Ecosystem engineering can happen at any scale, and due to its ubiquity, at any trophic level1. In some cases, physical modification of the environment by ecosystem engineers is relatively large compared to other physical processes operating in the ecosystem (e.g. dam building by beavers). However in most instances, ecosystem engineers are working at a more refined scale. Therefore, separating the effects of an ecosystem engineer from other biotic and abiotic factors is challenging. For example, factors such as species diversity, changes in species distribution, and numerous species interactions within an ecosystem make distinguishing between different biotic influences on soil processes difficult2.

Low food web complexity of Arctic biomes, which is primarily due to the bottom-up effect of decreased vegetation diversity and productivity, makes ecosystem engineering easier to study in Arctic communities3. Simultaneously, having low species diversity makes Arctic ecosystems highly susceptible to disturbances such as climate warming and human activities3. The loss (or introduction) of even a single species can cause drastic and cascading effects in Arctic ecosystems4. Therefore, studies on species’ non-trophic impacts (in an ecosystem engineering context) in Arctic biomes are also necessary.

Top predators can drastically change nutrient dynamics of an ecosystem through mechanisms such as decoupling carcass distribution from live-prey distribution. For example brown bears (Ursus arctos) feeding on salmon (Oncorhynchus spp.) redistribute marine-derived nutrients to terrestrial ecosystems, increasing the forest’s total inorganic nitrogen pools threefold5. Carcasses of moose (Alces alces) killed by grey wolves (Canis lupus) also create hot spots with up to 6 times more inorganic soil nutrients6.

The Arctic fox (Vulpes lagopus) has a native circumpolar tundra distribution, ranging from northern Greenland (88°N) to the southern edge of Hudson Bay, Canada (53°N). Arctic foxes are top predators, and within their continental range their main prey are microtine rodents including lemmings (Dicrostonyx and Lemmus spp.) and voles (Microtus and Myodes spp.)7,8,9. In years with low lemming density, Arctic foxes rely on geese and their eggs during summer10, and ringed seal (Phoca hispida) pups and carcasses of seals killed by polar bears (Ursus maritimus) during winter11.

Arctic foxes depend on well-established dens to shelter pups from the harsh Arctic climate and predators12. Suitable denning sites for the Arctic fox are mostly on elevated topographical features (e.g. ridges, banks, mounds, moraines) composed of coarse well-draining sediments, and greater depth to permafrost, allowing for easier excavation13,14. Development of a good den can take many years, with some dens estimated to be hundreds of years old7. However, high-quality den sites are limited15, and digging new dens is energetically costly and mainly done during peak population years16. Climate, soil type, and permafrost also further limit excavations of new dens spatially and temporarily13,12.

Arctic fox litter size averages 8–10 pups in Canada12, so active den sites receive high amounts of nutrients due to urine and faeces deposits as well as nutrient release from the remains of decaying prey items. Due to this nutrient addition, in many Arctic areas, Arctic fox dens have lush green vegetation and are readily spotted across the tundra landscape13,17,18 (Fig. 1). Despite the obvious differences in vegetation growth on Arctic fox dens, studies examining Arctic fox effects on soil are rare. Smith et al.19 found higher soil total nitrogen (N) levels at den sites compared to off-den areas, but no difference in soil total phosphorous (P). On the Aleutian islands, where the Arctic fox is an introduced species, predation of sea birds by Arctic foxes results in lower guano input, thus fox-inhabited islands have lower soil total N and extractable P (plant available P extracted with Bray extractant) percentages compared to fox-free islands20,21. A more thorough analysis of the effect of Arctic foxes as chemical ecosystem engineers on soil nutrient dynamics is necessary for better understanding their functional role in nutrient cycling processes in their native range. Specifically, by analysing soil inorganic N and extractable P (as opposed to the total N and P) and seasonal changes in these nutrients, our objective is to estimate the effect of Arctic fox denning activities on local nutrient dynamics.

Figure 1: Aerial photo of an Arctic fox den in Wapusk National Park, Canada, in August 2014, showing the contrast between the lush green vegetation on dens (dominated by Leymus mollis and Salix planifolia) and the background Dryas heath on beach ridges. For scale, a 1 × 1 m quadrat can be seen in the middle of the den. Full size image

Primary productivity usually varies more within a tundra site than among sites. This high local variation in primary productivity suggests that soil condition is one of main determinants of primary production in Arctic tundra ecosystems22. Primary productivity during short growing seasons in tundra ecosystems is often strongly limited by inorganic N availability in the soil, and followed closely by P, as shown by plant tissue analyses23 and fertilization experiments24,25. Measuring concentrations of the inorganic forms of N and P is necessary for a better understanding of the pool of nutrients available to plants in Arctic tundra where, because of cold temperatures and extremely high or low moisture levels, the decomposition rate of organic material is severely restricted26. Although Arctic soils are often rich in organic material and some Arctic plants can make use of the organic form of N, Arctic soils are still fairly poor medium for plant growth due to the fact that organic N in Arctic soil is mostly in insoluble form, and only a small proportion of the soluble organic N occurs in a form that is useable by Arctic plants27.

We predicted that, due to nutrient addition by Arctic foxes, inorganic N and extractable P levels at den sites would be higher than control sites, and as a result vegetation biomass would be higher at den sites. Additionally, we predicted that due to receiving nutrients from marine and allochtonous resources in Arctic fox diet (such as geese and seals), δ15N values would be elevated in plants growing on fox dens, whereas plants on control sites would have lower δ15N signatures, indicative of locally fixed N sources28,29. We also predicted that dens with pups in the previous year would have higher inorganic N and extractable P levels than dens that did not have pups.