Summary 1.Habitat fragmentation can affect pollinator and plant population structure in terms of species composition, abundance, area covered and density of flowering plants. This, in turn, may affect pollinator visitation frequency, pollen deposition, seed set and plant fitness. 2. A reduction in the quantity of flower visits can be coupled with a reduction in the quality of pollination service and hence the plants' overall reproductive success and long-term survival. Understanding the relationship between plant population size and ⁄ or isolation and pollination limitation is of fundamental importance for plant conservation. 3. We examined flower visitation and seed set of 10 different plant species from five European countries to investigate the general effects of plant populations size and density, both within (patch level) and between populations (population level), on seed set and pollination limitation. 4. We found evidence that the effects of area and density of flowering plant assemblages were generally more pronounced at the patch level than at the population level. We also found that patch and population level together influenced flower visitation and seed set, and the latter increased with increasing patch area and density, but this effect was only apparent in small populations. 5. Synthesis. By using an extensive pan-European data set on flower visitation and seed set we have identified a general pattern in the interplay between the attractiveness of flowering plant patches for pollinators and density dependence of flower visitation, and also a strong plant species-specific response to habitat fragmentation effects. This can guide efforts to conserve plant-pollinator interactions, ecosystem functioning and plant fitness in fragmented habitats.

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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.