Intraspecific variation in growth and defense among plant populations can be driven and maintained by variation in biotic conditions, such as herbivory as well as variation in resource availability. Plant defense theory postulates that increased defense is favored over growth in resource poor environments. The realized availability of resources depends not only on the abiotic conditions as such but also on the ability of individuals to compete for those resources that can be modified by the competitive environment. Introduction of species to novel environments affects simultaneously direct herbivory encountered by a plant and resource availability both directly and via changes in competitive environment. Selection in introduced ranges can shape the allocation patterns between growth and defense differently from that in the native range. We address the question of how growth and defense traits covary among populations across differential abiotic and biotic environments, and explore the genetic bases of these traits among multiple native and introduced populations of the garden lupine Lupinus polyphyllus. We focus specifically on genetically determined, evolved among-population differences in these traits by measuring several growth and resistance traits from plants grown from seed in a common garden.
Results/Conclusions
Plants from the introduced populations grew larger than plants from the native populations. Plant size was positively associated with total leaf alkaloid concentration (a putative defense trait) in the native populations but not in the introduced populations. Moreover, leaf trichome density was negatively associated (i.e. there was a trade-off) with size in the native populations only. There was a slight negative association with growth rate and resistance measured as consumption by a generalist mollusk Arianta arbustorum that was similar across all plant origins. Finally, when examining associations between different resistance traits, we found a trade-off between leaf trichome density and leaf total alkaloid concentration in plants from the native populations, but not in those from the introduced populations. These results suggests that trade-offs governing plant resource allocation to growth and defense may be fundamentally different between populations in the native vs. introduced range. Our results suggest that the lack of native herbivores and the altered resource availability in the introduced populations may facilitate the evolution of novel trait optimums without restrictions from trade-offs.