Fundamental resource specialization of herbivorous butterflies decreases toward lower latitudes

Aim: Revealing the drivers of variation in resource specialization is a long-standing goal of ecological and evolutionary research. It is generally assumed that the degree of resource specialization increases toward lower latitudes. Although herbivorous insects are one of the best-studied consumer groups, the factors determining their degree of specialization at large spatial scales are poorly understood. In this work, we investigated the processes determining spatial variation in fundamental resource specialization, which we defined as the host breadth when not limited by interspecific interactions at the same trophic level in the species pool. More specifically, we aimed to reveal the drivers of geographical variation in fundamental resource specialization through macroecological approaches. Location: The Japanese archipelago including hemiboreal, temperate, and subtropical zones. Taxon: Herbivorous butterflies. Methods: Species-specific fundamental host breadth was calculated based on both pooled geographical occurrence and host-use records. We focused on the latitudinal pattern and investigated significant drivers of the degree of specialization in regional species pools. As potential drivers, we focused on geography, current climate, diversity and body size of butterflies, and their changes along geographical gradients. Through Bayesian structural equation modeling, we investigated the complicated relationships between these variables and resource specialization with a comprehensive model fitted using indices of host breadth within each community. Results: We observed that fundamental resource specialization of butterfly communities at 10 km grid level tend to become more specialized toward higher latitudes. This observed pattern is contrary to the presumed general trend found in studies based on knowledge of realized resource specialization within local communities. We found that this pattern is driven mainly by factors related to climate, butterfly diversity, and body size in each community. Main conclusions: Our findings suggest that fundamental resource specialization may show different latitudinal patterns from the conventional prediction based on knowledge of realized resource specialization. Our results emphasize the importance of the current climate as a major factor regulating butterfly morphology and host breadth, regardless of whether the impact is direct or indirect.