Spatial Variation in the Effect of Ants on Extrafloral Nectary Plant

I investigated variation in the effect of ants at extrafloral nectaries on the plant reproductive success among three populations of Cassia fasciculata (Leguminosae) in northern Florida. Results show that the possession of extrafloral nectaries can lead to increased reproduction. Density of ants and removal of experimentally placed Drosophila larvae were far higher on C. fasciculata than on nonnectariferous background plants. Ants preyed on eggs and small larvae of lepidopteran herbivores. C. fasciculata plants from which ants were excluded had more larvae of the major seed pod predator Phoebis sennae (Pieridae; Lepidoptera), suffered higher pod predation (at one site), and had lower reproductive output (at two sites) than did control plants to which ants had access. Effects of extrafloral nectaries on reproduction of C. fasciculata varied among the three populations, due apparently to differences in density of ants and potential herbivores. At the site with the lowest density of ants and herbivores, pod set was not significantly different between control and ant exclusion groups. Where they were more numerous, ants had a small but significant positive effect. Only where the abundance of both ants and potential herbivores was high did ants visiting extrafloral nectaries strongly increase the reproductive success of individuals of C. fasciculata. The species of ants present and differences among herbivore species in susceptibility to ant attack may have also influenced the effect of extrafloral nectaries at any one site. Although the average effect of extrafloral nectaries throughout a plant species may be positive, the strength of this effect was found to vary widely among local populations.