Does Steeplebush (Spiraea Tomentosa) Facilitate Pollination of Virginia Meadow Beauty (Rhexia Virginica)?

Document Type


Publication Date

Winter 2004


Facilitation through pollinators is a rarely investigated topic. Rhexia virginica (Melastomataceae) is a bumblebee-pollinated, Atlantic coastal plain disjunct herb that is rare in Wisconsin. Spiraea tomentosa (Rosaceae), a native shrub often growing in the same location, blooms at approximately the same time of year, and bumblebees often fly between these species. We tested the hypothesis that the presence of Spiraea increases pollinator visits to Rhexia. We observed sites that varied in their densities of Spiraea inflorescences and Rhexia flowers and recorded bumblebee visits to Rhexia across these sites. Visits to Rhexia increase significantly in the presence of flowering Spiraea. However, bumblebee visits to Rhexia decline with increasing densities of Spiraea inflorescences, suggesting that facilitation is replaced then by competition for pollinators. Facilitation might occur because Rhexia offers only pollen as a reward whereas Spiraea offers nectar, and these are complementary resources for bees.


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