MEPS 597:39-46 (2018)  -  DOI: https://doi.org/10.3354/meps12584

ABSTRACT: A positive relationship between body size and trophic position amplitude is expected based on mobility and gape size. Using a dataset of marine consumers’ diet, we illuminated mechanisms driving the relationship between consumers’ body mass (a proxy for body size) and trophic position parameters within and among populations. We found that across a body size gradient, maximum trophic position increased, while minimum trophic position did not vary, resulting in a positive relationship between species body size and trophic position amplitude. However, using single individuals as data points, we found a positive correlation between trophic position and body mass. The correlation was stronger for larger species, indicating that as mean species body size increased, there was an increasing tendency for smaller individuals to forage on lower levels and larger individuals to forage mainly on higher levels. Therefore, the wider variation in trophic positions we observed for larger species is possibly related to an ontogenetic diet shift in size-structured populations. Our results corroborate the idea that on an individual level larger consumers do not show a positive relationship between body size and diet niche breadth, and support the notion that ontogenetic niche differences can drive the amplitude of trophic position among marine predator species.

KEY WORDS: Trophic chain · Ontogenetic diet shifts · Niche breadth · Consumer-resource interactions · Food web topology