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AbstractTo minimize competitive overlap, carnivores modify one of their critical niche axes: space, time, or resources. However, we currently lack rules for how carnivore communities operate in human-dominated landscapes. We simultaneously quantified overlap in the critical niche axes of a simple carnivore community-an apex carnivore ( Puma concolor ), a dominant mesocarnivore ( Lycalopex culpaeus ), and a subordinate small carnivore ( Lycalopex griseus )-in a human landscape featuring pastoralists and semidomestic carnivores (i.e., dogs, Canis familiaris ). We found that dominant species had strong negative effects on the space use of subordinate ones, which ultimately created space for subordinate small carnivores. Humans and dogs were strictly diurnal, whereas the native carnivore community was nocturnal and exhibited high temporal overlap. Dietary overlap was high among the native carnivores, but dogs were trophically decoupled, largely because of human food subsidies. Our results show that in landscapes with evident human presence, temporal and dietary partitioning among native carnivores can be limited, leaving space as the most important axis to be partitioned among carnivores. We believe that these findings-the first to simultaneously assess all three critical niche axes among competing carnivores and humans and their associated species (i.e., domesticated carnivores)-are transferable to other carnivore communities in human-modified landscapes.

Rationale: Ecologists increasingly determine the δ 15 N values of amino acids (AA) in animal tissue; "source" AA typically exhibit minor variation between diet and consumer, while "trophic" AA have increased δ 15 N values in consumers. Thus, trophic-source δ 15 N offsets (i.e., Δ 15 N T-S ) reflect trophic position in a food web. However, even minor variations in δ 15 N source AA values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDF T-S ). Diet digestibility and protein content can influence the δ 15 N values of bulk animal tissue, but the effects of these factors on AA Δ 15 N T-S and TDF T-S in mammals are unknown., Methods: We fed captive mice (Mus musculus) either (A) a low-fat, high-fiber diet with low, intermediate, or high protein; or (B) a high-fat, low-fiber diet with low or intermediate protein. Mouse muscle and dietary protein were analyzed for bulk tissue δ 15 N using elemental analyzer-isotope ratio mass spectrometry (EA-IRMS), and were also hydrolyzed into free AA that were analyzed for δ 15 N using gas chromatography-combustion-IRMS., Results: As dietary protein increased, Δ 15 N Consumer-Diet slightly declined for bulk muscle tissue in both experiments; increased for AA in the low-fat, high-fiber diet (A); and remained the same or decreased for AA in the high-fat, low-fiber diet (B). The effects of dietary protein on Δ 15 N T-S and on TDF T-S varied by AA but were consistent between variables., Conclusions: Diets were less digestible and included more protein in Experiment A than in Experiment B. As a result, the mice in Experiment A probably oxidized more AA, resulting in greater Δ 15 N Consumer-Diet values. However, the similar responses of Δ 15 N T-S and of TDF T-S to diet variation suggest that if diet samples are available, Δ 15 N T-S accurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ 15 N T-S values. The trophic-source offset of Pro-Lys did not vary across diets, and therefore may be more reliable for omnivores than other offsets (e.g., Glu-Phe)., (© 2021 John Wiley & Sons Ltd.)

Stable isotope analysis has revolutionized the way ecologists study animal resource use from the individual to the community level. Recent interest has emerged in using hydrogen isotopes ( 2 H/ 1 H) as ecological tracers, because they integrate information from both abiotic and biotic processes. A better physiological understanding of how animals assimilate hydrogen and use it to synthesize tissues is needed to further refine this tool and broaden its use in animal ecology. We conducted a controlled-feeding experiment using laboratory mice (Mus musculus) in which we varied the hydrogen isotope (δ 2 H) values of water and the proportions of dietary protein and carbohydrates among nine experimental treatments. For each tissue, we calculated the percent of hydrogen derived from water and the percent hydrogen derived from dietary protein versus carbohydrates using linear relationships and isotope mixing models based on accompanying carbon isotope (δ 13 C) data. The net discrimination (∆ 2 H Net ) between mice tissues and potential water and dietary sources of hydrogen differed among tissues. ∆ 2 H Net was positively correlated with dietary protein content in red blood cells (RBC) and muscle, but negatively correlated in liver and plasma. We also report the first estimates for hydrogen isotope discrimination factors (∆ 2 H) for different sources of hydrogen (∆ 2 H Water , ∆ 2 H Protein , and ∆ 2 H Carbs ) available for tissue synthesis. This research provides a foundation for understanding how diet quality (e.g., protein content) influences hydrogen isotope assimilation and discrimination in different tissues of a terrestrial mammal, which is a first step towards using δ 2 H as a tracer of resource use in free-ranging mammals.