Patterns of pollen quantity and quality limitation of pre‐zygotic reproduction in Mimulus guttatus vary with co‐flowering community context

Pollen quantity limitation has been widely recognized as one of the main causes of plant reproductive failure in nature. However, its negative effects on fruit and seed production have been often confounded with those of low pollen quality (i.e. low conspecific pollen viability and/or slow pollen tube growth rate). The lack of differentiation between these two aspects of pollen limitation has resulted not only in a potential overestimation of the incidence of pollen quantity limitation but has led to a poor understanding of the factors and mechanisms that affect pollen quality limitation. Knowledge of the relative importance and underlying causes of both aspects of pollen limitation (quantity and quality) is required to fully understand the ecological and evolutionary consequences of pollen limitation in natural populations. Co‐flowering community context (e.g. species richness and conspecific density), in particular, can be an important driver of overall pollen limitation. However, how pollen quantity and quality limitation vary separately with the community context has not previously been examined, even though they may vary differently as they arise from different mechanisms, e.g. insect visitation rate versus inbreeding depression. Here we evaluate the effect of co‐flowering diversity and conspecific density on the relative importance of pollen quantity and quality limitation for Mimulus guttatus pre‐zygotic reproduction (i.e. pollen tube success) at serpentine seeps in California over two years. We found overall pollen limitation of pre‐zygotic reproductive success at all seeps regardless of the co‐flowering context. However, plants in high‐diversity/low‐ conspecific density communities were mostly limited by pollen quantity, whereas plants in low‐diversity/high‐conspecific density ones experienced stronger quality limitation – a pattern that was consistent across years. These results not only highlight the importance of conducting comprehensive studies on pollen limitation that evaluate both, its quantity and quality aspects, but are the first to show how their relative contribution to overall pollen limitation can vary with extrinsic factors such as the co‐flowering community context.