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Total production of secondary metabolites in Inga comprises about 40–50% of leaf dry weight ( 15). We recorded the presence of defensive compounds, particularly several different classes of flavonoids, tannins, saponins, and metabolites containing amines. We included multiple classes of antiherbivore traits to capture as complete an understanding of the entire defensive profile as possible. We focused on expanding leaves as the majority of leaf damage occurs during this short window before leaves toughen ( 14). Following the notion that herbivores track or “chase” host defenses and not host species per se ( 11, 12), we would expect host choice at the level of plant species to mirror host defenses more than host phylogeny, a pattern that would diminish the role of plant phylogenetic relationships in the origin and structure of herbivore communities.Īt Los Amigos, we characterized the defensive traits of expanding leaves for 33 species of Inga. Furthermore, within a community, neighboring plants are more likely to differ in defenses than expected by chance even if they are closely related ( 3, 5, 6). Specifically, studies within several plant genera have found a poor pattern of congruence between their phylogenetic histories and the expression of defenses ( 3, 5, 6, 9, 10). However, recent work at the species level suggests that herbivores have selected for divergence in defenses in closely related host species. At these levels, phylogeny may be a good proxy for shared traits, and many resource acquisition traits show a phylogenetic signal. The Ehrlich and Raven model, and many subsequent studies, consider macroevolutionary processes across genera and families ( 7, 8). Here, we test hypotheses about herbivore host selection by extensively characterizing defenses of a speciose genus of trees co-occurring at one site, and by comparing phylogenies for both trophic groups. Although plant antiherbivore traits play a prominent role in determining host choice and need not track plant phylogeny, antiherbivore defenses are often not sufficiently considered. To begin to address these questions, we must understand the extent to which host choice is evolutionarily conserved. For herbivores, however, many questions remain with respect to factors shaping community structure, diversification, and coevolution. A number of recent, independent studies suggest that herbivore pressure contributes to the high local plant diversity, or coexistence, that is typical of plant communities in tropical rainforests ( 3– 6). Coevolutionary and escape-and-radiate models suggest that herbivores might drive speciation in plants ( 1, 2). Coevolutionary theory has long predicted that the arms race between plants and herbivores is the principal explanation for this great diversity ( 1). Although plants may evolve under selection by herbivores, we hypothesize that herbivores may not show coevolutionary adaptations, but instead “chase” hosts based on the herbivore’s own traits at the time that they encounter a new host, a pattern more consistent with resource tracking than with the arms race model of coevolution.īecause plants and their insect enemies are strikingly species-rich groups, understanding their interactions is a foundational issue in ecology and evolution. Hence, there is an apparent asymmetry in the evolutionary interactions between Inga and its herbivores. Together, these results suggest that plant defenses might be more evolutionarily labile than the herbivore traits related to host association. We found that closely related herbivores fed on Inga with similar defenses rather than on closely related plants. Finally, we compared the phylogeny and defenses of Inga to phylogenies for the major lepidopteran clades. Analyses at finer taxonomic scales showed that different lepidopteran clades select hosts based on different defenses, suggesting taxon-specific histories of herbivore–host plant interactions. Furthermore, host defensive traits explained 40% of herbivore community similarity. There was no correlation with phylogeny, a result consistent with our observations that the expression of defenses in Inga is independent of phylogeny. We found that similarity in herbivore assemblages between Inga species was correlated with similarity in defenses.
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We constructed phylogenies for both plants and insects and quantified host associations and plant defenses. We addressed these questions using the tree genus Inga and its lepidopteran herbivores in the Amazon. For herbivores, many questions remain regarding how plant defenses shape host choice and community structure. Coevolutionary models suggest that herbivores drive diversification and community composition in plants.