Carnivore Niche Partitioning in a Human Landscape.

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.

Dietary protein content and digestibility influences discrimination of amino acid nitrogen isotope values in a terrestrial omnivorous mammal.

RATIONALE: Ecologists increasingly determine the δ15 N values of amino acids (AA) in animal tissue; "source" AA typically exhibit minor variation between diet and consumer, while "trophic" AA have increased δ15 N values in consumers. Thus, trophic-source δ15 N offsets (i.e., Δ15 NT-S ) reflect trophic position in a food web. However, even minor variations in δ15 Nsource AA values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDFT-S ). Diet digestibility and protein content can influence the δ15 N values of bulk animal tissue, but the effects of these factors on AA Δ15 NT-S and TDFT-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 δ15 N using elemental analyzer-isotope ratio mass spectrometry (EA-IRMS), and were also hydrolyzed into free AA that were analyzed for δ15 N using gas chromatography-combustion-IRMS. RESULTS: As dietary protein increased, Δ15 NConsumer-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 Δ15 NT-S and on TDFT-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 Δ15 NConsumer-Diet values. However, the similar responses of Δ15 NT-S and of TDFT-S to diet variation suggest that if diet samples are available, Δ15 NT-S accurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ15 NT-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).

Assimilation and discrimination of hydrogen isotopes in a terrestrial mammal.

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 (2H/1H) 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 (δ2H) 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 (δ13C) data. The net discrimination (∆2HNet) between mice tissues and potential water and dietary sources of hydrogen differed among tissues. ∆2HNet 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 (∆2H) for different sources of hydrogen (∆2HWater, ∆2HProtein, and ∆2HCarbs) 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 δ2H as a tracer of resource use in free-ranging mammals.