Abstract
While introduced plants often have restricted distributions at high elevations, their impacts may be more extensive if they compete for native pollinators, potentially reducing pollinator services to native biotically pollinated plants. Conversely, introduced biotically pollinated plants might facilitate improved pollinator services to native plants by supporting higher pollinator densities and extending the flowering season. We examined weekly pollinator visitation to native and introduced plants, at five elevations over two flowering seasons on The Remarkables range, south-central South Island, New Zealand. In this area, introduced plants dominate the vegetation at lower elevations but are restricted to disturbed areas above treeline. We tested whether pollinator visitation rates and quantities of introduced pollen on insects or transferred to native flowers differed with elevation and community context, or with the local abundance of introduced flowers regardless of elevation. Introduced biotically pollinated plants produced more flowers and flowered later than most native species and were extensively utilised by native solitary bees. Weekly visitation rates to native flowers were higher in the first half of the flowering season than the second half, and were positively correlated with visitation rates to co-occurring introduced flowers. Introduced flower abundance did not affect visitation to native flowers but did significantly predict the occurrence of introduced pollen on native insects and stigmas of native flowers. Pollen contamination was also higher in the latter part of the flowering season. While an increase in introduced flowers at high elevations may benefit native pollinators by increasing the quantity and duration of floral resources available, the heavy use of introduced flowers by native bees will lead to at least localised stigma contamination, particularly for late flowering species. However, more information is required on foraging ranges of native pollinators and pollen limitation in native species to understand the consequences of introduced species expansion into alpine areas.