Assessment of leaf defense hypotheses at the intraspecific scale in three Helianthus (Asteraceae) species1

Interactions between plants and insects have shaped biodiversity at multiple ecological and evolutionary scales in both diverse taxonomic groups. Antagonistic interactions (such as herbivory) can impose strong selective pressures on plants, resulting in increased defense levels or diverse defense strategies. The nature and extent of plant defenses can vary both within and between species, potentially reflecting trade-offs between defense and growth strategy that are associated with environmental resources, known as the resource availability hypothesis. Global interspecific patterns may or may not reflect mechanisms acting within species at more restricted geographic scales. Here, we ask whether there is evidence for associations between growth strategies and defense levels, resource availability, and herbivore damage within three Helianthus species (H. giganteus, H. grosseserratus, and H. maximiliani) in a restricted region of the Upper Midwest in the USA. We measure growth traits, leaf defense traits, and leaf herbivore damage levels in wild sunflower populations to assess patterns across populations and in two common gardens to assess patterns within populations. We estimate associations between growth traits and defense traits, defense traits and environmental resources, and herbivore damage and environmental resources. Overall, we find that slower growth strategies are associated with increased levels of defenses, though these higher defense levels are not associated with lower-resource environments in wild populations, and some of these patterns are detected in the commons. We also find that herbivore damage levels are not associated with these resource levels. We conclude that defenses in these species are related to growth strategy even at this intraspecific scale and are largely in line with macroevolutionary patterns across the genus. Both defense levels and herbivore damage are not strongly related to resource availability, which may reflect limited amounts of resource variation in this restricted region and follow prediction within a general framework for intraspecific defense trait associations.