The distribution and evolution of antiherbivory defenses across taxa has been of long-standing interest to ecologists. What processes caused the variation in the efficacy of antiherbivory traits that we see today – evolutionary conservatism or convergence? Are these traits more effective today at mediating more specialist or generalist herbivory? This study examines the macroevolution of plant-herbivore interactions in oak trees by looking at host shifts of insects onto non-native oaks. Leaf damage, insect herbivore community, and 9 leaf defensive traits were measured across 57 species of mature, non-native oaks in a common garden. Each of these aspects of plant-herbivore interactions were mapped onto a phylogeny of oaks.
Results/Conclusions
Phylogenetic distance to the native oak (Quercus lobata) predicted the presence and abundance of specialist gallers, but did not predict chewing, mining, and phloem-feeding leaf (i.e. more generalist) damage nor the corresponding insect herbivore communities. Leaf defensive traits were evolutionarily labile, and multiple traits often co-occurred in species. Defensive traits significantly predicted damage from insects other than galls as well as generalist insect communities. These data suggest that putative leaf defense traits have evolved convergently and are likely adaptive under high pressure from generalist herbivores or in situations where leaf damage (from generalists) is especially harmful to the plant. These traits, however, are not necessarily adaptive in mediating specialist herbivory. This suggests that there are fundamental differences in the evolution of plant defenses against specialist versus generalist herbivores.
