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

Abstract

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 (10−3–105 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 (β1 = 0.33 ± 0.08) for small organisms (<200 kg) and a negative slope (β2 = −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 (β2 = −1.93) than expected by metabolic demand (β2expected = −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.