The Facultative Ant Mutualism of the Tropical Tree Genus Inga

Abstract

The tropical tree genus Inga (Fabaceae: Mimosoideae) is a useful model organism for the study of plant-herbivore interactions, which are in turn crucial for understanding the patterns and dynamics of biodiversity. The numerous and recently radiated Inga species show greater divergence in antiherbivore defenses than in other functional traits, suggestive of herbivore-driven diversification. Inga species combine chemical, physical, developmental and other forms of defense, all of which evolve independently of one another. One particularly intriguing defense involves the attraction of ants via extrafloral nectaries. This facultative mutualism introduces the benefits of ant aggression toward herbivores, but also carries risks, including exploitation by non-aggressive ants and the attraction of myrmecophile herbivores. This study explores sources of variability in this mutualism, finding that ants are unreliable in their presence and success in herbivore removal. Consistent with past research, the ants were observed to tolerate or farm certain herbivores, and the net impact of ants was not overtly beneficial. Extending past research to encompass observations of Inga seedlings, my results support the hypothesis that this early, more sensitive life stage would invest less in risky ant protection than would the more robust saplings. Delving into the Inga-ant mutualism is an opportunity to investigate complex trade-offs in multitrophic interactions and their role in creating and maintaining diverse tropical ecosystems.