Ethological and mechanical isolation promotes pollinator partitioning in sympatric Andean Salvia species

Abstract

Plants that co-occur and co-flower may suffer negative fitness consequences through pollinator sharing. However, if they have evolved specific adaptations such as being pollinated by only a subset of the available pollinator spectrum, they may co-exist without losing species integrity. Pollinator partitioning can be achieved by exhibiting specific floral traits, which promote the attraction of certain floral visitors (ethological isolation) and/or by specific flower proportions, which either only fit to part of the possible visitors or contribute to pollinator sharing without pollen mixing (mechanical isolation). In this study, we aimed to identify the floral traits and suite of pollinators associated with the reproductive isolation of three co-occurring and co-flowering Salvia species (S. cruikshanksii, S. tafallae, S. tubiflora) from the high elevation shrubland in the central Peruvian Andes. We investigated flower morphology, spectral reflectance (colour), and scent chemistry, as well as pollinator behaviour and breeding system. Salvia cruikshanksii and S. tafallae are adapted for bee pollination, whereas S. tubiflora is adapted to hummingbirds. Accordingly, all floral traits of S. tubiflora are distinct from those of the other two Salvia species. Although S. cruikshanksii and S. tafallae both attract bees, they do not share a single pollinator species. Reproductive isolation might be predominantly based on species-specific odour bouquets reinforced by visual signals (ethological isolation), and on differences in floral proportions between the Salvia species such as flower tube lengths (mechanical isolation). The three Salvia species demonstrate the significance of diversified floral traits in a species assemblage causing pollinator partitioning and allowing co-existence