Dietary variation in Icelandic arctic fox (Vulpes lagopus) over a period of 30 years assessed through stable isotopes

Identifying resources driving long-term trends in predators is important to understand ecosystem changes and to manage populations in the context of conservation or control. The arctic fox population in Iceland has increased steadily over a period of 30 years, an increase that has been attributed to an overall increase in food abundance. We hypothesized that increasing populations of geese or seabirds were driving this growth. We analyzed stable isotopes in a long-term series of collagen samples to determine the role of these different resources. The isotopic signatures of arctic foxes differed consistently between coastal and inland habitats. While δ15N displayed a non-linear change over time with a slight increase in the first part of the period followed by a decline in both habitats, δ13C was stable. Stable isotope mixing models suggested that marine resources and rock ptarmigan were the most important dietary sources, with marine resources dominating in coastal habitats and rock ptarmigan being more important inland. Our results suggest that seabirds may have been driving the arctic fox population increase. The rapidly increasing populations of breeding geese seem to have played a minor role in arctic fox population growth, as rock ptarmigan was the most important terrestrial resource despite a considerable decrease in their abundance during recent decades. This study shows that a long-term population trend in a generalist predator may have occurred without a pronounced change in main dietary resources, despite ongoing structural changes in the food web, where one species of herbivorous birds increased and another decreased.

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abstract: Stable isotope analysis has emerged as one of the primary means for examining the structure and dynamics of food webs, and numerous analytical approaches are now commonly used in the field. Techniques range from simple, qualitative inferences based on the isotopic niche, to Bayesian mixing models that can be used to characterize food‐web structure at multiple hierarchical levels. We provide a comprehensive review of these techniques, and thus a single reference source to help identify the most useful approaches to apply to a given data set. We structure the review around four general questions: (1) what is the trophic position of an organism in a food web?; (2) which resource pools support consumers?; (3) what additional information does relative position of consumers in isotopic space reveal about food‐web structure?; and (4) what is the degree of trophic variability at the intrapopulation level? For each general question, we detail different approaches that have been applied, discussing the strengths and weaknesses of each. We conclude with a set of suggestions that transcend individual analytical approaches, and provide guidance for future applications in the field.

title: The Influence of Coastal Access on Isotope Variation in Icelandic Arctic Foxes
abstract: 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.

title: The behavioural ecology of the Arctic fox (Alopex lagopus) in Iceland
abstract: ﻿During the last 20 years studies of various mammals and birds
have shown that social organization is highly variable between and
within species, and that such variation can frequently be explained
on the basis of variation in food habits and food dispersion. This
study is an attempt to discover the major ecological pressures
affecting social organization and reproductive fitness of Arctic
foxes in Iceland.
Foxes were both studied at an individual level, by monitoring
movements of radio-tagged foxes and observation of fox behaviour in
demarcated study areas, and at the population level, by analysis of
available foxhunting records, as well as material and information
provided by foxhunters.
The most common form of social organization was found to be a
breeding pair, while social groups of 3-4 individuals also occur,
particularly in coastal habitat, which is more productive and
spatially more heterogenous with respect to food dispersion, than
inland habitat.
The Arctic foxes were found to occupy well defined, but fairly
flexible group territories, whose borders were advertised by
olfactory, auditory and visual signals. In one coastal region, three
such territories ranged in size from 8.6 km2 to 18.5 km2, while
there was an indication that territories were larger in inland
habitats.
In coastal habitats, foxes feed mainly on oceanic prey and do not
show cycles in abundance. In inland habitat, foxes show a functional
response to the 10 year population cycle of ptarmigan (Lagopus
mutus), their main prey in winter. Litter sizes remain stable,
however, due to little interannual variation in the foxes' summer
diet, which consists mostly of migrant birds.
Mean litter size at birth is 5-3 ± 1.7 S.D. and mortality from
factors other than foxhunting is about 15% up to the age of weaning.
Annual mortality in adults is about 50%, mostly due to foxhunting.
Most vixens breed already as yearlings, while dog-foxes are more
likely to show delayed breeding.
In inland habitats there is balanced polymorphism with regard to
colour morph, but in coastal habitats blue foxes have an overall
selective advantage. Inland foxes are smaller than coastal foxes.
Both size and colour affect reproductive fitness.

