Divergent Herbivory Adaptation

Trade-offs are key to species divergence and maintenance. Trade-offs between investment in growth versus investment in defense against herbivores have been implicated in tropical plant species coexistence. However, the role of the growth-defense trade-off in recently diverged species coexistence has not been explored empirically in a pair of recently diverged species. Costus villosissimus and C. allenii are recently diverged, interfertile species primarily isolated by divergent habitat adaptation. Divergent drought adaptation involves differences in plant economic strategy (investment in growth). By the growth-defense trade-off, the faster-growing C. villosissimus should be less well-defended and experience more herbivory. However, we find the opposite pattern in the wild. To understand why, I characterized patterns of wild herbivory, evaluated differences in leaf toughness and chemistry, and evaluated the feeding preferences of primary herbivores in controlled feeding trials and experimental arrays. I found that patterns of growth and defense do follow the growth-defense trade-off, but patterns of wild herbivory are primarily dictated by beetle habitat preference. By adapting to a seasonally dry, open, high-light habitat, C. villosissimus avoids high levels of beetle herbivory, despite lower defenses (Harenčár et al.; in prep). In this way, the growth-defense trade-off and abiotic selection pressures act synergistically to promote ecogeogrpahic isolation and species maintenance between close relatives.