Are native nectar robbers against the alien? Effects of floral larceny on the reproductive success of the invasive yellow bells (Tecoma stans, Bignoniaceae)

Abstract

Floral larceny is expected to impact negatively the female fitness of alien plant species; however, the effects of nectar robbers are context-dependent and sometimes neutral. In this study, the effect of nectar robbers on the female fitness of the ornamental and invasive yellow bells, Tecoma stans (L.) Juss. ex Kunth (Bignoniaceae), was tested in two typical exotic habitats in northeastern Brazil: urban area and rural land. In order to quantify the intensity of floral larceny and identify the flower-visiting insects on T. stans, we sampled robbery in pre-anthesis flowers, and the frequency of illegitimate and legitimate visits in both sites. We also described the nectar-secreting pattern, and conducted experiments to test the effect of nectar robbing on plant female fitness. Our results indicated that nectar robbers provided neutral rather than negative impact on fruit and number of seeds in T. stans, mediated by the continuous nectar secretion and low natural fruit set. The populations of both habitats differed in the intensity of nectar robbing and pollinator activity. The main nectar robbers were stingless bees (Meliponini, Apidae) and carpenter bees (Xylocopini, Apidae). We found a higher predominance of primary and secondary nectar robbers in the urban area than in the rural environment. In addition, urban T. stans faced up to higher resource theft (nectar and pollen) and lower fruit set. This pattern may not be controlled by nectar production since the secretion pattern was similar between habitats. Nevertheless, T. stans may spread throughout rural environments with more opportunity to interact with pollinators, instead of nectar robbers.