title: Improved collagen extraction by modified Longin method
abstract: A re-evaluation of the Longin collagen-extraction method shows that a lower reflux temperature reduces degradation of protein ("collagen") remnants. This allows additional purification through ultrafiltration to isolate the >30kDalton fraction of the reflux product.

title: Stable isotope evidence for similarities in the types of marine foods used by Late Mesolithic humans at sites along the Atlantic coast of Europe
abstract: Human bone collagen stable isotope analysis of humans from coastal Mesolithic sites in Scotland, Denmark, France and Portugal indicates the importance of marine foods in the diet. We define the expected human δ 13 C and δ 15 N values of 100% marine and 100% terrestrial diets and conclude that at most sites isotope variability is due to differing proportions of these defined marine and terrestrial diets, rather than due to differences in the actual types of marine and terrestrial foods exploited. By comparing the European human values with marine faunal values, and values of marine-diet humans from North America, we propose that the marine component of human diet in the Late European Mesolithic was based mainly on marine fish, with only minor contributions from shellfish or marine mammals.

title: Replacement Rates for Human Tissue from Atmospheric Radiocarbon
abstract: Carbon-14, derived from the testing of thermonuclear weapons in the atmosphere of the Northern Hemisphere during 1961-62, has been found in human tissues including the brain in amounts which reflect the atmospheric concentration of carbon-14 as of several months earlier. In collagen of cartilage, the rate of uptake of carbon-14 is much slower than in other tissues; essentially no radioactive carbon was found in the collagen of 70-year-old adults that had been exposed to the comparatively high concentrations of carbon-14 in the atmosphere during the years 1954 to 1964. Individuals from the Southern Hemisphere show little increase in the carbon-14 content of their tissues at present, and detailed tests with individuals traveling to the Northern Hemisphere from the Southern allow closer scrutiny of the tissue replacement rates.

title: Variability in marine resources affects arctic fox population dynamics.
abstract: Terrestrial predators in coastal areas are often subsidized by marine foods. In order to determine the potential impact on terrestrial prey, the numerical response of predators to each food source must be determined. In winter, arctic foxes (Alopex lagopus) may forage on the frozen Arctic ocean and scavenge carcasses of seals killed by polar bears (Ursus maritimus), but the importance of this food source and its effect on the population cycles of arctic foxes and lemmings (their primary prey) are unclear. I estimated the marine component of the late winter diet of arctic foxes near Churchill, Manitoba, using stable-carbon isotope analysis, and compared these estimates to abundance of arctic foxes and collared lemmings (Dicrostonyx richardsoni). From 1994 to 1997, fox density varied with lemming abundance, but following a decline, fox abundance began increasing before lemmings. During this increase marine foods were consumed more than in other years, with over two-thirds of food intake from marine sources. Arctic and red fox (Vulpes vulpes) harvests in the 1980s to 1990s were correlated with published estimates of polar bear body mass, which varies with seal productivity. However, this relationship disappeared during high lemming years. Thus, variation in marine productivity affects arctic fox abundance, especially when their primary prey are scarce, and this numerical response of arctic foxes to marine resources and lemmings suggests that increased predation by arctic foxes subsidized by seal carrion may delay the recovery of low lemming populations.

