Insect-plant symbioses are ubiquitous and widespread, and include many examples of mutualism. Obligate ant-plant mutualisms are a common feature of the tropics and a classic example of mutualism. Ant-mutualists protect the plants against herbivory, and plants provide housing and food rewards for the ants. In ant-plant mutualisms, both partners also interact with other species. Cooperating or cheating within the mutualism may influence interactions with these third-parties, such as herbivores, through antagonistic or other subtle interactions with the ants and plants. This thesis investigates the effect of third-party interactions on the association between the ant-plant, Acacia drepanolobium, and its three obligate ant partners: Crematogaster mimosae, Crematogaster nigriceps and Tetraponera penzigi. Field experiments demonstrated that the identity of the ant species inhabiting the plant has a significant effect on levels of herbivory, both by vertebrates and by phytophagous insects. Aggressive ant inhabitants, particularly C. mimosae and C. nigriceps, are able to limit herbivory to a much greater degree than workers of the timid T. penzigi. Ant-association also has a strong effect on reproduction by the host, but no effect on infestation by seed-predators and parasitoids. Field experiments further indicated that the highly effective ant inhabitants, C. mimosae and C. nigriceps are able to 'eavesdrop' on a volatile plant alarm signal, methyl jasmonate (as part of a herbivore induced plant volatile signal), whereas workers of T. penzigi do not respond to these plant signals. Thus listening and responding to communication signals produced by African ant-acacias when they 'cry for help' is an important characteristic of high quality ant mutualists in this system. Association with third-party species includes parasitism by a lycaenid butterfly, Anthene hodsoni, an obligate associate of the aggressive ant C. mimosae, and behavioral experiments showed that females use ants as cues in laying eggs. Stable isotope analysis revealed that larvae are not simply phytophagous, but prey on the ants inside the domatia of the plant. At least 48 species of lycaenid butterfly have been documented to exploit acacias within the range of the ant-plants, 24 of these were observed on ant-plants as part of this study. Third-party species interactions with ants also result in resource partitioning and divergence of trophic niches: T. penzigi appears to be cultivating and dispersing fungi on a matrix of leaf-matter harvested from its host-plants; C. mimosae tends scale insects in the domatia; and both Crematogaster species feed on insects that they hunt both on and off the host tree. C. nigriceps relies more on prey than any of the other ants. Thus each partner manipulates the plant in a different way and occupies a distinct niche, mediated in large part through interactions with third-party species. Recognition of the complex roles of multiple interacting species in shaping what have classically been viewed as simple two party ant-plant mutualisms is essential in understanding their persistence and abundance in nature.
|Advisor:||Pierce, Naomi E.|
|School Location:||United States -- Massachusetts|
|Source:||DAI-B 72/09, Dissertation Abstracts International|
|Keywords:||Acacia drepanolobium, Ants, Crematogaster, Symbiosis, Tetraponera penzigi|
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