Divergent defensive strategies of young leaves in two species of inga

(I)n the recently radiated genus Inga (Fabaceae), few nucleotide substitutions have accumulated among species, yet large divergences have occurred in defensive phenotypes, suggesting strong selection by herbivores. We compared herbivory and defenses of young leaves for L goldmanii, a more derived species that follows a defense strategy, and L unibellifera, a more basal species that follows an escape strategy. The two species suffered similar rates of herbivory (22% of the leaf area eaten during expansion) but were attacked by different communities of herbivores. I. goldinanii relied heavily on extra-floral nectaries and on a diversity of effective secondary metabolites, while I. umbellfera minimized damage through rapid leaf expansion and synchronous flushing. The major classes of secondary compounds in both species were flavanoids and non-protein amino acids; however, there were large differences in structure, biosynthetic pathways, and efficacy against herbivores. Growth rates of lepidopteran larvae were significantly lower when fed artificial diets with crude extracts from L goldmanii as compared to L umbellifera. Flavanoids accounted for the majority of growth reduction in both species. L umbellifera had more unusual flavanoids and a non-protein amino acid not reported from plants, but the more common flavanoids found in L goldinanii were more bioactive against herbivores. I. goldmanii also had greater ant visitation to extrafloral nectaries, suggesting that there was no trade-off between biotic and chemical defenses. In contrast, young leaves of L umbellifera expanded more rapidly, minimizing the window of vulnerability before toughening. Resources for rapid expansion may have been reallocated from chloroplast development as L umbellifera delayed the greening process until after full leaf expansion. Leaves were also produced synchronously, which can satiate herbivores and reduce damage. These defense differences are reflected in almost completely nonoverlapping herbivore faunas and the more frequent occurrence of generalists on L umbellifera. To understand why defenses have evolved, it is important to view them in light of the herbivore community as well as in the context of the other co-occurring traits. We hypothesize that the effectiveness of chemical defenses determines whether a species follows the evolutionary path of defense or escape strategies.