Small-size bees reduce male fitness of the flowers of Ononis masquillierii (Fabaceae), a rare endemic plant in the northern Apennines

Most plant species are visited by a diverse array of visitors that may contribute differently to reproductive success. In particular, the size of visitors, relative to flower size, may have severe implications for plant fitness, as it can affect the mechanics of the pollination process. Ononis masquillierii, a rare endemic legume in northern Italy, is visited by numerous bee species of different families and sizes. To evaluate the effect of bee guilds on seed set, we compared the relative efficiency of four size classes of bees visiting flowers in eight plant populations occurring in the Apennines. Size classes were assorted by body weight. Reproductive success was positively related to plant population size and affected by the visitation frequency of the bee size guilds: higher visitation rates by larger bees (medium-large Andrenidae and large Apidae) increased seed set, whereas high visitation rates by smaller bees (small Halictidae and small-medium Megachilidae) reduced seed set. We conclude that the influence of bee size on seed set is related to the pollen foraging techniques of the different bee guilds. Larger bees paying short visits to flowers remove one 'batch' of pollen and transfer it to another flower. In contrast, smaller bees enter the flower entirely and activate the pump-like pollen dispensing mechanism several times, resulting in a higher degree of selfing. These behaviours may result in differential seed sets as the flowers are self-incompatible, i.e. seed set may be reduced after visitation by smaller bees. Such variations in pollinator quality among populations coupled with the ephemeral nature of the habitat of O. masquillierii may represent a risk for its persistence. Our results show that such a risk can be confronted only through the conservation of large plant populations and of large bees capable of flying long-distances.