator relationships. However, little is known about the reproductive biology and pollinator cohorts for many rare plants. Moreover, invasive species can have variable and context-specific impacts on plant-pollinator relationships. For example, invasive plants may compete for pollinators and increase the likelihood that other species receive heterospecific pollen. Impacts are expected to be greatest for species that overlap with the invasive plant in both flowering time and pollinator cohort. Clarifying whether these overlaps exist is important for setting conservation priorities for at-risk plant species, and for informing management actions that address the factors that threaten rare plant-pollinator relationships. In this study we investigate the pollination ecology of a rare and threatened plant endemic to southwestern Idaho, Astragalus mulfordiae, in the context of a widespread co-flowering invasive species, Sisymbrium altissimum. Our study sites fall along an urban-rural gradient and are representative of the range of A. mulfordiae habitats. We first fill knowledge gaps about A. mulfordiae’s dependence on pollinators and likely pollinator cohort through a pollinator exclusion experiment and insect observations. We then evaluate the overlap in insect visitors between A. mulfordiae and S. altissimum using visitation networks composed of 276 visitors. Our results show A. mulfordiae to be highly dependent on pollinators for reproduction; individual flowers had only a 2.1% chance of producing fruit when excluded from insects. Of the 155 insect visitors collected from A. mulfordiae, the majority were solitary bees from the genus Osmia. We found low overlap between the insects that visit A. mulfordiae and S. altissimum, and that across sites, insect visitors to S. altissimum had greater taxonomic diversity. These results highlight that spatial and temporal overlap in flowering does not necessarily imply overlap in insect visitation between an invasive species and a rare plant. They further suggest the need to consider additional influences on A. mulfordiae pollinator relationships, such as disturbances affecting the ground and stem nesting substrates commonly used by Osmia bees.
