Clinal variability of oil and nectar rewards in Monttea aphylla (Plantaginaceae): relationships with pollinators and climatic factors in the Monte Desert

Monttea aphylla flowers simultaneously produce oil and nectar, rewards known to differentially attract ecologically and functionally distinct pollinators. We examined whether geographical differentiation occurred for rewards, and whether this could be explained by spatially heterogeneous pollinator guilds and climate. Rewards were measured across the entire species range. Geographical patterns of reward quantity and their relationships with biotic and abiotic factors were examined using uni‐ and multivariate analyses. Latitude significantly explained population variation in nectar and oils, although in contrasting ways. Pollinator assemblages showed a prevalence of oil‐collecting Centris bees. Centris vardyorum, specialized on M. aphylla, showed a latitudinal pattern of visits. Oil production was not higher where plants associated with ecologically specialized bees (i.e. C. vardyorum), but this occurred instead where they were less frequent and co‐occurred with other bees that used many floral sources, including other plant species that produce oil. The multivariate analysis showed that the prevalence of C. vardyorum was the factor that contributed most significantly to the combined patterns of rewards. We suggest that large‐scale variation in rewards involves local optima throughout the species range, related to processes that operate in each ecoregion with their particular biotic and abiotic scenarios. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178 , 314–328.