The role of resource imbalances in the evolutionary ecology of tropical arboreal ants

Abstract

In numbers and biomass, ants (Hymenoptera, Formicidae) often dominate arthropod faunas of tropical rainforest canopies. Extraordinary ant abundance is due principally to one or a few species able to tap the high productivity of canopy foliage by feeding on plant and homopteran exudates. Prior studies of nitrogen isotopic ratios show that exudate-feeders derive much of their nitrogen (N) by processing large quantities of N-poor, but carbohydrate (CHO)-rich, exudates. CHOs in excess of those that can be coupled with protein for growth and reproduction (postulated as the colony's first priorities) may be directed at little cost and some profit to functions that increase access to limiting protein. High dietary CHO:protein ratios for exudate-feeders appear to subsidize ‘high tempo’ foraging activity, defence of absolute (level III) territories, and production of N-free alarm/defence exocrine products that enhance ecological dominance in contests with other ants. Among organisms (e.g. plants and Lepidoptera) symbiotic with ants, CHO:protein ratios of ant rewards may control both the identities of ant associates and the quality of ant-rendered services. Dietary ratios of CHO:protein play an important and previously unrecognized role in the ecology and evolution of ants generally. Modifications of worker digestive systems in certain ant subfamilies and genera represent key innovations for handling and processing large volumes of liquid food. The supreme tropical dominants are species released from nest site limitation and able to place their nests in the vicinity of abundant exudate resources. Polydomy appears to be typical of these species and should produce energetic savings by taking colony fragments to the resource.