So many visitors and so few pollinators: variation in insect frequency and effectiveness governs the reproductive success of an endemic milkwort

Plant–pollinator interactions are one of the most important and variable mutualisms having major implications for plant fitness. The present study evaluates the interactions between an endemic milk- wort, Polygala vayredae, and its floral visitors by studying the temporal variability, foraging behaviour and effectiveness of floral visitors in three populations during three consecutive years. The flowers were visited by a diverse array of insects, totalling 24 different species. However, only four species were effective pollinators, depositing pollen on stigmas after one visit, while the remaining species behaved as nectar robbers, secondary nectar robbers or nectar thieves and were completely ineffective for pollina- tion. Among the effective pollinators, two groups with distinct foraging behaviours were observed: the nectar collecting long-tongued bees Bombus pascuorum and Anthophora sp. and the pollen collectors Eucera longicornis and Halictus sp. No significant differences were observed among pollinators in their efficiency in pollen deposition on stigmas, but significant differ- ences were observed in the foraging behaviour between nectarivorous and pollen collectors. Varia- tion in the abundance and assemblage of floral visitors was observed at the temporal scale and among populations, with the effective pollinators being generally scarce. Consequently, the reproductive out- come in this species was low and significantly variable among populations and years. The results highlight the importance of studying floral visitor effectiveness when determining pollinator assemblages. Authors thank the Departamento de Medi Ambient of Generalitat de Cataluña and the Consorsi d’Alta Garrotxa for allowing this research and to Parc Natural de la Zona Volcànica de la Garrotxa for all the support during field seasons. Authors are grateful to Irina Stanescu for the assistance during 2006 field season and to José M. Gómez for the invaluable help with data analysis and comments during manuscript preparation. Authors are also grateful to the Associate Editor and two anonymous reviewers for the helpful comments to the manuscript. The Portuguese Foundation for Science and Technology financed the work of Sílvia Castro (FCT/BD/10901/2002 and FCT/BPD/41200/2007). Victoria Ferrero was funded by a Xunta de Galicia postdoctoral contract. The work was partially financed under the grants from the Xunta de Galicia (INCITE09-3103009PR), CGL200910466 of the Spanish Dirección General de Investigación, Ciencia y Tecnologíca (DGICYT) and FEDER funds from the European Union to Luis Navarro. published