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The marine side of a terrestrial carnivore: intra-population variation in use of allochthonous resources by arctic foxes - PubMed

The marine side of a terrestrial carnivore: intra-population variation in use of allochthonous resources by arctic foxes

Arnaud Tarroux et al. PLoS One. 2012.

Abstract

Inter-individual variation in diet within generalist animal populations is thought to be a widespread phenomenon but its potential causes are poorly known. Inter-individual variation can be amplified by the availability and use of allochthonous resources, i.e., resources coming from spatially distinct ecosystems. Using a wild population of arctic fox as a study model, we tested hypotheses that could explain variation in both population and individual isotopic niches, used here as proxy for the trophic niche. The arctic fox is an opportunistic forager, dwelling in terrestrial and marine environments characterized by strong spatial (arctic-nesting birds) and temporal (cyclic lemmings) fluctuations in resource abundance. First, we tested the hypothesis that generalist foraging habits, in association with temporal variation in prey accessibility, should induce temporal changes in isotopic niche width and diet. Second, we investigated whether within-population variation in the isotopic niche could be explained by individual characteristics (sex and breeding status) and environmental factors (spatiotemporal variation in prey availability). We addressed these questions using isotopic analysis and bayesian mixing models in conjunction with linear mixed-effects models. We found that: i) arctic fox populations can simultaneously undergo short-term (i.e., within a few months) reduction in both isotopic niche width and inter-individual variability in isotopic ratios, ii) individual isotopic ratios were higher and more representative of a marine-based diet for non-breeding than breeding foxes early in spring, and iii) lemming population cycles did not appear to directly influence the diet of individual foxes after taking their breeding status into account. However, lemming abundance was correlated to proportion of breeding foxes, and could thus indirectly affect the diet at the population scale.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Map of study area showing locations of capture sites relative to the goose colony and fox dens.

Locations of capture sites for breeding (•) and non-breeding (○) arctic foxes, monitored denning sites (X), and estimated average extent of the goose nesting colony during our study from 2003 to 2008 on Bylot Island (73°N, 80°W), Nunavut, Canada.

Figure 2
Figure 2. Temporal availability of various food sources used by arctic foxes in the study area on Bylot Island, Canada.

We used this phenological information to determine which prey was included in the mixing models for each fox dietary periods (Spring, Early-, and Mid-Summer). We assumed that δ13C and δ15N in whole blood represented the average diet during the previous month, hence the 1-month lag between dietary periods and their corresponding fox sampling periods (see methods and Fig. S1). Shaded areas are periods of availability (prey) or presence (sea ice).

Figure 3
Figure 3. Temporal variation of the isotopic system and the corresponding relative contribution of marine resources to individual arctic fox diets on Bylot Island, Canada.

Left panel – Isotopic biplots of the isotopic signature (δ13C, δ15N) of arctic foxes and their potential prey sampled between 2003 and 2008. Dashed grey lines show the 95% CI dispersion ellipses based on standard deviation of foxes’ isotopic ratios for each period of the pup rearing season. Prey sample sizes are indicated in parentheses, unless identical to the previous period (see also Table S1). Spring: diet from mid-April to mid-May; Early-Summer: mid-May to mid-June; Mid-Summer: mid-June to mid-July (Fig. 2). Right panel – Corresponding SIAR output distributions of the relative proportion of marine sources (seal) in the reconstructed diet of each individual and by period. We show the mean (white dot) as well as the 50, 75, and 95% Credible Intervals (dark gray, light gray, and white boxes, respectively) of the SIAR posterior probability distributions. For each period, continuous and dotted lines (in blue) show the mean and 95% Credible Intervals at the population level, respectively.

Figure 4
Figure 4. Comparison of δ15N (‰) within arctic fox breeding pairs.

Dots above the line show pairs where the male had a higher δ15N than the female. Pearson’s r = 0.67, t = 2.57, df = 8, p = 0.033.

Figure 5
Figure 5. Seasonal variation in δ15N (mean ‰ ±95% CI) of male and female arctic foxes.

Average δ15N (‰ ±95% CI) of arctic foxes on Bylot Island, Nunavut, based on their breeding status and period of the pup rearing season. Spring: mid-April to mid-May; Early-Summer: mid-May to mid-June; Mid-Summer: mid-June to mid-July (Fig. 2). Numbers in parentheses indicate sample sizes and all data from years 2003–2008 were pooled, except those from four individuals whose breeding status could not be determined (Table 1).

Figure 6
Figure 6. Proportion of breeding foxes vs. lemming trapping index during the study period.

Proportion of breeding foxes captured annually as a function of the lemming snap-trapping index. The curve represents predictions from a generalized linear mixed-effects model fitted to the data (±1SE, shaded area around the curve). Small vertical bars (gray) represent the original data for breeders (top) or non-breeders (bottom) to which the model was fitted. The bars were jittered (randomly displaced over small distances on the X-axis) in order to better show data concentration. The open circles show the actual proportion of breeders for a given year/lemming index value. Total number of foxes captured each year is also available in Table 1. Data include only year 2004 to 2008 and lemming abundance data from 2004 to 2006 are drawn from Morrissette et al. , based on our long term monitoring of lemming abundance on Bylot Island (see methods for details).

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Grants and funding

This study was partly funded by an National Sciences & Engineering Research Council Northern Research Internship to A. Tarroux. This study was also supported by (alphabetical order): Canada Foundation for Innovation, Canada Research Chairs, Centre d'Études Nordiques, Environment Canada, Fonds Québécois de la Recherche sur la Nature et les Technologies, Indian and Northern Affairs Canada, Natural Sciences and Engineering Research Council of Canada (NSERC), Networks of Centres of Excellence of Canada ArcticNet, Nunavut Wildlife Management Board, Parks Canada, Polar Continental Shelf Program, Université du Québec à Rimouski (UQAR), and Université Laval. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.