The Influence of Coastal Access on Isotope Variation in Icelandic Arctic Foxes

To quantify the ecological effects of predator populations, it is important to evaluate how population-level specializations are dictated by intra- versus inter-individual dietary variation. Coastal habitats contain prey from the terrestrial biome, the marine biome and prey confined to the coastal region. Such habitats have therefore been suggested to better support predator populations compared to habitats without coastal access. We used stable isotope data on a small generalist predator, the arctic fox, to infer dietary strategies between adult and juvenile individuals with and without coastal access on Iceland. Our results suggest that foxes in coastal habitats exhibited a broader isotope niche breadth compared to foxes in inland habitats. This broader niche was related to a greater diversity of individual strategies rather than to a uniform increase in individual niche breadth or by individuals retaining their specialization but increasing their niche differentiation. Juveniles in coastal habitats exhibited a narrower isotope niche breadth compared to both adults and juveniles in inland habitats, and juveniles in inland habitats inhabited a lower proportion of their total isotope niche compared to adults and juveniles from coastal habitats. Juveniles in both habitats exhibited lower intra-individual variation compared to adults. Based on these results, we suggest that foxes in both habitats were highly selective with respect to the resources they used to feed offspring, but that foxes in coastal habitats preferentially utilized marine resources for this purpose. We stress that coastal habitats should be regarded as high priority areas for conservation of generalist predators as they appear to offer a wide variety of dietary options that allow for greater flexibility in dietary strategies

Map of Iceland with the number of arctic foxes (adults|juveniles) that were sampled in coastal (≤3 km from the shore line) and inland (≥10 km from the shore line) habitats in each province.

<p>We accounted for potential spatial autocorrelation in isotope values from foxes from the same province by adding it as a random term in statistical models.</p

Individual isotope niche breadth (A), individual variation in individual isotope niche breadth (B) and the ratio of individual isotope niche breadth to population isotope niche breadth (C) of adult and juvenile foxes from coastal (open symbols) and inland (closed symbols) habitat on Iceland.

<p>Individual isotope niche breadth was estimated as the Euclidean distance between collagen and muscle within individuals, between individual variation in individual isotope niche breadth was calculated as the Euclidean distance of each individual difference between muscle and collagen to group centroids in a 2 dimensional isotope space, and the individual specialization index relates individual niche breadth (WIC) to the total isotope niche breadth of each sample group (TNW). We calculated this proxy for WIC/TNW (following nomenclature of Roughgarden <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032071#pone.0032071-Roughgarden1" target="_blank">[1]</a>) as the ratio of the average Euclidean distances of muscle and collagen samples to within individual centroids and the average Euclidean distances to group centroids. Groups were in all cases defined as age classes within each habitat. Figures presents mean ± 1 SE.</p

Isotope niche breadth of adult and juvenile foxes from coastal (open symbols) and inland (closed symbols) habitats on Iceland.

<p>Isotope niche breadth was estimated as the Euclidian distances to group centroids in a 2 dimensional isotope space formed by δ¹³C and δ¹³N. Figure presents mean ± 1 SE.</p

Results from a linear mixed model on the effects of habitat (coastal or inland), age of animal (adult or juvenile) and tissue (fur, muscle and collagen) on the Euclidean distance to group centroids in a 2 dimensional isotope space formed by respective δ¹³C and δ¹⁵N values in Icelandic arctic foxes.

<p>These distances can be interpreted as a measure of population wide isotope niche breadth.</p

Results from linear mixed models on the effects of habitat (coastal or inland), age of animal (adult or juvenile) and tissue (fur, muscle and collagen) on δ¹³C and δ¹⁵N in Icelandic arctic foxes.

<p>Results from linear mixed models on the effects of habitat (coastal or inland), age of animal (adult or juvenile) and tissue (fur, muscle and collagen) on δ¹³C and δ¹⁵N in Icelandic arctic foxes.</p

Average δ¹³C and δ¹⁵N values of potential prey available in coastal and inland habitats in Iceland.

1<p>) Data from interior Alaska <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032071#pone.0032071-Dalerum3" target="_blank">[56]</a>.</p><p>The prey table is not comprehensive and data are not intended for quantitative analyses, but rather to exemplify the wider isotope niche width that is available in coastal habitats.</p

Results from linear mixed models on the effects of habitat (coastal or inland) and age of animal (adult or juvenile) on three attributes of individual variation in δ¹³C and δ¹⁵N.

<p>Within individual isotope niche breadth was estimated as the Euclidean distance between collagen and muscle within individuals, between individual variation in individual isotope niche breadth was calculated as the Euclidean distance of each individual difference between muscle and collagen to group centroids in a 2 dimensional isotope space, and an individual specialization index that relates intra individual variation to the total isotope niche breadth of each sample group, calculated as the ratio of the average Euclidean distances of muscle and collagen samples to within individual centroids and the average Euclidean distances to group centroids. Groups were in all cases defined as age classes within each habitat.</p

Biplots of δ¹³C and δ¹³N values of fur (A, B), muscle (C, D) and collagen (E, F) samples from adult and juvenile arctic foxes from coastal (open symbols) and inland (closed symbols) habitats on Iceland.

<p>Biplots of δ¹³C and δ¹³N values of fur (A, B), muscle (C, D) and collagen (E, F) samples from adult and juvenile arctic foxes from coastal (open symbols) and inland (closed symbols) habitats on Iceland.</p