Though studies have revealed that plant chemical defenses can vary greatly at sub-individual scales and that this variation can be important for herbivores, it is unknown how common this variation is and how it compares in magnitude with variation at the larger scales that are traditionally examined. Across-scale comparisons are crucial for revealing the relative amount of variation in plant traits that will be overlooked by focusing solely on population or individual-level differences. To quantify how small-scale, sub-individual variation compares with variation at larger scales, I measured the concentration and composition of the secondary metabolites quinolizidine alkaloids (QAs) in Silvery lupine (Lupinus argenteus, Fabaceae) individuals at 5 nested ecological scales: among sites within the Colorado Front Range, among individuals within the sites, among branches within individuals, among pods within branches, and among seeds within pods. This study mostly focused on seeds, but I also compared leaf, branch, and seed-pod secondary metabolites, with fewer nested scales. Sub-individual variation in secondary metabolites, if significant, is likely to influence interactions with plant enemies that act on small scales. Thus, using the same nested ecological scales, I next tracked the fate of L. argenteus seeds to determine on what scales the diverse members of a community of pre-dispersal seed predating insects were acting.
Results/Conclusions
I found that the concentration of QAs varied considerably on sub-individual scales for leaves, pods, stems, and seeds of L. argenteus. In particular, QA concentration in leaves varied as much within individuals as among individuals or among sites and QA concentration in seeds varied more cumulatively on the sub-individual scales (within pods, among pods, and among branches of the same individual) than on larger scales (among individuals and among sites). QA composition (or chemical makeup), however, generally varied only on the largest scales. Seed damage results revealed that all insect seed predators were acting only on sub-individual scales, primarily on the scale of pods within branches. Two seed-feeding taxa also acted upon the scale of seeds within pods. These results suggest that variation in secondary metabolites on the often-overlooked scale of within individuals can rival variation at larger scales. Additionally, these sub-individual levels are the scales that seed-predators are acting upon within this system. Thus, the large variation at these scales may be important for determining seed survival.