Environmental mediation of plant-pollinator interactions and decline of the rare prairie endemic Besseya bullii (Plantaginaceae)

Studies in plant reproductive ecology have long recognized that characteristics of the local environment can act as a mediating force on pollinator foraging. This observation has potential management implications for plant conservation as: (1) reproductive success of pollinator-obligate species will be dependent on an environment that supports pollinator activity,and (2) changes in habitat quality due to degradation will likely affect this sensitive plant-pollinator dynamic. To address these concerns, we conducted a study using a rare, self-compatible prairie species, Besseya bullii (Plantaginaceae), which is thought to be declining due to habitat degradation. Data were collected for eight populations and characterized by a set of environmental variables: (1) canopy closure, (2) soil nutrient levels, and (3) precipitation. At each population, 10 inflorescences were randomly caged to exclude pollinators,and an additional 10 inflorescences were tagged but allowed access to pollinators. Pollinator visitation was estimated by comparing pollen loads on stigmas from the two treatments. Additionally, morphometric data was collected on inflorescences (e.g., height, floral density) and flowers (e.g., petal lengths, stamen/carpel lengths) to determine morphological differences among sites. After fruit had been produced, infructescences were collected to estimate fruit/seed set. Populations in more open habitats produced inflorescences with significantly greater floral density, which may have been more attractive to pollinators. Consequently, these populations also produced significantly higher fruit/seed set. When excluded from pollinators, all populations produced much lower fruit and seed set, which indicates reproduction in B. bullii is highly dependent on pollinator activity. Moreover, presence of heterospecific pollen in pollen loads collected from more forested habitats shows that low pollinator fidelity in these areas may potentially contribute to pollen-clogging and subsequently lower fruit/seed set. These results show that habitat quality can affect plant-pollinator interactions through floral development (i.e.,quality of floral display) and pollinator availability.