The negative impacts of non-native species have been well documented, but some non-natives can play a positive role in native ecosystems. One way that non-native plants can positively interact with native butterflies is by provisioning nectar. Relatively little is known about the role of phenology in determining native butterfly visitation to non-native plants for nectar, yet flowering time directly controls nectar availability. Here, we investigate the phenological patterns of flowering by native and non-native plants and nectar foraging by native butterflies in an oak savanna on Vancouver Island, British Columbia, Canada. We also test whether native butterflies select nectar sources in proportion to their availability. We found that non-native plants were well integrated into butterfly nectar diets (83% of foraging observations) and that visitation to non-natives increased later in the season when native plants were no longer flowering. We also found that butterflies selected non-native flowers more often than expected based on their availability, suggesting that these plants represent a potentially valuable resource. Our study shows that non-native species have the potential to drive key species interactions in seasonal ecosystems. Management regimes focused on eradicating non-native species may need to re-consider their aims and evaluate resources that non-natives provide.

Data was collected from 10 sites of Garry oak savanna in Victoria, British Columbia, Canada that were visited in rotation from May to August 2019. At the start of each visit to a site, two observers recorded butterfly nectar foraging for 60–90 minutes by walking a 300 m long route. When foraging occurred, the species of butterfly and flowering plant were recorded. Next, we estimated flower availability from all flowering forbs and shrubs either on the same or following day as the butterfly surveys by placing a 1 m² habitat quadrat at five random locations along the same 300 m route. Within each quadrat, we counted the number of open flowering units for each plant species. We also estimated the mean floral surface area for each flowering plant species observed across the season by measuring the dimensions of 10-20 individuals per species. Thus, flower availability was calculated by multiplying the number of flowering units counted with their mean floral surface area for each species during each visit to a site.