Evol Ecol Res 7: 435-452 (2005)     Full PDF if your library subscribes.

Ant–plant conflicts and a novel case of castration parasitism in a myrmecophyte

Laurence Gaume,1* Merry Zacharias2 and Renee M. Borges2

1Botanique et bioinformatique de l’architecture des plantes, UMR CNRS 5120, Boulevard de la Lironde – TA 40/PS 2, F-34398 Montpellier, cedex 5, France and  2Centre for Ecological Sciences, Indian Institute of Science, Bangalore 560 012, India

Author to whom all correspondence should be addressed.
e-mail: lgaume@cirad.fr


Questions: Do protective plant-ants perturb the pollination process and the reproduction of their host-plant? If they do, have partner selective mechanisms evolved against such conflicts?

Organisms: The semi-myrmecophyte Humboldtia brunonis and its ant associates.

Field site: Makut Reserve Forest, Western Ghats, South India.

Methods: We tracked insect and extrafloral nectar activity on inflorescences of several trees over a 24 h cycle. We repeatedly measured the extrafloral nectar produced by bracts of flowers throughout their phenology from the bud stage until and beyond flower opening. We studied the behaviour of ants towards the reproductive apparatus of the flowers and conducted ant exclusion experiments to test for any negative effect on herbivores or fruit production.

Conclusions: Pollinators did not visit inflorescences that had more than four ants on them. Ants, solely by their presence on bud bracts and bracteoles, intimidate other insects, both pollinators and herbivores. Some spatial and temporal mechanisms partially prevent negative ant–pollinator interactions. First, extrafloral nectar production on the bracts of flower buds, which attracts ants to inflorescences, was highest at night, attracting the largest numbers of ants at that time, whereas the major pollinators were active during the day. Second, this extrafloral nectar production declines after the first flower of each inflorescence opens. Third, the anthers and stigma are placed at the apex of a thin elongate axis, which offers a precarious foothold to ants. One ant species, Crematogaster dohrni, succeeds despite these difficulties by acting just before the flower opens, and damaging the flower when the style and stamens are still folded. This is the fourth case of castration behaviour of a plant-ant directed against its host-plant. Despite its anti-herbivore protection of flower buds, this plant-ant has a negative impact on fruit production in H. brunonis.

Keywords: ant–plant interactions, ant–pollinator conflict, castration parasitism, Crematogaster dohrni, defence–pollination conflict, extrafloral nectar, Humboldtia brunonis, myrmecophyte, partner selective mechanisms.

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