Precipitation‐mediated asymmetric fitness consequences of nectar robbing in distylous Primula florindae

Abstract Flowers with deep corollas are frequently associated with nectar robbing, a behaviour that often reduces pollen transfer and seed production. Robbing scars, however, may further affect plant reproductive success through rainwater entry into corolla tubes. In distylous species, the reciprocal positioning of stigmas and anthers in long‐ and short‐styled morphs may create unequal exposure to incoming water and thereby impose morph‐specific precipitation‐mediated fitness costs. We used 2‐year field manipulations in one focal population of distylous Primula florindae , manipulating nectar robbing in both years and additionally manipulating pollination success in a full‐factorial design in 2024, to investigate how robbing associated with rainwater affects female fitness and interacts with legitimate visitation to shape phenotypic selection on floral traits. We further combined precipitation data, floral trait measurements and behavioural observations of the dominant visitor Bombus kashmirensis to explore the ecological context associated with the observed shift from legitimate visitation to nectar robbing. Our manipulations revealed that robbing significantly reduced seed production independently of pollen limitation. This reduction was stronger in long‐styled plants, and the contrast between morphs was more pronounced in the wetter year, consistent with differential water exposure in pendant flowers: the long‐styled stigma lies below upward‐facing robbing holes, where water may accumulate and submerge it, whereas the short‐styled stigma lies deeper within the corolla tube but spatially above this accumulation zone. Furthermore, legitimate visitation and nectar robbing exerted complex, interactive selective pressures across floral traits, with illegitimate visitation driving accentuated, morph‐specific divergent selection on corolla length. However, neither type of visitation alone, nor both types combined, fully explained the net selection on this trait, suggesting additional selective agents. Observational evidence indicated that robbing onset was associated with elevated pre‐onset rainfall, unusually long corollas and greater foraging efficiency during secondary robbing, although these relationships remain inferential. In summary, our study provides experimental evidence that nectar robbing can turn floral damage into a precipitation‐mediated fitness cost, offering new insights into how nectar robbing affects plant fitness. More broadly, our results suggest that climatic factors can mediate plant–visitor interactions along the mutualism–antagonism continuum and contribute to context‐dependent adaptive trajectories. Read the free Plain Language Summary for this article on the Journal blog.