Recipient and donor characteristics govern the hierarchical structure of heterospecific pollen competition networks

1. Pollinator sharing can have negative consequences for plant fitness via competition for visits as well as with the arrival of heterospecific pollen. Plant traits and relatedness of donor and recipient species have been suggested to drive the observed variation in plant fitness effects of both processes, but how they shape the structure of interspecific pollen competition networks has been overlooked at the community level.
2. To understand the importance of reproductive traits and relatedness to the impact of heterospecific pollen, we conducted a controlled glasshouse experiment with an artificial co-flowering community. We performed 2,200 hand pollination crosses by experimentally transferring conspecific pollen alone or with 50% and 100% foreign pollen among 10 species belonging to three different plant families.
3. Relative to conspecific pollen alone, there was a significant reduction in seed set with 50% heterospecific pollen for 67% of the crosses. This effect is driven largely by recipient traits and the interaction between recipient and donor traits under specific circumstances of trait matching. In general, species with shorter styles, smaller stigmas and lower pollen:ovule ratios were more impacted by heterospecific pollen. These traits and their differences among species led to a hierarchical (or transitive) structure of pollen competition with clear winners and losers. However, phylogenetic distance among recipient and donor species did not explain the effects.
4. Synthesis. Our study shows that specific traits and trait combinations between donor and recipient species are important in determining seed production outcomes with heterospecific pollen receipt. Moreover, the differences in traits between species lead to a competitive structure with clear 'winners' or 'losers' species. The results of this study highlight the importance of specific traits in understanding the position of the competitive hierarchy of the species and the mechanisms underlying heterospecific pollen impacts upon plant reproductive success.