Abstract
Release from specialist insect herbivores may allow invasive plants to evolve traits associated with decreased resistance and increased competitive ability. Given that there may be genetic trade-off between resistance and tolerance, invasive plants could also become more tolerant to herbivores. Although it is widely acknowledged that light availability affects tolerance to herbivores, little information is available for whether the effect of light availability on tolerance differ between the introduced and native populations. We conducted a common garden experiment in the introduced range of Alternanthera philoxeroides using ten invasive US and ten native Argentinean populations at two levels of light availability and in the presence or absence of a specialist stem-boring insect Agasicles hygrophila. Plant biomass (total and storage root biomass), two allocation traits (root/shoot ratio and branch intensity, branches biomass/main stem biomass) and two functional traits (specific stem length and specific leaf area), which are potentially associated with herbivore resistance and light capture, were measured. Overall, we found that A. philoxeroides from introduced ranges had comparable biomass and tolerance to specialist herbivores, lower branch intensity, lower specific stem length and specific leaf area. Moreover, introduced populations displayed higher shade tolerance of storage root biomass and lower plastic response to shading in specific stem length. Finally, light availability had no significant effect on evolution of tolerance to specialist herbivores of A. philoxeroides. Our results suggest that post-introduction evolution might have occurred in A. philoxeroides. While light availability did not influence the evolution of tolerance to specialist herbivores, increased shade tolerance and release from specialist insects might have contributed to the successful invasion of A. philoxeroides.
Highlights
Rapid adaptation to the novel environments in introduced ranges is one of the mechanisms by which invasive species could spread and become extremely successful invaders after a lag time [1]
A relevant hypothesis is the evolution of increased competitive ability (EICA) hypothesis [2] which posits that release from specialist enemies may have caused evolution in exotic plants, lowered resistance to herbivores and increased growth and competitive ability
We focused on testing the following effects of each trait: 1) significant effects of plant origin could indicate differences between native and introduced ranges; 2) significant effects of origin by shading interaction on biomass would show differences in shade tolerance between plant origins; 3) significant effects of origin by shading interaction on allocation and functional traits would show differences in plasticity to shading between plant origins; 4) significant effects of origin by herbivore interaction on biomass can indicate differences in herbivory tolerance due to plant origin; 5) significant interactions of origin, shading, and herbivory can indicate the difference in effect of light availability on tolerance between native and introduced populations
Summary
Rapid adaptation to the novel environments in introduced ranges is one of the mechanisms by which invasive species could spread and become extremely successful invaders after a lag time [1]. Comparisons of herbivory tolerance between native and introduced populations of invasive plants have shown conflicting results, some invasive plants have evolved increased tolerance to specialist insects [10,11,12]; others have showed no differences in tolerance between native and introduced populations [12, 13] While these studies have been focused on the novel biotic condition: release from specialist enemies, invasive species can experience novel resource availability (e.g. light, water and nutrients) [14], which may lead to evolution in functional traits, and influence plant responses to herbivory [15]
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