Mutualism between Three Species of Tropical Piper (Piperaceae) and Their Ant Inhabitants

Abstract

Three species of the genus Piper (P. cenocladum G.DG., P. fimbriulatum G.DG., P. sagittifolium G.DG.) from the wet evergreen forest of Costa Rica are associated with ants of the genus Pheidole in what appears to be a coevolved mutualistic relationship. The ants live in petiolar cavities and in the stems which they hollow out; the plant produces lipid-rich food bodies inside the petiolar cavities on which the ants feed. The ants appear to increase the competitive fitness of the plants by removing encroaching vines. More importantly, it seems that the plant may receive a nutritional benefit from the ants' presence by absorbing nutrients released from decaying nest material inside the stem. Ant-plant mutualisms involving several plant families have been described in the Old and New World tropics (Hocking, 1970; Janzen, 1967, 1969a, 1972, 1973a, 1975; Rehr et al., 1973; Whiffin, 1972). The swollen-thorn acacia and its obligate ant inhabitant, Pseudomyrmex, is the most thoroughly studied ant plant, or myrmecophyte (Janzen, 1967, 1969a, b). However, most of the ant-plant associations described in the New World tropics are facultative. It is often the case that the plant produces extrafloral nectaries and is associated with aggressive ants that feed on the nectar source and in turn presumably protect the plant from herbivory (Bequaert, 1922; Carroll, 1974; Whiffin, 1972). We describe a situation involving three species of plants of the genus Piper (P. cenocladum C. DC, P. fimbriulatum C. DC, P. sagittifolium C. DC) that are always associated with ants of the genus Pheidole in what appears to be a coevolved mutualistic relationship. The occurrence of ants in these Piper species has been previously noted (Burger, 1972), but there are no* descriptions of the benefits that both ants and plant derive from the relationship. Piper cenocladum and P. fimbriulatum are slender shrublike plants that grow commonly to a height of 4 m, although P. fimbriulatum sometimes reaches a height of 7-8 m. Both species live in the understory of the wet evergreen forest of Costa Rica between sea level and approximately 1100 m altitude. Piper cenocladum is endemic to Costa Rica and is found only on the Caribbean coast, while P. fimbriulatum is found only on the Pacific coast of Costa Rica and in western Panama (Burger, 1971). Piper sagittifolium is a smaller herbaceous plant seldom exceeding 1.5 m in height and has only been recorded from the 1 Present address : Department of Zoology, University of Michigan, Ann