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- How availability and quality of nectar and honeydew shape an Australian rainforest ant community (2003)
- Ant communities visiting nectar and honeydew sources were studied in a tropical lowland rainforest in North Queensland, Australia. The study focused on the hypothesis whether the distribution and composition of nectar and honeydew diets influence resource partitioning and competition in the ant community, and thus regulate community composition. Ants were the most common consumers on all extrafloral nectaries, while they constituted only a minority of floral visitors. In total, 43 ant species were observed to consume nectar from extrafloral nectaries (34 plant species) or from flowers (14 plant species), and wound sap exudates (three plant species). Six nectar-foraging ant species attended trophobionts (including at least 12 species of homopterans and two species of lycaenid caterpillars) for honeydew. Ant species showed a significant compartmentalisation of nectar use across plant species, although most ant species visited a broad spectrum of plants that strongly overlapped between different ants. Trophobioses were much more specialised at the study site, and some ant species attended certain trophobionts exclusively. On each plant individual, only a single ant colony was observed attending trophobionts. In contrast, simultaneous co-occurrences between different ant species foraging for nectar on the same plant individuals were common (observed in 23% of the surveys), although these proportions varied strongly across plant and ant species. The two most dominant ant species (Oecophylla smaragdina and Anonychomyrma gilberti) had mutually exclusive territories, and they were each associated with a significantly different assemblage of other ant species on nectar plants. This community pattern corresponds with the concept of ant mosaics that is based on dominance hierarchies. Honeydew and nectar sources varied substantially in carbohydrate and amino acid concentration and composition (HPLC analyses). There was a strong relationship between the composition of these resources and their use by ants, in particular by the dominant O. smaragdina. Among all 32 nectar and honeydew sources analysed, resources actually consumed by this ant were characterised by relatively similar amino acid profiles and higher total sugar concentration. The most common diets of O. smaragdina included two honeydew sources (Sextius ‘kurandae’ membracids on Entada phaseoloides and Caesalpinia traceyi legume lianas) and two extrafloral nectars (Flagellaria indica and Smilax cf. australis) that had the broadest spectrum of amino acids. Furthermore, these trophobioses on lianas showed a significantly higher per capita recruitment of this ant species (number of workers per individual homopteran) compared to trees. F. indica and S. cf. australis extrafloral nectaries were also commonly monopolised by O. smaragdina in a similar way as trophobioses; co-occurrences were significantly rarer than at other nectar sources. Field experiments on nectar preferences were performed using artificial sugar and amino acid solutions in pairwise comparisons. Preferences among sugars were largely concordant between ant species. For most ant species, sucrose was more attractive than any other sugar, and attractiveness increased with sugar concentration. Most ant species also preferred sugar solutions containing mixtures of amino acids over pure sugar solutions. However, choices between different single amino acids in sugar solutions varied substantially and significantly between species. Preferences between solutions were significantly reduced in the presence of competing ant species. Thus the experiments show that both variability in gustatory preferences, especially for amino acids, and conditional effects of competition may be important for resource selection and partitioning in nectar feeding ant communities. Stable carbon and nitrogen isotope composition was analysed for 50 ant species, and additionally for associated plants, homopterans and other arthropods from the study site. Nitrogen isotope ratios (d15N) of ants were not correlated with those of plant foliage from which the ants were collected. Instead, d15N may represent a powerful indicator of trophic position of omnivorous ants like in other foodweb studies, suggesting that members of the ant community spread out in a continuum between largely herbivorous species, feeding on nectar or honeydew, and predatory taxa. Variability between colonies of the same species was also pronounced. d15N values of O. smaragdina colonies from mature forests, where most of their nectar and honeydew sources are found, indicate lower trophic levels than isotope signatures of colonies from open secondary vegetation. This study demonstrates that the distribution and quality of honeydew and nectar sources have a strong structuring impact in diverse tropical ant communities. Amino acids were found to play a key role for ant species preferences and competition, and for nitrogen fluxes to colonies of the arboreal ant fauna.