Pollination systems: Measuring specialization in the genus Oenothera

Background/Question/Methods Although generalists and specialists are often discussed as alternative states, the biological reality may better be viewed as a continuum of specialization to generalization.Since not all plant visitors are pollinators, calculating the degree of pollination specialization based solely on visitation can also be misleading. Failure to account for pollen load can lead to inaccurate assumptions regarding the number of pollinators with which a plant species actually interacts. The focus of this study was to clarify the degree of specialization using a more rigorous assessment of pollination systems. The genus Oenothera has long been used as a model system for studying reproductive biology, and provides a diversity of pollination systems and an accumulated set of data suitable for addressing the question: How specialized are Oenothera pollination systems? Both visitation rate and pollen load analysis were used to quantify the pollination systems of 19 Oenothera species. For each Oenothera species, I counted the number, type, and behavior of pollinators. Pollinators were collected and pollen load counted. To correct for disparity between frequency and efficacy of pollinators, pollen transfer was be calculated by combining visitation rate with pollen load. The degree of pollinator specialization of a plant species, which I have termed the “S-score,” is defined as the number of pollen-carrying insect taxa that account for 95% of the pollen flow. Results/Conclusions   Results show that 1. visitation alone highly overestimates the number of pollinators (p=.0059) and is therefore inadequate for determining pollination specialization, and 2. the majority of these Oenothera species, when examined in detail, were found to exhibit pollination systems that were distributed across the continuum from highly specialized to generalized. These results are in conflict with previous studies that depict most plant species as having generalized pollination systems.