COS 85-8
The pollination ecology of floral metal accumulation
The 'elemental defense hypothesis' posits that metal hyperaccumulation by plants is adaptive, in that it reduces damage rate by enemies, such as herbivores and pathogens. While metal hyperaccumulation has been shown to protect plants from enemies, it is unclear whether plant mutualists, including pollinators, are also affected. Furthermore, some insect herbivores are known to specialize on metal hyperaccumulating plants, but pollinators have not been similarly studied. To determine the influence of floral metal accumulation on pollinator attraction and diversity, we surveyed populations of two sympatric species of Streptanthus (Brassicaceae): S. polygaloides, a serpentine-endemic nickel (Ni) hyperaccumulator, and S. tortuosus, a non-accumulating, non-serpentine species. We first documented Ni concentrations in pollinator rewards (anthers and nectar) of plants in situ. We then recorded pollinator diversity and visitation rates. Finding differences in pollinator visitation and diversity between the plant species, we sought to determine if metal accumulation was responsible. To do this, we presented arrays of S. polygaloides that were either grown in Ni-supplemented or control soils to pollinators at both serpentine and non-serpentine sites to determine whether (1) pollinator visitation is reduced by Ni accumulation and (2) pollinator response to Ni-rich flowers varies by background soil context (i.e., serpentine vs. non-serpentine).
Results/Conclusions
We found that S. polygaloides, but not S. tortuosus, accumulated Ni in anthers and nectar. In fact, Ni in anthers reached hyperaccumulation levels. Visitation to flowers in natural populations was higher to S. tortuosus than S. polygaloides. In addition, while pollinator diversity (i.e., the number of taxa) was similar for both species, the composition of the pollinator assemblage differed, with each species hosting numerous unique, unshared insect taxa. For experimental plants, we found that Ni-supplemented S. polygaloides received fewer visits than control plants. In particular, small and large bees displayed decreased visitation to Ni-treated plants relative to controls but only at non-serpentine sites. Our findings show that Ni accumulation by flowers reduces pollinator visitation rate and changes pollinator composition. Furthermore, the distinct pollinator community associated with S. polygaloides, as well as the reduction of visitation by bees only at non-serpentine sites, suggests that some pollinators, similar to herbivores, are tolerant of metal-rich floral resources. These results provide novel insights into the ecology of metal hyperaccumulation, and suggest that metal hyperaccumulation may not only alter plant-animal interactions with plant antagonists, but also mutualists.