Nectar robbing: a common phenomenon mainly determined by accessibility constraints, nectar volume and density of energy rewards

Nectar robbers use a hole made in the perianth to extract nectar. Since robbers may modify plant fitness, they play an important role by driving evolution on floral traits, shaping population structure and influencing community dynamics. Although nectar robbing is widespread in angiosperms, the causes and ecological implications of this behaviour on large ecological scales are still unexplored. Our aim is to study the frequency of nectar robbing in plants of temperate and tropical regions and examine its association with plant traits. We characterised the levels of nectar robbing in 88 species of Mediterranean, Alpine, Antillean and Andean plant communities and identified the most important nectar robbers. We analysed associations between the levels of robbing and floral morphology, production and density of energy rewards, mechanisms of protection against nectar robbers, plant life form and geographic origin. Nectar robbing was present at all sampling sites. Within communities two patterns of robbing levels related to the diversity and specialization of robbers were detected. In most communities one plant species presented very high levels of robbing while other species had intermediate to low robbing levels. There, nectar robbers are opportunists, robbing highly rewarding plants. In the Andean community the high specialization of several co-existing flowerpiercers produced an even pattern of robbing levels in the plant community. Plants with long flowers, abundant nectar and a high energy density are more likely to be robbed by both insects and birds. A high aggregation of the flowers within the plants and the presence of long calyxes and bracts are associated to low robbing rates by insects and to a lesser extent by birds. Besides the morphological constraints that operate on a single flower basis, nectar robbing is a phenomenon dependent upon the density of energy rewards reflecting the presence of mechanisms on higher ecological scales.