Divergence and diversity in the defensive ecology of Inga at two Neotropical sites

Abstract

Summary 1. Tropical plants express multiple defensive traits that enable them to resist, deter or escape herbivores. In this study, we approached plant defensive adaptations by focusing on variation and interaction between sets of plant defensive strategies from a community perspective. 2. We compared two sites: the aseasonal Amazonian forest of Yasuni, Ecuador, and the seasonal forest of Barro Colorado National Monument (BCNM), Panama. We suggest that differences in ant abundance between these sites influence the divergence, local assembly and interaction of chemical and indirect ant-mediated defences in 18 tree species of the genus Inga (Fabaceae: Mimosoideae). 3. Relative ant abundance and visitation to extra floral nectaries were significantly higher at Yasuni than at BCNM. Correspondingly, Yasuni Ingas had low leaf extract bioactivity compared with BCNM Ingas, suggesting that Yasuni Ingas rely more on indirect ant defences, while BCNM Ingas rely more on chemical defences. 4. Yasuni Ingas varied widely in their ant visitation and extract bioactivity, and the strategies were negatively correlated in a trade-off like pattern. In comparison, BCNM Ingas had a positive correlation and little variation in their ant visitation, suggesting that their defence ecology was driven by environmental characteristics other than ants. 5. An experiment planting the same Inga species at both sites supported the prediction of higher herbivore pressure at Yasuni than at BCNM. However, leaf damage to naturally occurring Ingas was similar between sites, suggesting that the adaptations at the two sites were equally effective. Damage levels were also similar among species with contrasting defensive strategies, suggesting that ant and chemical strategies were also equally effective at a single site. 6. Synthesis. While tropical forests are thought to be a cradle of specialized interactions that promote diversification and allow the coexistence of a large number of species, the observed divergence in the defence ecology of Inga between these two sites suggests that general community characteristics can also promote diversity by favouring local and geographical diversity in defensive solutions.