Strength of asymmetric competition between predators in food webs ruled by fluctuating prey: the case of foxes in tundra

In food webs heavily influenced by multi‐annual population fluctuations of key herbivores, predator species may differ in their functional and numerical responses as well as their competitive ability. Focusing on red and arctic fox in tundra with cyclic populations of rodents as key prey, we develop a model to predict how population dynamics of a dominant and versatile predator (red fox) impacted long‐term growth rate of a subdominant and less versatile predator (arctic fox). We compare three realistic scenarios of red fox performance: (1) a numerical response scenario where red fox acted as a resident rodent specialist exhibiting population cycles lagging one year after the rodent cycle, (2) an aggregative response scenario where red fox shifted between tundra and a nearby ecosystem (i.e. boreal forest) so as to track rodent peaks in tundra without delay, and (3) a constant subsidy scenario in which the red fox population was stabilized at the same mean density as in the other two scenarios. For all three scenarios it is assumed that the arctic fox responded numerically as a rodent specialist and that the mechanisms of competition is of a interference type for space, in which the arctic fox is excluded from the most resource rich patches in tundra. Arctic fox is impacted most by the constant subsidy scenario and least by the numerical response scenario. The differential effects of the scenarios stemmed from cyclic phase‐dependent sensitivity to competition mediated by changes in temporal mean and variance of available prey to the subdominant predator. A general implication from our result is that external resource subsidies (prey or habitats), monopolized by the dominant competitor, can significantly reduce the likelihood for co‐existence within the predator guild. In terms of conservation of vulnerable arctic fox populations this means that the likelihood of extinction increases with increasing amount of subsidies (e.g. carcasses of large herbivores or marine resources) in tundra and nearby forest areas, since it will act to both increase and stabilize populations of red fox.     

Invasion of Siberian Pine Populations in Mountain Tundra in the Northern Urals

A new method for determining the upper forest border (UFB) as the border of the “root-closed” stand and regrowth as its potential vanguard is proposed. The mass dispersal of Pinus sibirica populations by the nutcracker (Nucifraga caryocatactes) in the UFB ecotone in the Northern Urals (Pavdinskii Kamen’ and Tretii Bugor Mountain) from the middle-mountain taiga to the mountain forest tundra and tundra at a distance reaching 1 km is found. It is done based on the original reconstruction method of the dynamics of population number in P. sibirica annual seedlings and on an analysis of their relations with the dynamics of the seed bearing and population number of the nutcracker. The trustworthy consortive relations of the number of Pinus sibirica generations in the mountain forested tundra and tundra zones, with its seed bearing and nutcracker number in the previous year in the middle-mountain taiga zone, are found. A significant increase, 3.7 times, in the number of seedling generations during the last 49 years is found in connection with the increase in the summer air temperature in the subarctic to 2.0°C. A hypothesis of the formation of the united genetic metapopulation of P. sibirica in the forest–tundra mountain ecotone zone as a consequence of its seed ornitochory from the highly different populations is formulated. A forecast of the mosaic formation of the vanguard “thin forests” in 20–25 years and the root-closed curtains of the forest in 40–50 years in the mountain tundra of the Northern Urals at the current rate of the rise in temperature is made.     

Population persistence in a landscape context: the case of endangered arctic fox populations in Fennoscandia

