Extrafloral nectar secretion does not mediate ant predatory activity on Pityrocarpa moniliformis plants in a seasonally dry forest

Abstract

Plants bearing extrafloral nectaries (EFNs) and their attendant ants are part of a mutualistic, facultative and generalized interaction. The final balance of this interaction can vary greatly over time and space, making the outcome context-dependent. In this study, we investigated how nectar volume and concentration mediate ant attraction and ant predatory activity on a legume species, as well as evaluate their daily variation in a seasonally dry tropical forest in northeastern Brazil. Our study model was the tree species Pityrocarpa moniliformis (Benth.) Luckow & R. W. Jobson (Fabaceae), and we used simulated herbivores (termites) to assess the predatory activity of ants. We found that ants interacted more on the EFNs of plants that produce higher nectar volumes, but not to those producing higher nectar concentrations. Ant predatory activity was not influenced by the volume or concentration of extrafloral nectar. Regarding the daily variation in nectar secretion, we found higher volumes secreted at nighttime, but no difference in nectar concentrations between day and night. Ant predatory activity was similar throughout the day and night. Our results indicate that ants are more sensitive to variations in nectar volume than concentration and that the volume secreted is higher at night; both probably due to the water restriction of the Caatinga. However, ant predatory activity is not related to nectar secretion volume or concentration, highlighting the complexity of mechanisms mediating the interaction between EFN-bearing plants and ants.