Weighing Defensive and Nutritive Roles of Ant Mutualists Across a Tropical Altitudinal Gradient

The diversity of mutualistic interactions influences many ecological components of community structure, including biodiversity and ecosystem stability. However, mutualistic interactions are not well resolved because of a historical bias toward examining antagonistic interactions. Here we examine both antagonistic and facilitative interactions between tropical plants and arthropods by characterizing the biotic interactions between a common myrmecophytic shrub, Piper immutatum Trel. (Piperaceae), the ants hosted by this plant, Pheidole sp. (Formicidae: Myrmicinae), and their associated communities of herbivorous and predatory arthropods. To determine if ant mutualists affect the altitudinal distribution of Neotropical myrmecophytes, P. immutatum interactions with arthropods were quantified across a tropical elevational gradient. Piper immutatum was most abundant in lower montane forests (1000-1600 m asl) and disappeared above 1600 m asl, and colonies of Pheidole sp. inhabited 90 percent of the sampled plants. The myrmecophyte was then transplanted within and beyond its altitudinal range, excluding ants from half of the transplanted plants. Plant survival was affected primarily by elevation, with only 20 percent surviving above 1600 m asl. Ant exclusion did not significantly affect plant mortality. Nevertheless, ant colony size did affect both herbivory and nutrient availability for surviving P. immutatum, with nutrient availability having a stronger effect than antiherbivore defense on growth and biomass. This approach of studying the contributions of ant mutualisms across the myrmecohpyte's habitat range yields an improved picture of the role of mutualistic interactions in determining community structure.