Anthropogenic fragmentation of habitat and populations is recognized as one of the most important factors influencing loss of biodiversity. Since it is difficult to quantify demographic parameters in small populations, we need alternative methods to elucidate important factors affecting the viability of local populations. The Fennoscandian arctic fox inhabits a naturally fragmented alpine tundra environment, but historic anthropogenic impacts have further fragmented its distribution. After almost 80 yr of protection, the population remains critically endangered. Both intrinsic factors (related to the isolation and size of sub‐populations) and extrinsic factors (related to environmental conditions influencing patch quality and interspecific competition) have been proposed as explanations for the lack of population growth. To distinguish between these hypotheses, we conducted a spatially explicit analysis that compares areas where the species has persisted with areas where it has become locally extinct. We used characteristics of the fragments of alpine tundra habitat and individual arctic fox breeding dens (including both currently active dens and historically active dens) within the fragments to evaluate the importance of habitat characteristics and connectivity in explaining variation in persistence within a fragment. The number of reproductive events in a fragment was related to the size of the fragment, but not more than expected following a 1:1 relationship, suggesting little effect of fragment size on the relative number of reproductions. The likelihood of a den being used for breeding was positively associated with factors minimising interspecific competition as well as increasing within‐fragment connectivity. These results support the idea that the failure of Fennoscandian arctic fox to recover is caused by demographic factors that can be related to fine‐scale Allee or Allee‐like effects, as well as environmental influences related to increased competition and exclusion by red foxes.     

Red fox takeover of arctic fox breeding den: an observation from Yamal Peninsula, Russia

Here, we report from the first direct observation of a red fox (Vulpes vulpes) intrusion on an arctic fox (Vulpes lagopus) breeding den from the southern Arctic tundra of Yamal Peninsula, Russia in 2007. At the same time, as a current range retraction of the original inhabitant of the circumpolar tundra zone the arctic fox is going on, the red fox is expanding their range from the south into arctic habitats. Thus, within large parts of the northern tundra areas the two species are sympatric which gives opportunities for direct interactions including interference competition. However, direct first-hand observations of such interactions are rare, especially in the Russian Arctic. In the present study, we observed one red fox taking over an arctic fox breeding den which resulted in den abandonment by the arctic fox. On July 19, eight arctic fox pups were observed on the den before the red fox was observed on the same den July 22. The pups were never seen at the den or elsewhere after the red fox was observed on the den for as long as we stayed in the area (until August 10). Our observation supports the view that direct interference with red fox on breeding dens may contribute to the range retraction of arctic foxes from the southern limits of the Arctic tundra in Russia.     

内蒙古达赉湖地区赤狐生境选择及生境景观特征分析

近年来,干旱、鼠害及人为对草原的过度开发利用等对达赉湖地区赤狐的数量及分布范围造成了很大影响。借助于主成分分析、Bailey’s判别分析、遥感和地理信息系统对达赉湖自然保护区赤狐的生境选择及生境的景观特征进行了研究。在赤狐家域内采用样线法和样方法,共设置10 m×10 m样方245个(实验样方101个,对照样方144个),并测定样方内的7个生境因子:植被类型、隐蔽级、食物丰富度、雪深、距水源距离、距围栏距离、距居民点距离。对样方内数据进行主成分分析得出,食物和隐蔽性是赤狐生境选择的主要因子,距居民点距离和距围栏距离是赤狐生境选择的次要因子,雪深和距水源距离是赤狐在生境选择中未表现出选择和利用的因子。利用Bailey’s方法和基于赤狐的生境分布图的景观统计得出:赤狐偏好选择柳灌丛和芦苇塘两种生境,这两种生境的总面积约为1093.47 km2,占研究区总面积的14.05%;赤狐随机利用河道、典型草原和草甸草原3种生境类型,这3种生境的总面积约为4721.79 km2,占研究区总面积的60.67%。;回避的生境为冰面和沙化草地,总面积约为1968.09 km2,占研究区总面积的25.29%。基于地理信息系统和Fragstats的景观特征分析得出,赤狐最适宜生境的面积最少,斑块数量最多,平均斑块面积最小,平均形状指数最小,平均斑块距离最大;其次是较适宜生境和不适宜生境;一般适宜生境的面积最多,斑块数量最少,平均斑块面积和平均形状指数最大,平均斑块距离最小。赤狐多分布于斑块较大的适宜其生存的生境或分布于由这些斑块形成的生境斑块镶嵌体中。     

Natal den selection by sympatric arctic and red foxes on Herschel Island,Yukon, Canada

