Body size is more important than diet in determining\xa0stable-isotope estimates of trophic position in crocodilians

Francisco Villamarín,Timothy D Jardine,Stuart E Bunn,Boris Marioni,William E Magnusson

doi:10.1038/s41598-018-19918-6

Abstract

The trophic position of a top predator, synonymous with food-chain length, is one of the most fundamental attributes of ecosystems. Stable isotope ratios of nitrogen (δ15N) have been used to estimate trophic position of organisms due to the predictable enrichment of 15N in consumer tissues relative to their diet. Previous studies in crocodilians have found upward ontogenetic shifts in their ‘trophic position’. However, such increases are not expected from what is known about crocodilian diets because ontogenetic shifts in diet relate to taxonomic categories of prey rather than shifts to prey from higher trophic levels. When we analysed dietary information from the literature on the four Amazonian crocodilians, ontogenetic shifts in dietary-based trophic position (TPdiet) were minimal, and differed from those estimated using δ15N data (TPSIA). Thus, ontogenetic shifts in TPSIA may result not only from dietary assimilation but also from trophic discrimination factors (TDF or Δ15N) associated with body size. Using a unique TDF value to estimate trophic position of crocodilians of all sizes might obscure conclusions about ontogenetic shifts in trophic position. Our findings may change the way that researchers estimate trophic position of organisms that show orders of magnitude differences in size across their life span.

Highlights

The trophic position of a top predator is an important component of food-web structure because it reflects the number of steps that energy takes to reach it from basal resources
The relationship between snout-vent length (SVL) and trophic position assessed using nitrogen stable isotopes (TPSIA) in P. trigonatus was best explained by a quadratic functional response, suggesting a plateau at maximum TP for mid-sized individuals (TPSIA = 2.27 + 0.05*SVL − 0.0003*SVL2; F2,42 = 9.42; r2 = 0.31; p < 0.001)
For the remaining three species, the relationship between SVL and TPSIA was best explained by a linear functional response: P. palpebrosus (TPSIA = 3.16 + 0.01*SVL; F1,34 = 38; r2 = 0.53; p < 0.001), C. crocodilus (TPSIA = 2.39 + 0.02*SVL; F1,29 = 25.94; r2 = 0.47; p < 0.001), M. niger (TPSIA = 1.33 + 0.02*SVL; F1,7 = 4.01; r2 = 0.36; p = 0.085)

Summary

Introduction

The trophic position of a top predator is an important component of food-web structure because it reflects the number of steps that energy takes to reach it from basal resources. Previous studies that analyzed δ 15N values in crocodilians, including individuals of broad size ranges, found ontogenetic increases in δ 15N values that were considered to mirror changes in trophic position caused exclusively by dietary shifts[14,34,35] Most of those studies included information on stomach-content analyses, none explicitly estimated the proportional contribution of each prey category to the diet of crocodilians of different sizes, which would have allowed direct estimation of prey trophic levels. We estimated growth rates of the four species based on their size, using equations published in the literature[40,41,42,43] in order to elucidate other possible mechanisms responsible for ontogenetic shifts in δ 15N values Using those sources of evidence, we aim to determine to what extent ontogenetic shifts in diet are coupled with changes in δ 15N values of tissues of Amazonian crocodilians and whether size-related metabolic changes can explain part of those changes

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Conclusion