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Consequences of ants and extrafloral nectar for a pollinating seed-consuming mutualism: ant satiation, floral distraction or plant defense?

Authors


J. N. Holland, Dept of Ecology and Evolutionary Biology, Rice Univ., 6100 S. Main St., Houston, TX 77005, USA. E-mail: jhollandiii@gmail.com

Abstract

Non-pollinating consumers of floral resources, especially ants, can disrupt pollination and plant reproductive processes. As an alternative food resource to flowers, extrafloral nectar (EFN) may distract and satiate ants from flowers, thereby reducing their antagonistic effects on plants. Yet, EFN may actually attract and increase ant density on plants, thus increasing the disruption of pollination and/or their defense of plants. In this study, we tested the effects of ants and EFN on pollinating seed-consuming interactions between senita cacti and senita moths in the Sonoran Desert. Prior study of senita showed that EFN can distract ants from flowers, but consequences for plant–pollinator interactions remain unstudied. In our current study, ant exclusion had no effect on pollination or oviposition when moths were abundant (>85% flower visitation). Yet, in an ant by EFN factorial experiment under lower moth abundance (<40% visitation), there was a significant effect of ant exclusion (but not EFN or an ant × EFN) on pollination and oviposition. In contrast with our predictions, ant presence increased rather than decreased pollination (and oviposition) by moths, indicating a beneficial effect of ants on plant reproduction. While ant density on plants showed a saturating response to continuous experimental variation in EFN, in support of ant satiation and distraction, the probability of pollination and oviposition increased and saturated with ant density, again showing a beneficial effect of ants on plant reproduction. Ants showed no significant effect on fruit set, fruit survival, or fruit production of oviposited flowers in the ant exclusion experiment. Ants did not affect the survival of moth larvae, but there was a marginally significant effect of ants in reducing wasp parasitism of moths. We suggest that EFN may not only distract ants from disrupting plant–pollinator interactions, but they may also enhance plant–pollinator interactions by increasing pollination and reducing wasp parasitism. Though often considered dichotomous hypotheses, ant distraction and plant defense may be synergistic, though the mechanism(s) for such positive ant effects on plant–pollinator interactions needs further study.

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