Abstract
Ant defense against floral enemies incurs a two-fold ant-pollinator conflict, both via pollinator deterrence and nectar or pollen collection by non-pollinating protective ants. Some ant-plants have physical barriers whereas others produce ant-repellent chemicals to avoid ant visitation to flowers and subsequent pollination interference. Passiflora coccinea is a hummingbird-pollinated myrmecophilous plant in which floral enemy repellence occurs without limiting ant access to open flowers. To test the hypothesis that ant activity is restricted within flowers to prevent contact with anthers, we compared ant defense response between reproductive (anthers and stigmas) and non-reproductive (bracts, corona and perianth) floral structures by combining an observational survey with an experimental approach. A few insect species were found to visit flowers without providing pollination service, mostly pollen-collecting bees and nectar-thieving butterflies landing on petals. Ants always attacked floral visitors that landed on non-reproductive structures, but they never attacked insects visiting reproductive structures as ants never accessed anthers. Our results suggest that the differential ant defense response is an adaptative process to prevent ant-pollinator conflict. The eventual mechanism that regulates this process could be closely linked to the corona of filaments that protects nectar chambers, simultaneously restricting ant access to nectar and pollen.
Highlights
Animal-pollinated plants have to deal with a diverse assemblage of illegitimate floral visitors that consume substantial amounts of flower tissues, alter nectar availability and degrade pollinator attractiveness properties, which may reduce plant reproductive success (Galen 1999; McCall & Irwin 2006; Irwin et al 2010; Moreira et al 2019)
We found significant differences in ant defense response between bracts and perianth (P < 0.001) and between these structures and reproductive organs or corona (P < 0.01 for all pairwise comparisons)
By combining an observational survey with an experimental approach, our study shows compelling novel evidence that floral visitors that land on flower reproductive organs of P. coccinea are never attacked by ants, as they do not ever access these structures
Summary
Animal-pollinated plants have to deal with a diverse assemblage of illegitimate floral visitors (e.g. florivores, nectar thieves) that consume substantial amounts of flower tissues, alter nectar availability and degrade pollinator attractiveness properties, which may reduce plant reproductive success (Galen 1999; McCall & Irwin 2006; Irwin et al 2010; Moreira et al 2019). Plants have developed a set of defense mechanisms and strategies to avoid or, at least, minimize the damage produced by floral enemies Some of these defensive systems have generally evolved to deter herbivores of vegetative tissues, they are commonly and efficiently used to protect reproductive organs, even produced only during the flowering period in some species (Agrawal 1998; Karban & Baldwin 2007; Dutton et al 2016). Indirect defense provides protection against herbivory and, in some cases, exclusively against florivory and nectar-thieving from flowers via mutualistic interaction with natural predators (Heil & McKey 2003; Trager et al 2010; Lortzing et al 2016). Myrmecophilous plants face a two-fold trade-off as ant-mediated protection against floral enemies raises two non-exclusive ant-pollinator conflicts
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