title: Diet and habitat of the saiga antelope during the late Quaternary using stable carbon and nitrogen isotope ratios
abstract: Saiga antelope ( Saiga tatarica ) is one of the typical late Pleistocene species of the cold and arid mammoth steppe that covered a large area of northern hemisphere. The species is currently endangered and persists only in small areas of Central Asian steppe and desert ecosystems. The investigation of the ecology of the Pleistocene saiga using stable isotope ratios (δ 13 C, δ 15 N) aimed to decipher how different their diet and habitat were from those observed nowadays in relict populations. Up to 76 samples of bone collagen of ancient saiga from Western Europe, Siberia and Eastern Beringia were analysed and compared with 52 samples of hair and bone collagen of modern specimens from Kazahkstan, Russia and Mongolia. The δ 13 C values of the ancient saiga do not exhibit a clear trend over time. They cover the same range of values as the modern ones, from a C3-dominated to a C3-C4-dominated mixed diet (including probably Chenopodiaceae). In contrast, the δ 15 N values of fossil saigas are more variable and lower on average than the extant ones. The lowest δ 15 N values of ancient saiga are found around the Last Glacial Maximum, reflecting the influence of the cold conditions at that time. On the other hand, fossil saiga occupying the same regions as the historical and modern populations exhibit high δ 15 N values similar to the modern ones, confirming ecological continuity over time. Modern saiga is thus occupying just one of its potential diverse habitats they used in the past. Therefore, the extant saiga is not a refugee species confined to a suboptimal habitat. During the late Pleistocene, the saiga occupied a separate niche compared with the other ungulates of the mammoth steppe. However, this species could also adapt to a lichen-dominated diet normally seen in reindeer, leading to an isotopic overlap between the two species in south-western France and Alaska around the Last Glacial Maximum. This adaptation allowed a geographical expansion that does not correspond to a habitat-tracking episode. Hence, the realized niche currently observed for the saiga is reduced compared with their potential capacity for adaptation, a crucially important factor for the conservation of this endangered species.

title: Carnivore-specific stable isotope variables and variation in the foraging ecology of modern and ancient wolf populations: case studies from Isle Royale, Minnesota, and La Brea
abstract: We use carbon and nitrogen isotope data collected from two North American gray wolf (Canis lupus L., 1758) populations (Isle Royale and northern Minnesota) to both calculate carnivore-specific isotopic variables and investigate wolf foraging ecology. The isotopic enrichments of 13C and 15N that occur between mammalian carnivores and their prey have not been well defined in modern populations. We use bone collagen from the Isle Royale National Park wolf, moose (Alces alces (L., 1758)), and beaver (Castor canadensis Kuhl, 1820) populations to determine trophic enrichment factors of 1.3‰ ± 0.6‰ for δ13C and 4.6‰ ± 0.7‰ for δ15N. We apply these carnivore-specific fractionation factors to a case study from the fossil record, and reconstruct the diets of late-Pleistocene dire wolves (Canis dirus (Leidy, 1858)) from the La Brea tar pits. We use the Minnesota wolf tissue (collagen, hair, muscle) isotopic data to estimate carnivore population subsample sizes needed to replicate the mean values of the whole populat...

title: Collagen-to-collagen prey-predator isotopic enrichment (Δ13C, Δ15N) in terrestrial mammals - a case study of a subfossil red fox den
abstract: Trophic Enrichment Factor (TEF) is the main parameter used in isotopic trophic ecology. TEF values can be derived from specimens subjected to experimental feeding or from free-ranging specimens whose dietary behavior is well monitored, and it can be measured for different tissues of animal body. Direct collagen-to-collagen TEF is a key parameter for fossil material and needs to be well constrained in order to ascertain the reliability of the palaeodietary models. In this paper, we present isotopic results for a subfossil bone accumulation related to red fox (Vulpes vulpes) activity, discovered in an abandoned mine in Potok-Senderki (Poland). The objective was to report δ13C and δ15N collagen data for red foxes and their prey. These data were used to calculate a prey-predator collagen-to-collagen TEF and provided important information for interpreting stable isotope fractionation in terrestrial food webs. We used different taphonomic indexes to calculate the fox mean diet. The presence of juvenile and adult individuals of fox allowed us to specify the difference in isotopic enrichment according to the age class of the predator. Δ13C and Δ15N values calculated here for fox were similar to TEF values presented previously for wolf and lynx, but characterized by wider standard deviation.

