Effects of floral metal accumulation on floral visitor communities: Introducing the elemental filter hypothesis

• Premise of the study: For plant species that occur in heavy‐metal‐rich soil, floral metal accumulation may produce an “elemental filter” that reduces pollinator visitation rate and species richness and changes pollinator species composition relative to closely related species growing on normal soils. Consequently, metal hyperaccumulation may contribute to pollinator‐mediated reproductive isolation between closely related plant species that differ in metal accumulation. • Methods: To test these ideas, we characterized plant–pollinator interactions in a sympatric pair of species that differ in metal accumulation ( Streptanthus polygaloides , a nickel (Ni) hyperaccumulator, and S. tortuosus , a nonaccumulator). To test the elemental filter hypothesis, we presented arrays of S. polygaloides that were grown in either Ni‐treated or control soils to insects at both S. polygaloides and S. tortuosus sites and recorded visitation. • Key results: Naturally occurring S. polygaloides hyperaccumulated Ni in anthers and accumulated Ni in nectar, while S. tortuosus did not. Floral visitation rates in natural populations were higher to S. tortuosus than S. polygaloides . In addition, while floral visitor richness was similar, few pollinator taxa were shared between the two plant species. Nickel‐treatment of S. polygaloides reduced visits by bees, but only for arrays presented at S. tortuosus sites. • Conclusions: We show that the Ni hyperaccumulator S. polygaloides hosts a distinct floral visitor community, indicating that metal accumulation creates a filter for pollinators, similar to that documented for herbivores. Our study highlights a novel mechanism by which the abiotic environment can alter plant–pollinator interactions, and consequently plant reproduction and speciation.