Competition for pollination has served as a model for the integration of ecological and evolutionary perspectives in the study of species interactions. Its study has elucidated both obvious and more subtle mechanisms, and has documented a range of outcomes. However, the potential for this interaction to inform our understanding of both pure and applied aspects of pollination biology has only begun to be realized.

Invasive species are frequently regarded as superlative competitors that can vegetatively crowd out natives, but little is known about whether invasives can compete for pollination services with native plants. We hypothesized that, when the showy invasive species Lythrum salicaria (purple loosestrife) was present, pollinator visitation and seed set would be reduced in a native congener, L. alatum (winged loosestrife). To test this hypothesis, we constructed mixed and monospecific plots of the two species. Over two years of study, we found that L. salicaria significantly reduced both pollinator visitation and seed set in L. alatum. Furthermore, pollinators moved frequently between the two plant species, which may cause heterospecific pollen transfer. Thus, reductions in both pollen quantity and pollen quality may reduce L. alatum seed set. If similar patterns occur in the field, invasive plants may be an even greater threat to natives than previously thought.

Competition for pollination can be an important factor in plant reproduction, but little attention has been given to the effect of the growing number of invasive plant species on pollination of native species. As a first step in understanding this threat, we used hand pollination to investigate the effects of pollen from an invasive species (Lythrum salicaria) on seed set in a sympatric and co-flowering native congener (L. alatum). Dispersal of fluorescent dyes in the field confirms that pollinators (bumble bees and honey bees) transfer pollen between species. To determine the potential effect of such interspecific pollen transfer on seed set of the native, we pollinated 773 flowers on 20 plants with one of three treatments: legitimate conspecific pollen, a mixture of conspecific and foreign pollen, and foreign pollen. The mixed-pollen treatment resulted in 28.8% lower seed set relative to conspecific pollination. Foreign crosses resulted in extremely low seed set. Observations of pollen germination indicate that events at the stigmatic surface contribute to the reduction in seed set for mixed pollination. Our results indicate that the impacts of invasive species may extend beyond vegetative competition to include competition for pollination.

Invasive species are frequently regarded as superlative competitors that can vegetatively crowd out natives, but little is known about whether invasives can compete for pollination services with native plants. We hypothesized that, when the showy invasive species Lythrum salicaria (purple loosestrife) was present, pollinator visitation and seed set would be reduced in a native congener, L. alatum (winged loosestrife). To test this hypothesis, we constructed mixed and monospecific plots of the two species. Over two years of study, we found that L. salicaria significantly reduced both pollinator visitation and seed set in L. alatum. Furthermore, pollinators moved frequently between the two plant species, which may cause heterospecific pollen transfer. Thus, reductions in both pollen quantity and pollen quality may reduce L. alatum seed set. If similar patterns occur in the field, invasive plants may be an even greater threat to natives than previously thought.

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