In the twentieth century, red fox (Vulpes vulpes) expanded into the Canadian Arctic, where it competes with arctic fox (Vulpes lagopus) for food and shelter. Red fox dominates in physical interactions with the smaller arctic fox, but little is known about competition between them on the tundra. On Hershel Island, north Yukon, where these foxes are sympatric, we focused on natal den choice, a critical aspect of habitat selection. We tested the hypothesis that red fox displaces arctic fox from dens in prey-rich habitats. We applied an approach based on model comparisons to analyse a 10-year data set and identify factors important to den selection. Red fox selected dens in habitats that were more prey-rich in spring. When red foxes reproduced, arctic fox selected dens with good springtime access, notably many burrows unblocked by ice and snow. These provided the best refuge early in the reproductive season. In the absence of red foxes, arctic foxes selected dens offering good shelter (i.e. large isolated dens). Proximity to prey-rich habitats was consistently less important than the physical aspects of dens for arctic fox. Our study shows for the first time that red foxes in the tundra select dens associated primarily with prey-rich areas, while sympatric arctic foxes do not. These results fit a model of red fox competitively interfering with arctic fox, the first detailed study of such competition in a true arctic setting.     

How ecological neighbourhoods influence the structure of the scavenger guild in low arctic tundra

Aim How the ecological neighbourhoods of coast and forest affect arctic tundra ecosystems is a pressing question as the circumpolar tundra belt is shrinking under global warming. Mobile facultative scavengers are likely to negatively impact tundra biodiversity as dominant competitors or predators, if they spill over into tundra. Here, we provide the first quantitative assessments of the structure of a scavenger guild in low arctic tundra with emphasis on how it changes along spatial gradients from neighbouring ecosystems (i.e. forest and coast) and with altitude (i.e. productivity gradients). We also assess the likelihood of interactions between guild members that may negatively impact vulnerable tundra species. Location North‐eastern part of Norway. Methods Extensive records of scavenger prevalence were obtained by deploying automatic digital cameras at experimental carcasses in tundra regions covering several thousand square kilometres and three winters in northern Norway. Main conclusions We found short‐range neighbourhood effects of forest and coast within the tundra scavenger guild. Species richness declined steeply with decreasing distance from the neighbouring ecosystems, in particular subarctic forest, and with increasing altitude. Bird species with strongholds in forest (golden eagle Aquila chrysaetos and hooded crow Corvus cornix) or along the coast (white‐tailed eagle Haliaeetus albicilla) were mostly responsible for short‐range neighbourhood effects on guild structure. However, the two most abundant guild members, the common raven Corvus corax and the red fox Vulpes vulpes, exhibited no spatial patterns within the range of neighbourhoods and altitudes examined. There was a clear diurnal segregation in the use of carcasses between birds and mammals reducing the likelihood of direct interactions between these two taxa. Presence of red fox appeared to exclude the arctic fox Vulpes lagopus, the only endemic tundra species within the guild, from carcasses.     

Scavenger community structure along an environmental gradient from boreal forest to alpine tundra in Scandinavia

Scavengers can have strong impacts on food webs, and awareness of their role in ecosystems has increased during the last decades. In our study, we used baited camera traps to quantify the structure of the winter scavenger community in central Scandinavia across a forest–alpine continuum and assess how climatic conditions affected spatial patterns of species occurrences at baits. Canonical correspondence analysis revealed that the main habitat type (forest or alpine tundra) and snow depth was main determinants of the community structure. According to a joint species distribution model within the HMSC framework, species richness tended to be higher in forest than in alpine tundra habitat, but was only weakly associated with temperature and snow depth. However, we observed stronger and more diverse impacts of these covariates on individual species. Occurrence at baits by habitat generalists (red fox, golden eagle, and common raven) typically increased at low temperatures and high snow depth, probably due to increased energetic demands and lower abundance of natural prey in harsh winter conditions. On the contrary, occurrence at baits by forest specialists (e.g., Eurasian jay) tended to decrease in deep snow, which is possibly a consequence of reduced bait detectability and accessibility. In general, the influence of environmental covariates on species richness and occurrence at baits was lower in alpine tundra than in forests, and habitat generalists dominated the scavenger communities in both forest and alpine tundra. Following forecasted climate change, altered environmental conditions are likely to cause range expansion of boreal species and range contraction of typical alpine species such as the arctic fox. Our results suggest that altered snow conditions will possibly be a main driver of changes in species community structure.     

