Tropical tree communities can have astounding alpha diversity, even within individual genera. For example, there are 45 species within the genus Inga (Fabaceae) alone in 25 ha of rainforest in Ecuador. Negative frequency dependence enforced by natural enemies is often cited as a process that facilitates the coexistence of tropical tree species. One of the central assumptions underlying this process is that coexisting species vary in their mechanism of defense against natural enemies, and further, we might predict that coexisting species diverge more in defense traits than in communities assembled at random. Testing these ideas requires quantifying defense traits across species. While many recent studies in tropical tree communities have focused on characterizing species for traits related to resource acquisition, few have focused on traits related to defense against natural enemies. We present one such study.
We focus on members of the diverse neotropical tree genus Inga (Fabaceae). We characterize species in French Guiana, Peru, and Panama for various traits that likely play a role in herbivore defense. We first construct a phylogeny for these species and assess how these various defense traits evolve. We than use plot inventory data to determine if coexisting species are more divergent for these defense traits than expected if communities are assembled at random.
Results/Conclusions:
We find evidence for a large diversity of defense syndromes within the genus. There seem to be three principal axes of defense, which are orthogonal to each other and across which species show great variation: chemical defenses, developmental defenses (e.g. rapid expansion), and ant defenses (through production of nectar at extrafloral nectaries). Furthermore, these different defense axes seem to be evolving rapidly and independently of each other. In analyses of plot data, we find that coexisting species diverge more in defense traits than expected if communities are assembled at random. This seems largely driven by differences in chemical defenses.
Our results suggest that negative frequency dependence, enforced by herbivores, may play a role in facilitating the coexistence of closely related plant species. Further work is aimed at characterizing other aspects of this process, such as whether natural enemies are indeed specializing on different species of Inga.
