Gape and energy limitation determine a humped relationship between trophic position and body size

We found a segmented pattern, increasing for small sizes and decreasing for larger sizes, in the relationship between trophic position and body size. This pattern provides support for a recently developed theoretical model whose derivation was based on consumers' metabolic requirements and on basic assumptions about feeding relationships. We combined original and published information about stable nitrogen isotopes, a proxy of trophic position, for a broad range of animal body sizes (10(-3)-10(5) kg) inhabiting the southwestern Atlantic Ocean. Linear, polynomic, and piecewise segmented models were fit to species trophic position and body mass. The segmented model had the best fit, presenting a positive slope (beta(1) = 0.33 +/- 0.08) for small organisms (<200 kg) and a negative slope (beta(2) = -1.93 +/- 0.16) for larger ones. This suggests that there are morphological restrictions to prey consumption in smaller organisms and energetic constraints to trophic position in larger ones. Furthermore, the predator-prey body mass ratio (BMR = 1.31; 95% CI = 0.9-2.40) estimated here is similar to previous reports of direct observations (BMR = 1.64 and 1.82). However, the trophic position of larger organisms decreases at a faster rate (beta(2) = -1.93) than expected by metabolic demand (beta(2expected) = -0.16 to -0.82), suggesting that additional processes should be considered. Our results suggest that large species could be more vulnerable to global change than previously thought.