Reciprocal gender effects of a keystone alpine plant species on other plants, pollinators, and arthropods

The term facilitation generally describes positive interactions between plants, and a common approach in these studies is to identify a dominant plant to structure sampling. Unfortunately, whilst this field has rapidly expanded community ecology, it rarely includes other trophic levels such as insects and pollinators. Here, we combine facilitation, pollination, and reciprocity measures to explore the general hypothesis that sexual dimorphism in a benefactor plant species mediates its impact. The following three predictions were tested and supported using the gynodioecious alpine cushion plant Silene acaulis (L.) Jacq.: (i) that the trait set of a gynodioecious benefactor plant varies between genders; (ii) that dimorphism changes the facilitation of other plants, arthropods, and pollinators; and (iii) that insect selectivity, particularly pollinators, reciprocally impacts the reproductive output of the two genders. Female S. acaulis cushion plants produced significantly more flowers but they were smaller than those of hermaphrodites. Hermaphrodite cushions facilitated other plant species and pollinators more effectively than females, whilst females strongly facilitated more arthropods. Finally, female plants have significantly higher reproductive output as estimated by fruit and seed production, and this was directly related to visitation rate by pollinators. Hence, this study clearly establishes the value of combining some of the common themes of pollination biology such as sexual dimorphism, floral morphology, and measuring reproduction with the study of positive plant–plant interactions.