Abstract
An obligate symbiosis between Ocotea dendrodaphne Mez and O. atirrensis Mez & Donn.Sm. (Lauraceae) and their cryptic ant symbiont, Myrmelachista flavocotea, is common in lowland wet forests of Costa Rica, yet it is unclear whether the association is typically mutualistic or parasitic. Ants impose costs by tending trophobionts inside the plant body and further compromise the structural integrity of their host by hollowing its stems. Benefits to the host, including anti-herbivore defense and nutrient provisioning, must outweigh these costs for the association to be mutualistic, but benefits in this system are largely unknown. We adopted a stable isotope approach to investigate trophic relationships among Ocotea hosts, coccoid trophobionts, and worker and larval ants in the understory of mature forest at La Selva Biological Station, Costa Rica. In addition, a natural stable isotope marker (tuna) was made available to M. flavocotea colonies to determine whether ants are fortuitous scavengers and/or provision their host. In this system, we found unusual patterns of isotopic fractionation. For coccoids, nitrogen isotopes are only slightly enriched, and carbon isotopes were depleted relative to the host signature. Moreover, nitrogen signatures of the timid Myrmelachista ants are high, suggesting a substantial degree of carnivory and/or scavenging in this species. In the provisioning study, ant colonies provided with baits demonstrated a significant shift in carbon isotopic composition relative to untreated colonies indicating uptake and assimilation of the bait. These results suggest that these timid ants are patrolling the plant for food and concentrating nutrients in Ocotea’s hollowed stems. Further studies are required to confirm if M. flavocotea provides a net benefit to its host by providing nutrients, reducing herbivory, or by removing detrimental debris from the plants surface.
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Acknowledgments
We owe thanks to E. Pollard and T. Millican of the UASIL for assistance with stable isotope analyses, J. Ludlam and D. Magoulick for statistical advice, V. Carmona and the class of OTS 01-1 for help with field collections, T. Kondo for identifying mealy bugs, D. McClearn for logistical support, Organization for Tropical Studies for use of La Selva Biological Station and facilities, and classmates of The Evergreen State College, 2006. Funding for this project comes from an Undergraduate Research Award from the University of Arkansas Honors College to KP, an OTS Glaxo Wellcome Fellowship to PB, and National Science Foundation grant DEB-0640015 to JTL. A portion of this work was completed while CLS was a fellow of the National Research Council at the US EPA/NHEERL/WED laboratory in Corvallis, OR, USA. The manuscript was greatly improved by the comments of two anonymous reviewers.
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McNett, K., Longino, J., Barriga, P. et al. Stable isotope investigation of a cryptic ant-plant association: Myrmelachista flavocotea (Hymenoptera, Formicidae) and Ocotea spp. (Lauraceae). Insect. Soc. 57, 67–72 (2010). https://doi.org/10.1007/s00040-009-0051-z
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DOI: https://doi.org/10.1007/s00040-009-0051-z