1. Mutualistic interactions are characterized by conditional outcomes that depend on both the biotic and the abiotic context. However, limited information is available on the factors that affect the strength of ant–plant interactions among sympatric congeneric species.
2. We compared the benefits gained from attracting ants via extrafloral nectaries – i.e. lowered herbivory and increased seed set – of three co-occurring varieties in the Chamaecrista desvauxii complex (Leguminosae) in a cerrado area in Uberlândia, Brazil. Using whole-individual exclusion experiments, we tested the hypotheses (i) that the relative strength of those benefits is higher in the variety with the largest extrafloral nectaries and (ii) that those benefits are conditional on the presence of predispersal seed predators.
3. Extrafloral nectaries are larger, produce more nectar and attract more ants in var. brevipes than in the other two varieties included in the study. Var. modesta has intermediate-sized nectaries, while a third, undescribed variety has small nectaries, and both attract relatively few ants.
4. For var. brevipes, extrafloral nectary (EFN) removal significantly increased folivory and attack on fruits by sucking insects, decreasing the relative number of flowers, fruits and seeds produced per individual. For the other two varieties, in contrast, ant effects were reduced, and ants did not significantly improve reproductive success. In addition, effects of EFN removal were less pronounced or absent when seed predators were excluded from fruits of var. brevipes.
5. Synthesis. We showed experimentally that benefits from interactions of three co-occurring varieties of Chamaecrista desvauxii with ants are context-dependent both within and among taxa. Variation in the strength of mutualisms among sympatric taxa may potentially reinforce ecological reproductive isolation and contribute to diversification in this group.