The presence of a showy invasive plant disrupts pollinator service and reproductive output in native alpine species only at high densities

1. Alien invasive plants possessing attractive flowers can affect the interactions between native plants and their pollinators. The few studies conducted so far have reported positive, negative and neutral effects of the presence of an invasive species on the pollinator visitation rates and seed output of native species. However, the role played by the density of the alien species has been seldom explored. 2. While high densities of the invasive species can negatively effect the pollinator visitation rates and seed output of the native species, due to sequestration of pollinators by the invasive species, at lower densities the invasive can attract pollinators that otherwise would not visit patches of native species, positively affecting their reproduction. 3. Using observations and pollinator exclusions at a site at 2800 m altitude in the central Chilean Andes, we show that the alien herb Taraxacum officinale (Asteraceae), possessing showy floral capitula, shares pollinators to a high degree (> 90%) with two co-occurring native Asteraceae: Hypochaeris thrincioides and Perezia carthamoides. Pollinator exclusion indicated that both natives are highly dependent on insect visitation to produce seeds. 4. Using manipulative supplemental hand-pollination and experiments, where we varied the density of T. officinale, we determined that seed output in H. thrincioides and P. carthamoides is pollen-limited and that its magnitude is not affected by the presence of the exotic. Further, while the presence of one individual of T. officinale around focal native individuals showed neutral or facilitative effects on pollinator service or seed output in the two native species, the presence of five individuals of T. officinale negatively affected these reproductive variables. 5. Synthesis. Our results suggest that the effects of the presence of alien invasive plant species with attractive flowers vary with their density. This demonstrates the need to experimentally test for the potential impacts of introduced species at different densities before prematurely coming to conclusions regarding their assumed negative or positive effects in native ecosystems.