Mutulisms, interspecific interactions from which both species benefit, are now recognized as important drivers of species diversification and community structure; however, ecological studies of mutualism have been hampered by the difficulties of assessing net benefits to either partner. We develop a plant physiological approach to measuring whole plant benefits of ant association in specialized ant-plants. Plant physiological status, a correlation between maximum photosynthesis and water use efficiency, may serve as an integrated, whole plant measure of the many benefits of ant association. The Cecropia -Azteca association has become a model system for the study of mutualism in the New World tropics. In Costa Rica, C. obtusifolia hosts ants in the alfari and constructor species groups. Ants in the alfari group are known to be more aggressive and are thought to provide greater benefits than their congeners, although there are no long-term studies to substantiate this. The objective of this study was to determine whether plant physiological status differs among ant symbionts of C. obtusifolia. Specifically we asked whether the correlation between nitrogen concentration and δ13C of C. obtusifolia trees differs with alfari or constructor symbionts. Forty-five plants and their associated ants were collected along three elevation transects on the Pacific slope of the Cordillera Central in western Costa Rica. Oven-dried samples of ant and plant tissues were analyzed with a Finnigan Delta+ isotope ratio mass spectrometer. Plant physiological status was evaluated as the correlation between leaf nitrogen concentration and δ13C of ant hosts. Results/Conclusions

Plant δ13C (-27.76 per mil) and nitrogen concentration (2.72%) did not differ between plants with different ant symbionts (ANOVA: P=0.108). The correlation between nitrogen concentration and the δ13C of the host differed significantly between ant groups (ANCOVA: P< 0.05); photosynthetic capacity/water use efficiency was greater for the alfari than the constructor hosts. These findings suggest that alfari ants provide greater overall benefits to their host than the constructor group ants, and are consistent with behavioral observations and by previous diet analysis of these ant groups. This work suggests that plant physiological status, the correlation between nitrogen concentration and δ13C, reflects the relative benefits of ant association and is useful as an integrated measure of ant-provided benefits