Defense with benefits? Ducking plants outperformed erect plants in the goldenrod Solidago gigantea in the absence of herbivory.

Abstract

Despite the fact that herbivores can be highly detrimental to their host plants' fitness, plant populations often maintain genetic variation for resistance to their natural enemies. Investigating the various costs (e.g., allocation tradeoffs, autotoxicity, and ecological costs) that may prevent plants from evolving to their fullest potential resistance has been a productive strategy for shedding insight into the eco-evolutionary dynamics of plant-herbivore communities. Recent studies have shown that some individuals of goldenrod (Solidago spp.) evade apex-attacking herbivores by a temporary nodding of their stem (i.e., resistance-by-ducking). Although ducking provides an obvious fitness benefit to these individuals, nonducking (erect) morphs persist in goldenrod populations. In this study, I investigated potential costs of ducking in Solidago gigantea in terms of tradeoffs involving growth and reproduction in a common garden experiment using field-collected seeds. The S. gigantea population contained substantial genetic variation for stem morph, with 28% erect and 72% ducking stems. In the absence of herbivory, ducking plants were taller, had thicker stems, and produced an average of 20% more seeds than erect plants. This study suggests that resistance-by-ducking, instead of being costly, actually comes with additional, nondefense-related benefits. These results support the conclusion that the factors that constrain the evolution of resistance in plant populations are likely to be more subtle and complex than simple tradeoffs in resource allocation.