Effects of long‐term fox baiting on species composition and abundance in an Australian lizard community

Abstract We report on the effects of almost a decade of 1080‐fox baiting on a lizard community in a mosaic Australian habitat. Replicated comparisons of baited versus non‐baited control areas with near‐identical histories of bush fires, grazing and climate showed a higher density of red fox tracks (Canis vulpes) in the non‐baited areas. Furthermore, the fox‐baited areas showed a more than five times higher density of sand goannas (Varanus gouldii), a species that strongly overlaps the red fox in food niche breadth and is itself a direct target of fox predation, in particular its eggs and young. Exclusion of predators from a natural habitat led to significant increases in the density of small lizards, suggesting that predation can drive lizard population dynamics in this ecosystem. Replicated pitfall‐trapping in three habitats in the control areas (with high fox and low goanna density) versus the baited areas (with low fox and high goanna density) showed that fox baiting had positive effects on the density of diurnal scincid lizards in open grassland, whereas the control areas showed higher density of nocturnal gecko lizards. Our interpretation is that fox removal may result in a shift in the top predator towards the sand goanna. Historically, this indigenous, endemic species was the natural top predator. It has co‐evolved with its prey and that may have moulded it into a more efficient lizard predator per encounter than the introduced fox.     

The ecology of the meadow pipit <Emphasis Type="Italic">Anthus pratensis</Emphasis> (Linnaeus, 1758) at the northeastern limit of the breeding area

The ecology of the meadow pipit was studied for many years in nature in the Lower Ob region and on the Yamal Peninsula. Experimental studies were carried out in laboratory conditions at natural and alternative day durations. The factors affecting the distribution of the species at the northeastern limit of its area were analyzed. The meadow pipit is widespread in the taiga zone; it does not need special adaptations for its existence in forest tundra and southern taiga. However, the species has no adaptations that would permit it to exist in northern tundra. Current climate changes define the northern border of the area. The species has no obvious obstacles for spreading to the east.     

Dynamics of voles and small mustelids in the taiga landscape of northern Fennoscandia in relation to habitat quality

In the forests of northern Fennoscandia during the I980's, the dynamics of microtine rodents changed from multiannual high amplitude fluctuations (cycles) to, depending on species, fluctuations with a strong seasonal component or fluctuations with smaller amplitude and lower frequency. Microtine and predator data from the Pallasjarvi area, Finnish Lapland, suggest that this transition took place at different rates in different parts of the taiga landscape. Generally, densities in forest habitats have been primarily seasonal since 198S-86. In mesic spruce taiga and in drier forest habitats microtines had a prolonged peak in 1981-83 and a crash in 1984-83. At the timberline, however, microtine populations dropped from peak to low densities already in 1982-83 but the final crash did not occur until spring 1985. The synchronous decrease in microtines densities in all habitat types in 1984-85 coincided with increase in weasel activity. Activity of other carnivores was consistently high in mesic lowland habitats. The data support following three conjectures. 1) Periodic abundance of least weasels is crucial for sustained vole cycles. 2) Predominance of stoats and other generalist predators lead to less regular fluctuations with a strong seasonal component where density declines occur in autumn and early winter. 3) In barren tundra areas, the vegetation cannot sustain high densities of microtines and. consequently, predation is not a necessary condition for population crashes.     