title: Intrapopulation Variability Shaping Isotope Discrimination and Turnover: Experimental Evidence in Arctic Foxes
abstract: Tissue-specific stable isotope signatures can provide insights into the trophic ecology of consumers and their roles in food webs. Two parameters are central for making valid inferences based on stable isotopes, isotopic discrimination (difference in isotopic ratio between consumer and its diet) and turnover time (renewal process of molecules in a given tissue usually measured when half of the tissue composition has changed). We investigated simultaneously the effects of age, sex, and diet types on the variation of discrimination and half-life in nitrogen and carbon stable isotopes (δ15N and δ13C, respectively) in five tissues (blood cells, plasma, muscle, liver, nail, and hair) of a top predator, the arctic fox Vulpes lagopus.  We fed 40 farmed foxes (equal numbers of adults and yearlings of both sexes) with diet capturing the range of resources used by their wild counterparts. We found that, for a single species, six tissues, and three diet types, the range of discrimination values can be almost as large as what is known at the scale of the whole mammalian or avian class. Discrimination varied depending on sex, age, tissue, and diet types, ranging from 0.3‰ to 5.3‰ (mean  = 2.6‰) for δ15N and from 0.2‰ to 2.9‰ (mean  = 0.9‰) for δ13C. We also found an impact of population structure on δ15N half-life in blood cells. Varying across individuals, δ15N half-life in plasma (6 to 10 days) was also shorter than for δ13C (14 to 22 days), though δ15N and δ13C half-lives are usually considered as equal.  Overall, our multi-factorial experiment revealed that at least six levels of isotopic variations could co-occur in the same population. Our experimental analysis provides a framework for quantifying multiple sources of variation in isotopic discrimination and half-life that needs to be taken into account when designing and analysing ecological field studies.

title: Stable isotope analysis: modelling lipid normalization for muscle and eggs from arctic mammals and birds
abstract: Lipids are more depleted in 13 C than proteins. Variable lipid contents in tissues affect therefore the measurements of stable carbon isotope ratios. Model based (also called mathematical) normalization has been suggested to correct d 13 C values using the ratio of carbon to nitrogen (C ⁄ N) as a proxy for lipid content. This approach has not been thoroughly validated for terrestrial animals and it is not clear to what extent it is generally applicable or species ⁄ tissue specific. 2. Ratios of stable carbon isotopes (d 13 C) were obtained for muscle samples of 22 mainly terrestrial arctic mammal and bird species and for egg samples of 32 bird species from nine sites in the circumpolar Arctic. We used linear and nonlinear equations to model the difference in d 13 C between samples from which lipids had been extracted chemically and bulk tissue samples. Models were compared on the basis of a model selection criterion (AIC) and of prediction error as estimated by cross-validation. 3. For muscle samples, a linear and a nonlinear equation performed equally well. The observed values were also well predicted by a previously published general equation for aquatic organisms. For egg samples, a nonlinear equation fitted separately to waterfowl and non waterfowl bird species fitted the data best. Prediction errors were, however, larger than for muscle samples. 4. The generality of the inferred normalization equations was assessed by applying them to a second data set from a similar ecosystem, but produced in the frame of another study. The predicted lean d 13 Cv alues were within 0AE5& of the observed values for 73% of the muscle samples, but only for 27% of the egg samples. 5. Based on our results, we recommend model based normalization of d 13 Cv alues as an economic way to deal with varying lipid contents in muscle samples of mammals and birds. For egg samples, on the contrary, model based predictions had large errors. Therefore, we recommend chemical lipid extraction in order to estimate lipid-freed 13 C values for egg content.

