Understanding the scale at which ecological processes occur is a fundamental goal in the field. In plant ecology, research has focused on variation in plant traits at the scale of individuals or larger. This is especially true for chemical defenses. Several studies, however, have revealed how chemical defenses can vary greatly at very small scales, such as within-leaves, and that this variation can be important for herbivores. It is unknown how common variation is at these smaller scales and how it compares with variation at the scales that are traditionally examined. Across-scale comparisons have not often been made for plant defensive chemistry, yet are crucial for revealing the relative amount of variation in plant traits that has been overlooked. In this study, we measured variation in seed toxins (quinolizidine alkaloids) in Silvery lupine (Lupinus argenteus, Fabaceae) individuals at 5 nested spatial 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.
Results/Conclusions
We found that chemical defense varied as much at the smaller scales (within pods, among pods, and among branches of the same individual) as at the larger scales (among individuals and among sites). For developing lupine seeds, the processes that determine survival (selection by seed-predating insects or infection by mold, for example) occur on a very small scale, so variation at even the smallest scales may be important for determining patterns of seed survival. These results suggest that studies of natural selection at the individual level may mislead if sub-individual variation is ignored.