Why ant pollination is rare: new evidence and implications of the antibiotic hypothesis
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The antibiotic hypothesis proposes that ant pollination is rare at least in part because the cuticular antimicrobial secretions of ants are toxic to pollen grains. We tested this hypothesis by comparing the effects of ants and bees on pollen in two regions: a tropical rainforest in Amazonian Peru and temperate forests and old fields in Canada. We found support for three predictions that follow from the antibiotic hypothesis. (1) For all 10 ant and 11 plant species in our study, contact with ants significantly reduced pollen germination, confirming the generality of this effect. (2) Contact with two bee species did not have similar effects; pollen exposed to bees germinated as well as control pollen. (3) Consistent with the presumed greater abundance of entomopathogens in the tropics, which may have selected for stronger antibiotic secretions in tropical ants, tropical ants had more negative effects on pollen than temperate ants. We speculate that the antibiotic hypothesis contributes not only to the rarity but also to the biogeography of ant pollination, and we discuss whether the negative effects of ants on pollen have resulted in selection for floral defenses against ants.
KeywordsAnt–plant interactions Cuticular antimicrobial secretions Disease resistance Insect immunity Hymenoptera Tropical rainforests
We thank Lina Arcila Hernández, Adam Cembrowski, Antonio Coral, Eddie Ho, Gabriel Miller, and Jon Sanders for field/laboratory assistance; Frank Azorsa Salazar, Laurence Packer, and Claus Rasmussen for identifying our Peruvian ants and bee; Takashi Makino, Alison Parker, and James Thomson for their comments and for supplying the B. impatiens; the residents of Port Whitby for allowing us to collect flowers; the directors and staff at CICRA/the Amazon Conservation Association and the Koffler Scientific Reserve at Jokers Hill for logistics and permission to work at these sites; and the Peruvian Ministry of Agriculture for issuing a research permit (No. 299-2011-AG-DGFFS-DGEFFS). We are also grateful to the AE and two anonymous reviewers for their comments on an earlier draft. A Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant to MEF funded this research; EMD was also supported by an NSERC Undergraduate Student Research Award.
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