Discrimination factors of carbon and nitrogen stable isotopes from diet to hair in captive large Arctic carnivores of conservation concern

Abstract

Rationale: Stable isotope analysis is widely used to reconstruct diet, delineate trophic interactions, and determine energy pathways. Such ecological inferences are based on the idea that animals are, isotopically, what they eat but with a predictable difference between the isotopic ratio of a consumer and that of its diet, coined as the discrimination factor. Providing correct estimates of diet-consumer isotopic discrimination in controlled conditions is key for a robust application of the stable isotopes technique in the wild.

Methods: Using a Finnigan Mat Delta Plus isotope-ratio mass spectrometer, we investigated isotopic discrimination of carbon and nitrogen isotope ratios (δ¹³ C and δ¹⁵ N values) in guard hairs of four Arctic predators; the wolf (n = 7), the wolverine (n = 2), the grizzly bear (n = 2), and the polar bear (n = 3). During a 3-month trial, carnivores were fed a mixed diet. The δ¹³ C and δ¹⁵ N values, and the mass (g) of diet items, were monitored weekly for each individual to determine their Total Diet Average ratios.

Results: Diet-hair isotopic discrimination (Δx) varied according to species, ranging [1.88 ± 0.69‰: 3.2 ± 0.69‰] for δ¹³ C values, and [1.58 ± 0.17‰: 3.81 ± 0.22‰] for δ¹⁵ N values. Adult wolves Δ¹³ C average (2.03 ± 0.7‰) was lower than that of young wolves (2.60 ± 0.8‰) and any other species (combined average of 2.59 ± 0.28‰), except for the wolverine (2.12 ± 0.23‰). Wolves Δ¹⁵ N averages (juveniles: 3.51 ± 0.34‰, adults: 3.68 ± 0.28‰) were higher than those of any other species (combined average: 2.50 ± 0.58‰).

Conclusions: The discrimination factors for δ¹³ C and δ¹⁵ N values calculated in this study could be used in ecological studies dealing with free-ranging animals, with implications for non-invasive research approaches. As in other controlled discrimination studies, we recommend caution in applying our discrimination factors when the population structure is heterogeneous.