Resource specialization is a key determinant of interspecific interactions and subsequently, the structure and dynamics of ecological communities. In this regard, herbivore diet breadth has been linked to an array of ecological and evolutionary processes, including speciation and extinction, response to environmental change, nutrient cycling, and energy flow in multi-trophic food webs. However, despite the recognized importance of diet and the magnitude of variation in specialization among co-occurring taxa, we lack a general understanding of the ecological and evolutionary constraints on host range at broad scales.
We use a large-scale dataset of herbivore-host plant interactions between temperate trees and larval Lepidoptera to examine the eco-evolutionary correlates of resource specialization in herbivores. We quantify the diet breadth of 500 micro- and macro- Lepidoptera species and 20 families surveyed from 32 common North American tree genera. We ask whether key traits – namely body size and feeding strategy – are related to diet, and consider the role of geographic distribution in shaping these relationships. In doing so, we highlight the many dimensions of dietary generalism and unique natural history that has evolved within North American Lepidoptera.
Results/Conclusions
We document considerable variation in herbivore diet breadth at the species- and family-level; 12% of species were strict specialists that feed from a single plant genus, whereas the majority of species demonstrate some degree of dietary generalism. Diet varied to some degree among Lepidoptera families, however was strongly constrained by feeding strategy such that compared to free-feeding caterpillars, concealed feeders (borers, shelter-builders) tend to have fewer hosts, be less variable in host use, and are found on less commonly consumed hosts. An exception to this pattern are gregarious feeding caterpillars who typically have a diet that reflects the resource environment and are characterized by high phylogenetic dissimilarity among hosts. These patterns between feeding strategy and specialization contribute to a non-linear relationship between caterpillar body size and diet in which the most specialized taxa are found at the extremes - both the smallest and largest body sizes - and the most generalized taxa are middle of the road. However, when caterpillar geographic distribution is controlled for, the overall relationship between body size and diet breadth is positive due to the restricted range and climatic niche of the largest and most specialized taxa. These findings provide the basis for understanding broad scale patterns in specialization and interaction structure.