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Simple Summary: The red panda, with an exclusive bamboo diet, has been classified as Ailurus fulgens and Ailurus styani. However, the quantitative comparison of the trophic niches of these two species is still an unsolved problem. Using stable isotope analysis, we found that Ailurus styani had similar δ13C values in the hair keratin to Ailurus fulgens, but significantly higher δ15N values due to different altitude distribution. Furthermore, the two red panda species had comparable trophic niche widths. Three genetic populations of A. styani endured various levels of competitive pressure. The Sichuan red panda occupied a relative lower trophic level than local carnivores and herbivores, but a bit higher than giant pandas, and the foraging strategies and habitat partition was attributed to the difference of niche width of the two pandas. We suggest more differentiated and refined conservation and management of the red panda and its habitat, and this study provides a new insight into intraspecific trophic niche studies. The red panda (Ailurus), a rare and endangered mammal native to the Himalayan–Hengduan Mountains, has a specialized bamboo diet. Combining morphological and genomic evidence, red pandas have been classified as Ailurus fulgens and Ailurus styani. However, previous studies focused on ecological aspects such as foraging behaviors, habitat use and threats within specific distributions; hence, there is still a gap in quantitative comparative studies on the trophic niches of these two species. In this study, we calculated and compared the isotopic trophic niche widths of A. styani and A. fulgens by measuring carbon and nitrogen stable isotopes in hair keratin of 102 red pandas, then conducting a fecal analysis to explore the diet of a population. The results showed that (1) there was no significant difference in δ13C values between A. styani (−24.73 ± 1.26‰, N = 86) and A. fulgens (−24.43 ± 1.20‰, N = 16); however, A. styani had 1.4‰ higher average δ1⁵N value than that of A. fulgens, and the isotopic niche widths converted by Bayesian ellipse model were A. styani SEAC = 4.40‰2, A. fulgens SEAC = 4.70‰2, which might be caused by their lower and narrower altitude distribution. (2) Niche widths of three genetic populations in A. styani were XXL-LS (5.16‰2), EH-GLG (3.40‰2), QL (2.31‰2), and the comparison of the fecal composition suggested their diet diversity, which might imply a degree of competitive pressure with sympatric animals. (3) Sichuan red pandas occupied a lower trophic position compared to other herbivores but higher than giant pandas in the same ecosystem, and the niche width of red pandas was two times larger than that of sympatric giant pandas, which was related to the foraging strategies and microhabitat selection of the giant and red panda. This research provides basic stable isotopic data for red pandas, offers scientific support for the differentiated and refined conservation and management of the red panda and its habitat, and develops the application of stable isotope analysis in the ecological studies of endangered species. [ABSTRACT FROM AUTHOR]

Abstract Trophic position and niche width are fundamental components of a species’ ecology, reflecting resource use, and influencing key demographic parameters such as somatic growth, maturation, and survival. Concepts about a species’ trophic niche space have important implications for local management and habitat protection, and can shed light about resilience to changing climate for species occurring over broad spatial scales. For elusive marine animals such as sea turtles, trophic niche is challenging to study, and researchers often rely on other metrics, such as isotopic niche, as a proxy. Here, stable isotope analysis (δ13C and δ15N values) was conducted on bulk skin tissue of 718 green turtles (Chelonia mydas) distributed among 16 foraging areas in the eastern Pacific from the USA to Chile, a range spanning ~10,000 km. Compound‐specific nitrogen isotope analysis of amino acids (CSIA‐AA) was applied to 21 turtles among seven sites. Isotopic niche space was determined via Bayesian ellipse area (BEA) and convex hull area (CHA) analyses of bulk isotope values, which were also used along with amino acid δ15N values to determine trophic position (TP). Substantial variability in bulk tissue δ13C and δ15N values was found within and among sites, and amino acid δ15N values confirmed this was largely due to spatial differences in baseline nitrogen isotopic compositions, but also to a lesser extent from TP differences among the green turtle foraging populations. Isotope niche space varied among sites, influenced by the diversity of prey types and relative input of terrestrial‐ vs. marine‐derived nutrients; BEAs were the most suitable measurement of isotopic niche space due to the larger influence of outlying values with the CHA approach. Amino acid isotope‐derived TP estimates that accounted for local habitat conditions (e.g., mixed seagrass/macroalgae diet) performed the best among several approaches; TP ranged from 2.3 to 3.6, which indicates an omnivorous diet for most populations. In addition to providing additional spatial resolution for δ13C and δ15N isoscapes in the eastern Pacific, especially in coastal habitats, this study further establishes CSIA‐AA as an effective tool to study the trophic ecology of sea turtles across a variety of food webs and habitats.

Trophic position and niche width are fundamental components of a species' ecology, reflecting resource use, and influencing key demographic parameters such as somatic growth, maturation, and survival. Concepts about a species' trophic niche space have important implications for local management and habitat protection, and can shed light about resilience to changing climate for species occurring over broad spatial scales. For elusive marine animals such as sea turtles, trophic niche is challenging to study, and researchers often rely on other metrics, such as isotopic niche, as a proxy. Here, stable isotope analysis (δ13C and δ15N values) was conducted on bulk skin tissue of 718 green turtles (Chelonia mydas) distributed among 16 foraging areas in the eastern Pacific from the USA to Chile, a range spanning ~10,000 km. Compound‐specific nitrogen isotope analysis of amino acids (CSIA‐AA) was applied to 21 turtles among seven sites. Isotopic niche space was determined via Bayesian ellipse area (BEA) and convex hull area (CHA) analyses of bulk isotope values, which were also used along with amino acid δ15N values to determine trophic position (TP). Substantial variability in bulk tissue δ13C and δ15N values was found within and among sites, and amino acid δ15N values confirmed this was largely due to spatial differences in baseline nitrogen isotopic compositions, but also to a lesser extent from TP differences among the green turtle foraging populations. Isotope niche space varied among sites, influenced by the diversity of prey types and relative input of terrestrial‐ vs. marine‐derived nutrients; BEAs were the most suitable measurement of isotopic niche space due to the larger influence of outlying values with the CHA approach. Amino acid isotope‐derived TP estimates that accounted for local habitat conditions (e.g., mixed seagrass/macroalgae diet) performed the best among several approaches; TP ranged from 2.3 to 3.6, which indicates an omnivorous diet for most populations. In addition to providing additional spatial resolution for δ13C and δ15N isoscapes in the eastern Pacific, especially in coastal habitats, this study further establishes CSIA‐AA as an effective tool to study the trophic ecology of sea turtles across a variety of food webs and habitats. [ABSTRACT FROM AUTHOR]