title: Model selection and multimodel inference : a practical information-theoretic approach
abstract: The second edition of this book is unique in that it focuses on methods for making formal statistical inference from all the models in an a priori set (Multi-Model Inference). A philosophy is presented for model-based data analysis and a general strategy outlined for the analysis of empirical data. The book invites increased attention on a priori science hypotheses and modeling. Kullback-Leibler Information represents a fundamental quantity in science and is Hirotugu Akaike's basis for model selection. The maximized log-likelihood function can be bias-corrected as an estimator of expected, relative Kullback-Leibler information. This leads to Akaike's Information Criterion (AIC) and various extensions. These methods are relatively simple and easy to use in practice, but based on deep statistical theory. The information theoretic approaches provide a unified and rigorous theory, an extension of likelihood theory, an important application of information theory, and are objective and practical to employ across a very wide class of empirical problems. The book presents several new ways to incorporate model selection uncertainty into parameter estimates and estimates of precision. An array of challenging examples is given to illustrate various technical issues. This is an applied book written primarily for biologists and statisticians wanting to make inferences from multiple models and is suitable as a graduate text or as a reference for professional analysts.

title: The blunting of Occam's Razor, or to hell with parsimony
abstract: The Principle of Parsimony, and the apparently related matter of the supposed inadmissibility of untestable hypotheses, are examined in the light of certain examples taken from the history of scien...

title: Sea-ice use by arctic foxes in northern Alaska
abstract: The extensive use of sea-ice by three arctic foxes (Alopex lagopus) in northern Alaska was documented using satellite telemetry during the winter of 2005–2006. Here we present the first detailed data on movements of individual foxes while on the sea-ice. Two juvenile males and one juvenile female traveled long distances (904, 1,096, and 2,757 km) and remained on the sea-ice for extended periods of time (76, 120, and 156 days). Average distances traveled per day ranged from 7.5 to 17.6 km and foxes achieved maximum rates of travel of up to 61 km/day. These findings verify the use of sea-ice by arctic foxes and raise concerns that the diminishing arctic ice cover may negatively impact populations by limiting access to marine food sources.

title: Northern nomads: ability for extensive movements in adult arctic foxes
abstract: In July 2008 we outfitted reproductively active adult arctic foxes with satellite tracking collars on Bylot Island, Nunavut, Canada and recorded their movements over a complete annual cycle. We present the tracking data from two individuals, one female and one male, who traveled extensively from February to July 2009, covering minimum distances of 4,599 and 2,193 km, respectively. We recorded high and sustained travel rates on both land and sea ice that reached 90 km/day for the female and 88 km/day for the male. Our data confirm that arctic foxes can move extensively and demonstrate sustained travel rates that are 1.5 times those previously measured for the species. Our study is the first presenting detailed year-round satellite tracking of adult arctic foxes and has implications for our understanding of navigational abilities, foraging ecology, trophic interactions with lemming populations, and genetic population structure of arctic foxes.

title: Arctic fox dispersal from Svalbard to Canada: one female’s long run across sea ice
abstract: We report the first satellite tracking of natal dispersal by an Arctic fox (Vulpes lagopus) between continents and High-Arctic ecosystems. A young female left Spitsbergen (Svalbard Archipelago, Norway) on 26 March 2018 and reached Ellesmere Island, Nunavut, Canada, 76 days later, after travelling a cumulative distance of 3506 km, bringing her ca. 1789 km away (straight-line distance) from her natal area. The total cumulative distance travelled during the entire tracking period, starting when she left her natal area on 1 March 2018 and ending when she settled on Ellesmere Island on 1 July 2018, was 4415 km. This is among the longest dispersal events ever recorded for an Arctic fox. Crossing extensive stretches of sea ice and glaciers, the female moved at an average rate of 46.3 km/day ± 41.1 SD. The maximum movement rate was 155 km/day and occurred on the ice sheet in northern Greenland. This is the fastest movement rate recorded for this species. The northernmost location recorded was on the sea ice off northern Greenland at a latitude of 84.7°N. The Arctic fox was of the blue colour morph typical for coastal environments, where Arctic foxes are adapted to food webs without lemmings but with substantial inputs of marine food resources. The Arctic fox settled on Ellesmere Island in a food web with lemmings, thereby switching ecosystems. Our observation supports evidence of gene flow across Arctic regions, including those seasonally bridged by sea ice, found in studies of the circumpolar genetic structure of Arctic fox populations. 
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title: The status and trends of seabirds breeding in Norway and Svalbard
abstract: ...................................................................................................................................... 3