Pollen limitation is prevalent in many species, and can be especially worrisome in fragmented landscapes. Reproduction in the purple coneflower, Echinacea angustifolia (Asteraceae), which grows in small remnant patches of tallgrass prairie in North America, is pollen limited, but not pollinator limited. 

Pollen limitation of Echinacea increases with isolation of individual plants, decreases with size of population, and has a strong negative impact on reproduction. However, pollinator visitation does not explain the reduced reproductive success. Wagenius and Lyon (2010), found that plants in the densest and largest populations of Echinacea receive fewer pollinator visits yet have greater reproductive success than plants in small isolated populations, which receive more visits but have decreased reproductive success. Among the many prairie plants pollinated by native solitary generalist bees, interspecific co-flowering plants may either enhance pollination by attracting more pollinators, or reduce reproductive success through competition or interference with compatible pollen receipt (Feldman 2008, Mitchell et al 2009).

Results/Conclusions

We investigated the community of co-flowering species surrounding a plant (floral neighborhood). We measured the effect of floral neighborhood on pollinator visitation, pollinator diversity, and pollination success of Echinacea. We randomly selected 8 focal plants at each of 10 remnant prairie sites in Douglas Co., Minnesota and observed and collected insect pollinators four times during summer 2009. We also identified and counted inflorescences of nearby co-flowering species. 

We found strong evidence that floral neighborhood composition influences pollinator visitation of Echinacea. Forty species co-flowered with Echinacea, nine of which are invasive. Co-flowering species richness ranged from 14 in the largest remnant to five in the smallest. Alfalfa, Medicago sativa (Fabaceae), the most abundant exotic co-flowering species, occurred at seven of the sites, while leadplant, Amorpha canescens (Fabaceae), the most abundant native species (besides Echinacea), was only found at three remnants. The presence of alfalfa within a focal plant’s floral neighborhood increased the probability of a pollinator visit by 7% (according to a glm with binomial response p<0.03). In contrast, native leadplant decreased pollinator visits by 9% (p<0.02). There is no evidence that alfalfa, leadplant, and Echinacea interact in their effect on pollinator visits. We also collected pollen from Echinacea insect pollinators and flower parts to see if patterns in floral neighborhood composition and pollinator visitation are reflected in pollen loads on pollinators or stigmas.