Context-dependent variation in a plant-pollinator mutualism

Background/Question/Methods In the plant-pollinator mutualism, both species that are involved benefit to some degree. This benefit, however, can vary in strength both spatially and temporally. We examined the relationship between the plant-pollinator interaction and ecological context for two species of invasive thistles, Carduus nutans and C. acanthoides, in pure and mixed constructed arrays of different densities and sizes in the field. We recorded pollinator movements within each array, and considered the fitness consequence of these movements for both plant species. Interactions between invasive species are relatively understudied; thus examining their indirect interactions through their shared pollinators is of particular importance. Results/Conclusions Preliminary results reveal that the visitation behavior of insect pollinators varied with the distribution of flowering plants. Visitation rate to pure C. acanthoides arrays was higher than that to pure C. nutans arrays, with mixed arrays receiving an intermediate rate of visitation. Within arrays, the number of flower heads (display size) of an individual was a much stronger predictor of visitation than the distance between plants. In pure C. acanthoides arrays, pollinators preferred to visit plants with larger displays; however, there was no apparent preference for display in pure C. nutans arrays or the mixed arrays. Instead, in mixed arrays, pollinators preferred to visit C. acanthoides individuals regardless of display size. In mixed arrays, the average duration of a visit to a C. nutans flower head decreased compared to in pure C. nutans arrays. In contrast, visits to C. acanthoides flower heads, the preferred species, were the same in single species and mixed arrays. Thus, when C. acanthoides is present, C. nutans experiences not only reduced visitation but also decreased duration of visits, while C. acanthoides is relatively little affected by the presence of C. nutans. Understanding the change in pollinator foraging behavior with ecological context can shed light on patterns of variation in the strength of a particular plant-pollinator mutualism. For our two invasive thistles, these shifts in pollinator behavior may potentially reduce the fitness of C. nutans in the presence of its congeneric invader C. acanthoides, with repercussions for its invasion success.