Incorporating indirect pathways in body size-trophic position relationships

Body size, trophic position (TP), and trophic niche width are important elements of food webs; however, there is still debate regarding their interrelationships. Most studies have tested these correlations using datasets restricted to carnivores and bivariate models that disregard potential indirect effects of other factors, their interactions, and phylogeny. We analyzed relationships among TP, consumer size, maximum food item size, food item size variation (a proxy for trophic niche width), and two other traits (gut length and mouth width) using confirmatory path analysis of an extensive dataset for freshwater fishes that encompass both carnivorous and non-carnivorous species. Consumer size was associated with maximum food size, food size variation, mouth width, and gut length, all of which mediated indirect relationships between body size and TP. Mouth gape was associated with maximum food size, and consumers that fed on larger food items had higher TP. Consumers with relatively long guts generally fed on small and homogeneous food items near the base of the food web. Models were consistent whether or not accounting for phylogeny, but varied according to trophic guilds. However, the body size of both carnivorous and non-carnivorous was not directly associated with TP. Therefore, the incorporation of functional traits and their intermediate pathways is critical for understanding size-based trophic relationships of animals that encompass diverse feeding strategies. Our results caution approaches that rely on body size as a surrogate for TP, especially in systems where plants and detritus are consumed directly by a significant number of animals, such as in most freshwater ecosystems.