Disentangling the determinants of trophic structure is central to ecology. The capacity to capture subjugate and consume a prey (i.e. gape limitation) is a relevant limitation to acquire energy for most organisms, especially those in smaller size ranges. This generates a size hierarchy of trophic positions in which large organisms consume small ones. Body size is tightly correlated to gape limitation and explains a large fraction of variance in the body size-trophic position relationship. However, a considerable fraction of variance still remains to be explained. Consumer search space dimensionality (2D or 3D) and feeding strategies, temperature and the size structure of primary producers can alter the trophic structure, but tests based on information from natural food webs are scarce.

We generated specific predictions about the body size trophic position relationship and evaluated them using information from a subtropical South Atlantic coastal marine ecosystem: benthic realm (2D, rocky shore and sandy beach) and the pelagic realm (3D). We characterized this marine coastal food web based on stable isotopes of carbon and nitrogen from 256 samples from primary producers (macroalgae and phytoplankton) to large predators (sand shark) in summer and winter. Consumer body size encompassed 6 orders of magnitude in weight from 10^-2 to 6x10⁴ g.

Isotopic signal corresponded to an integration of carbon sources from basal consumers to top predators. The body size-trophic position relationship showed a linear positive association with different slopes for the benthic and pelagic environments. This implies a smaller predator prey size ratio for pelagic (3D) with respect to benthic consumers (2D) as theoretically expected. No seasonal differences were found in slopes and most of the overall variance in benthic environments was largely explained by feeding strategies of the different taxonomic groups.

We provide an integrated evaluation on the role of body size, consumer search space and feeding strategy to understand the determinants of trophic position.

Results demonstrate that integrating Gape Limitation hypothesis, the dimensionality of consumer search space and feeding strategies into a formal robust framework to understand trophic structure is feasible even in complex natural ecosystems.

Study area and field collections

Samples were collected from Punta del Diablo, a coastal marine ecosystem on the South Atlantic coast of Uruguay (33°54’S - 53°30’W and 34°02’S – 53°32’W). Two surveys were conducted: one during austral summer (12^th February) and a second one in winter (14^th September) of 2015, at five sampling stations in both pelagic and benthic realms. Biological samples were collected with multiple gears to capture different size groups and to perform stable isotope analyses (SIA).

Laboratory analysis

Samples were analyzed at the Center for Stable Isotopes, University of New Mexico, USA (https://csi.unm.edu/). Carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios were measured by Elemental Analyzer Continuous Flow Isotope Ratio Mass Spectrometry using a Costech ECS 4010 Elemental Analyzer coupled to a ThermoFisher Scientific Delta V Advantage mass spectrometer via a CONFLO IV interface.

Detailed descriptions of the methodologies for SIA analysis, the study area, and the associated data analysis are available in Leoni et al. (2024), Journal of Animal Ecology.