Habitat loss exacerbates regional extinction risk of the keystone semiarid shrub <Emphasis Type="Italic">Ziziphus lotus</Emphasis> through collapsing the seed dispersal service by foxes (<Emphasis Type="Italic">Vulpes vulpes</Emphasis>)

Habitat loss and landscape degradation affect animal-mediated seed dispersal, often collapsing the regeneration of endangered plant species and habitats in anthropogenic landscapes. We first compared the role of red fox and other vertebrates as seed disperser for the keystone scrub Ziziphus lotus. Because it turned out that foxes are the major Z. lotus dispersers, we investigated how fox activity and dispersal service relate to habitat loss and landscape alteration in the threatened Ziziphus semiarid scrublands, a priority habitat for conservation in Europe. Considering its opportunistic behavior, we hypothesized that landscape features should affect moderately fox abundance, while influence in a large extent its dispersal service. Accordingly, we predicted that a substantial decline in Ziziphus fruit consumption rather than in disperser activity would be responsible for seed dispersal collapse under severe habitat loss. We evaluated fox activity and dispersal service in 17 populations of Z. lotus spread through the range of its habitat in Spain and found within landscapes with different land-use intensity. We certified the collapse of the dispersal service by fox under severe habitat loss and confirmed that fox activity was less affected by habitat loss or landscape alteration than consumption of Ziziphus fruits. Consequently, the decline of consumption of Ziziphus fruits under severe habitat loss triggers the collapse of its seed dispersal. Results suggest that without increase of the remnant areas other managements may not suffice to achieve seed dispersal and habitat restoring. Dispersal service and natural regeneration in many Ziziphus habitat remnants will possibly cease in the future if habitat loss continues.     

Biogeneous carbon fluxes in the boreal forests of Central Siberia

The assessments of the carbon pool and rate of plant biomass production, phytodetritus destruction, new formations of humic matters, and removal of water-soluble decomposition products for the forest ecosystems of the forest tundra and the northern and southern parts of the Central Siberian taiga were given. The rates of the main processes (organic-matter production and degradation) were demonstrated to be balanced in the ecosystems of the forest tundra. The larch forests of the northern taiga serve as a stock for a C atmosphere, which are equivalent to 32–34% of net primary production (NPP). The secondary birch growth where the understory needle-leaved trees have been formed and the primary old-growth fir forests are characterized by the balance of the main carbon fluxes in the southern taiga. The birch forests where the understory trees are just being formed and the fir forests at the age of 50–90 years serve as a stock for an average of 26% of carbon extracted as dioxide to make NPP.     

Forces structuring tree squirrel communities in landscapes fragmented by agriculture: species differences in perceptions of forest connectivity and carrying capacity

The North American red squirrel ( Tamiasciurus hudsonicus ) has expanded its range into the central hardwoods of the United States in conjunction with increasing forest fragmentation and declining gray squirrel ( Sciurus carolinensis ) populations. We used translocation experiments and patch occupancy data to test for interspecific differences in mobility and sensitivity to habitat loss and modification by agriculture. We released squirrels in fencerows to test the hypothesis that gray squirrels display inferior mobility relative to red and fox ( S. niger ) squirrels. Elapsed time to movement from fencerows for 76 individuals increased with distance to forest patches and harvesting of crops. Gray and red squirrels took longer to move from fencerows than fox squirrels, and gray squirrels were less successful at moving from fencerows than red and fox squirrels. Ecologically scaled landscape indices revealed the degree to which interspecific differences in mobility and individual area requirements accounted for the occurrence of these species across landscapes. Gray squirrel distribution was constrained both by individual area requirements and dispersal ability. Occurrence of red and fox squirrels was related to patch size but was unaffected by landscape connectivity.     

