Local adaptation to hummingbirds and bees in Salvia stachydifolia: insights into pollinator shifts in a Southern Andean sage

Abstract

Background and Aims Differences among populations in pollinator assemblages can lead to local adaptation mosaics in which plants evolve different floral morphologies and attractive traits. Mountain habitats may promote local adaptation because of differences in environmental conditions along the altitude, causing changes in pollinators, and because mountaintops can act as isolated habitats. We studied if the differences in floral shape, size and nectar traits in Salvia stachydifolia can be attributed to variations in the relative contribution of hummingbirds and insects. Methods We studied eight populations of S. stachydifolia in natural and under common garden conditions, to assess whether population differences have a genetic component. We recorded pollinators, their behaviour and visitation rates, and characterised pollinator assemblages. In addition, we measured nectar volume and concentration, and collected flowers to describe floral shape and size variation using geometric morphometric methods. Then, we applied an unsupervised learning algorithm to identify ecotypes based on morphometric traits. Finally, we explored whether populations with different pollinator assemblages had different climatic and/or elevation preferences. Key Results We found that variation in the identity of the main pollinators was associated with differences among populations in all traits, as expected under a local adaptation scenario. These differences persisted in the common garden, suggesting that they were not due to phenotypic plasticity. We found S. stachydifolia populations were pollinated either by bees, hummingbirds or with mixed pollination. We identify two ecotypes that correspond to the identity of the main pollinator guilds, without climatic or altitudinal correspondence. Conclusions Variation in S. stachydifolia floral traits did not follow any evident association with bioclimatic factors, suggesting that populations may have diverged as the product of historical isolation on mountaintops. We suggest that differences among populations pointed to incipient speciation and an ongoing pollinator shift.