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Facultative ant-plant associations: which ants provide protection?

One third of tropical forest plants bear extrafloral nectaries to attract ants. Ants, in turn, provide protective services for plant’s young leaves by preying on herbivores. Although these diverse and abundant ant-plant associations are classically viewed as mutualisms, the paucity of data suggest that ants may not always benefit plants. This project focused on a field experiment to investigate how the presence/absence of nectaries, ant identity, and distance between focal plants and nests of two locally abundant ant species (Azteca spp. and Ectatomma tuberculatum) affected ant visitation and subsequent herbivore attacks.

Results demonstrated that artificial nectaries effectively attracted ants to plants. However, the focal ant species were only observed on plants near their nests, irrespective of the nectary presence. Total ant abundance fluctuated over time, indicating that ant visitation is highly variable across spatial and temporal scales. Herbivore attacks were measured by placing an artificial caterpillar near the artificial nectaries. Attacks were related to ant visitor identity, indicating that some ants benefit plants, while others do not. Nectaries are not the only sources of sugar rewards offered to ants in the tropics. Treehoppers produce honeydew that seems to distract ants from foraging on nectaries. To determine if nectaries and treehoppers co-occur in tropical plants, plants were surveyed in 20 plots and the presence/absence of ants, nectaries, and treehoppers were recorded. Herbivory was quantified in plants that were experimentally assigned to nectaries and/or treehopper treatments. Surveys indicated that treehoppers do not occur on plants with nectaries, suggesting that plants with nectaries may repel treehoppers. Herbivory levels did not differ between treatments, indicating that herbivory may be related to random effects such as the distribution of herbivores.

About 1-2% of vascular plants worldwide are reported as bearing nectaries. Surveys from Panama suggest that nectary production in the tropics is much higher, with one third of tropical forest plants bearing nectar to attract ants. To determine the taxonomic distribution of nectaries and domatia- (i.e., nesting structures in plants) bearing plants in the Central Amazon, the number the plant species with nectaries and domatia was estimated using published findings and herbarium specimens to map them in a phylogenetic tree constructed from a mega-tree based on the Angiosperm Phylogeny Group. Results demonstrate that less than 20% of vascular plant species are reported as having nectaries or glands that produce nectar and less than 2% bear domatia. Both traits emerged numerous times across different species without a phylogenetic signal. This finding indicates the evolution of nectaries is not a prerequisite for the appearance of domatia. Instead, both traits, independently or together, have been favored in tropical plants over evolutionary time.

While conducting fieldwork, the investigator released a short documentary that was shown to undergraduate students taking of the “Rain Forest Biology and Conservation” course at the University of Utah (http://www.youtube.com/watch?v=nhUm4HDvctk). Six Brazilian students were trained for the project as field assistants. Findings were also shared through talks in Brazil and the US, and interviews for the local Amazonian television news and the State Department of Science and Technology. Additional information and photos are available at gsinimbu.wix.com/home and flickr.com/photos/gsinimbu (tagged as “CREOI”). ($9,806)