Zonal types of host-parasite complexes of arthropods and small mammals in a flat part of Western Siberia

Within a flat part of Western Siberia distribution of the small mammals and parasitic arthropods connected with them is accompanied by formation zonal host-parasite complexes dated for corresponding landscape zones or subzones: tundra complex dated for a tundra zone; forest complex dated for a forest zone (including subzones northern taiga, middle taiga, southern taiga and subtaiga); forest-steppe complex dated for a northern forest-steppe subzone; steppe complex dated for a southern forest-steppe subzone and steppe. Parasitic specificity of each type of a complex is defined by arthropods of different systematical and ecological groups: a tundra complex is defined by epizoite gamasid mites (Mesostigmata), forest and forest-steppe complexes is defined by tick (Ixodides) and fleas (Siphonaptera), steppe complex is defined by fleas and nidicolous gamasid mites.     

Interpretation of the last‐glacial vegetation of eastern‐central Europe using modern analogues from southern Siberia

Aim Interpretation of fossil pollen assemblages may benefit greatly from comparisons with modern palynological and vegetation analogues. To interpret the full‐ and late‐glacial vegetation in eastern‐central Europe we compared fossil pollen assemblages from this region with modern pollen assemblages from various vegetation types in southern Siberia, which presumably include the closest modern analogues of the last‐glacial vegetation of central Europe. Location Czech and Slovak Republics (fossil pollen assemblages); Western Sayan Mountains, southern Siberia (modern pollen assemblages). Methods Eighty‐eight modern pollen spectra were sampled in 14 vegetation types of Siberian forest, tundra and steppe, and compared with the last‐glacial pollen spectra from seven central European localities using principal components analysis. Results Both full‐ and late‐glacial pollen spectra from the valleys of the Western Carpathians (altitudes 350–610 m) are similar to modern pollen spectra from southern Siberian taiga, hemiboreal forest and dwarf‐birch tundra. The full‐glacial and early late‐glacial pollen spectra from lowland river valleys in the Bohemian Massif (altitudes 185–190 m) also indicate the presence of patches of hemiboreal forest or taiga. Other late‐glacial pollen spectra from the Bohemian Massif suggest an open landscape with steppe or tundra or a mosaic of both, possibly with small patches of hemiboreal forest. Main conclusions Our results are consistent with the hypothesis that during the full glacial and late glacial, the mountain valleys of the north‐western Carpathians supported taiga or hemiboreal forest dominated by Larix, Pinus cembra, Pinus sylvestris and Picea, along with some steppic or tundra formations. Forests tended to be increasingly open or patchy towards the west (Moravian lowlands), gradually passing into the generally treeless landscape of Bohemia, with possible woodland patches in locally favourable sites.     

Reproduction and seedling establishment of Picea glauca across the northernmost forest‐tundra region in Canada

The northern boundary of boreal forest and the ranges of tree species are expected to shift northward in response to climate warming, which will result in a decrease in the albedo of areas currently covered by tundra vegetation, an increase in terrestrial carbon sequestration, and an alteration of biodiversity in the current Low Arctic. Central to the prediction of forest expansion is an increase in the reproductive capacity and establishment of individual trees. We assessed cone production, seed viability, and transplanted seedling success of Picea glauca (Moench.) Voss. (white spruce) in the early 1990s and again in the late 2000s at four forest stand sites and eight tree island sites (clonal populations beyond present treeline) in the Mackenzie Delta region of the Northwest Territories, Canada. Over the past 20 years, average temperatures in this region have increased by 0.9 °C. This area has the northernmost forest‐tundra ecotone in North America and is one of the few circumpolar regions where the northern limit of conifer trees reaches the Arctic Ocean. We found that cone production and seed viability did not change between the two periods of examination and that both variables decreased northward across the forest‐tundra ecotone. Nevertheless, white spruce individuals at the northern limit of the forest‐tundra ecotone produced viable seeds. Furthermore, transplanted seedlings were able to survive in the northernmost sites for 15 years, but there were no signs of natural regeneration. These results indicate that if climatic conditions continue to ameliorate, reproductive output will likely increase, but seedling establishment and forest expansion within the forest‐tundra of this region is unlikely to occur without the availability of suitable recruitment sites. Processes that affect the availability of recruitment sites are likely to be important elsewhere in the circumpolar ecotone, and should be incorporated into models and predictions of climate change and its effects on the northern forest‐tundra ecotone.     

