Ant-pollinator interactions in Turnera velutina: ecological costs and evolutionary consequences for the ant-plant pollination

Abstract

Ant-plants recruit ants to defend them against herbivores, but most of them also require pollinator for successful seed set. Interactions between patrolling ants and pollinators on ant-plants have received relatively little attention. Negative ant-pollinator interactions are expected for several reasons. First, ants and pollinators benefit from plant investment in different functions (defence and reproduction, respectively) which can lead to plant trade-offs. Second, although more aggressive ants are likely to be better defenders, they may also deter pollinators, affecting plant fitness. However, ant-plants may have mechanisms to manage ant-pollinator interactions, maximising the benefits from their services whilst minimising the costs. I used field experiments to investigate the ecological costs and evolutionary consequences of ant patrolling for the pollination biology of a facultative ant-plant, the Mexican endemic Turnera velutina (Passifloraceae), addressing the following questions: a) Which are the most aggressive and best ant defenders of T. velutina? b) Is there direct (pollinator deterrence) or indirect (nectar trade-offs) ant-pollinator conflict? c) What are the ecological costs and evolutionary consequences of myrmecophily for the host plant pollination and mating system? d) Do ant-plants have adaptations to cope with both mutualists, avoiding conflict? Answers: a) Cephalotes sp. ants was detected as parasitic non-defenders, and the remaining ant species were ranked as: Capmponotus planatus < Crematogaster sp. < Paratrechina longicornis < Brachymyrmex sp. < Dorymyrmex bicolor. b) I found evidence for direct but not for indirect conflict. c) Ant patrolling affected pollinator visit duration, pollen loads, out-crossing rates, and male fitness, leading to negative effects on pollinator foraging efficiency, but such changes had positive effects for plant fitness increasing outcrossing and male fitness. d) Extrafloral nectar also serves to bribe ants away from reproductive structures during the crucial pollination period, reducing the probability of ant-occupation of flowers, reducing ant-pollinator conflict, and increasing plant reproductive success.