Ecological Correlates of a Neotropical Mutualism: Ant Assemblages at Costus extrafloral Nectaries

Abstract

The species richness and composition of ant assemblages at the extrafloral nectaries of four Costus spp. (Zingiberaceae) in Panama were quantified to determine (1) the relative specificity of these ant—plant mutualisms, and (2) the ecological factors that influence ant assemblages. Although inflorescences of Costus spp. commonly produce extrafloral nectar for several months (species means 1—2 mo, range to 4 mo), there was little turnover in ant—species composition on individual inflorescences. A total of 34 ant species in five subfamilies were observed, but plant species varied significantly in ant species richness. The number of ant species expected from equal sampling (40 inflorescences per Costus sp.) was 9 for C. pulverulentus, 13 for C. scaber, 15 for C. allenii, and 20 for C. laevis. Among—species differences in ant species richness were not a function of plant distribution, the volume or composition of extrafloral nectar, or the duration of nectar flow. The number of ant species was positively correlated with inflorescence height, which varied from 0.6 to 2.0 m. The differences in ant species richness with inflorescence height were due primarily to the addition of arboreal ants, which increased from 3 species representing 3% of all ant species observations on the shortest plant, C. pulverulentus, to 12 species representing 62% of all ant observations on the tallest plant, C. laevis. The number of terrestrial ant species varied only from six to eight. Cumulative ant—species curves as a function of number of inflorescences sampled (N = 41—80) did not plateau for any Costus sp., indicating that a large number of ant species can utilize extrafloral nectar in this system. The high ant species richness and the similarity in nectar characteristics among Costus spp. suggest these are relatively generalized mutualisms with limited coevolution between the plants and their attendant ants. The brief period of extrafloral nectar production (° = 36—55 d), coupled with spatial variation in ant species distributions, probably limit the degree of specialization among mutualists. This is in contrast to more specialized ant—plant mutualisms where plants are relatively predictable sources of food and/or nest sites for ants.