Arctic tundra soils are low in available nitrogen and phosphorus, so any concentrated input shows. Fox dens are used repeatedly over decades and sometimes centuries, and the accumulated deposition of urine, faeces and uneaten prey raises soil nutrient concentrations well above the surrounding tundra.
Surveys on Canada's Herschel Island measured markedly greater vegetation biomass on dens than on adjacent control plots, with grasses and forbs replacing sparse tundra cover. The patches are distinct enough to be picked out in aerial and satellite imagery, and they persist after a den falls out of use. Herbivores are drawn to the richer forage, so the foxes alter local grazing patterns as well as plant growth.
The fox's other well-known trait, its winter coat, has a specific measured basis: it has the best insulation recorded for any mammal, and it does not begin to raise its metabolic rate until air temperatures fall far below freezing. The seasonal colour change is genuine, but the blue morph, common on some islands, stays dark all year.