In the absence of coevolved natural enemies, plants are expected to experience selection away from costly herbivore defense towards growth and reproduction (Evolution of Increased Competitive Ability hypothesis), yet no one has demonstrated EICA for an indirect defense trait. Likewise, we have little understanding of how constitutive and induced levels of defense vary among native and invasive plant populations and if this depends on whether a trait is direct versus indirect. Previous work has demonstrated that indirect defenses are under selection from both the herbivore community and the enemy of herbivores community, suggesting that indirect defense traits may be especially responsive to changes in biotic interactions during species invasions. Because indirect defense involves a third trophic level, plants with these traits may be especially prone to ecological mismatches that lead to evolutionary change, as the benefits of defense are dependent on multiple interactions versus one interaction. We conducted a greenhouse experiment in the introduced range to test whether invasive populations have reduced constitutive and induced investment in an indirect defense trait, extrafloral nectar (EFN) production, compared to native populations of Chinese tallow tree, Triadica sebifera, through an experimental leaf damage treatment. We compare this to field data collected on EFN production of native and invasive populations grown in replicate common gardens in the native (China) and invasive (US) range.
Results/Conclusions
Overall, native populations invested more in indirect defense: Native populations had a greater number (+16%) and percentage of leaves producing EFN (35% vs. 28%), produced more EFN (63% greater volume), and produced more sugar (+33%) compared to invasive populations, independent of damage treatment. Of these traits, number of leaves producing EFN and volume of EFN exhibited a tradeoff between constitutive and induced investment but these did not depend on plant origin. When grown in the field, all populations produced more nectar in the introduced range where plants were larger but native populations produced a greater proportion of leaves with EFN than invasive populations. Our results are the first to support the EICA hypothesis for an indirect defense trait. This suggests that tri-trophic interactions such as indirect defense are under similar selection as direct defense traits within introduced populations. Despite reduced investment in EFN production, invasive populations still retain the ability to produce EFN, which may enable invasive plants to defend against herbivores in the introduced range