Recent plant-pollinator network studies have revealed high pollinator sharing among plant species potentially leading to increased levels of pollen transfer among them (heterospecific pollen transfer). Evaluation of pollen transfer in wild plant communities has in fact revealed heterospecific pollen receipt is common, yet experimental hand-pollinations have revealed high among-species variation in the magnitude of its effect on recipient species (0-80% reduction in seed set). To date the causes underlying the variability of these effects are unknown. It has been proposed that the magnitude of heterospecific pollen effects may be determined by individual characteristics of pollen donor and recipient species. Here, we further propose that heterospecific pollen effects can be mediated by the specific combination of donor and recipient characteristics, a likely but so far untested prediction. Thus, we conducted a hand-pollination experiment with 10 co-flowering species to determine if the effects of heterospecific pollen receipt are mediated by the pollen donor or recipient species alone, or if the effects are determined by the interaction between them. We further assessed species traits potentially mediating interactive effects in heterospecific pollen receipt by evaluating the relationship between heterospecific pollen effect size and three different predictors reflecting a unique combination of pollen donor and recipient characteristics (degree of flowering overlap, phylogenetic distance and pollen-stigma size ratio).
Results/Conclusions
Heterospecific pollen effect size varied between -1.1 and 0.39 among donor-recipient species pairs. Furthermore, our results show, for the first time, that variation in the magnitude of the heterospecific pollen receipt effect is significantly determined by the specific combination of donor and recipient species (pollen donor by recipient species interaction; F15, 303 = 1.85, P =0.02). However, we were unable to uncover the specific combination of traits mediating these effects. Nonetheless, our study provides strong evidence suggesting that an understanding of heterospecific pollen receipt effects based on recipient or donor characteristics alone may be insufficient. This study is an important step towards a complete understanding of the ecological and evolutionary consequences of heterospecific pollen receipt in co-flowering communities.