末次冰期以来北海道、萨哈林岛及亚洲东北部落叶松林演变史

萨哈林岛（库叶岛）和北海道的末次冰期沉积丰富的落叶松花粉,为了搞清楚落叶松花粉百分比和实际森林密度的关系,我们系统地分析了从苔原,森林苔原以及泰加林的表土样品,根据孢粉资料恢复了两个岛屿3万年以来的植被和气候,两个岛上3万年前后 以云杉与落叶松共生的森林为主,松和冷杉现在比现代相对较冷和干的气候条件下,萨哈林在末次冰盛期的草原和泰加林主要由落叶松和松组成,和现在西伯利亚东北部的泰加林相似,在北海道末次冰盛期的草原和泰加林主要由落叶松,松以及云杉组成,气候更加干冷,在新仙女木期森林苔原分布到中萨哈林岛,而北海道的草原和泰加林以落叶松为主,气候可能比末次冰盛期还要冷,在1万年前两个岛的落叶松含量都开始下降,6000年前在北海道消失,萨哈林岛的落叶松则残存至今。     

Foraging ecology and spatial behaviour of the red fox (Vulpes vulpes) in a wet grassland ecosystem

We investigated diet composition, habitat selection and spatial behaviour of the red fox (Vulpes vulpes) in relation to the availability of wader nests in a coastal polder area in southwest Denmark. The predatory role of the red fox in wet grassland ecosystems has profound implications for conservation status of declining populations of grassland breeding waders. However, few studies have focussed on the foraging ecology and behaviour of the red fox in these landscapes. Faecal analyses revealed that fox diet consisted of birds (43 % of prey remains?/?32 % of biomass), rodents (39 %?/?21 %), sheep (mainly as carrion, 14 %?/?41 %) and lagomorphs (4 %?/?7 %). Charadriiformes (including waders) comprised 3–12 % of prey remains throughout the year. Telemetry data and spotlight counts indicated that foxes did not select areas with high densities of breeding waders, suggesting that foxes did not target wader nests while foraging. Foxes maintained stable home ranges throughout their lives, indicating that the area sustained a permanent fox population all year round. The population densities, estimated from spotlight surveys, were 0.74 visible foxes km^?2 (95 % CI; 0.34–1.61) on the preferred breeding habitat for waders and 1.21 km^?2 in other open habitats such as cultivated fields. Our results indicate that red fox predation on wader nests is incidental, consistent with the notion that red foxes are generalist predators that opportunistically subsist on many prey groups.     

Arctic fox versus red fox in the warming Arctic: four decades of den surveys in north Yukon

During the last century, the red fox (Vulpes vulpes) has expanded its distribution into the Arctic, where it competes with the arctic fox (Vulpes lagopus), an ecologically similar tundra predator. The red fox expansion correlates with climate warming, and the ultimate determinant of the outcome of the competition between the two species is hypothesized to be climate. We conducted aerial and ground fox den surveys in the northern Yukon (Herschel Island and the coastal mainland) to investigate the relative abundance of red and arctic foxes over the last four decades. This region has undergone the most intense warming observed in North America, and we hypothesized that this climate change led to increasing dominance of red fox over arctic fox. Results of recent surveys fall within the range of previous ones, indicating little change in the relative abundance of the two species. North Yukon fox dens are mostly occupied by arctic fox, with active red fox dens occurring sympatrically. While vegetation changes have been reported, there is no indication that secondary productivity and food abundance for foxes have increased. Our study shows that in the western Arctic of North America, where climate warming was intense, the competitive balance between red and arctic foxes changed little in 40?years. Our results challenge the hypotheses linking climate to red fox expansion, and we discuss how climate warming’s negative effects on predators may be overriding positive effects of milder temperatures and longer growing seasons.