Taxonomic revision of the Stoat is still absent and most of the recent checklists still recognise a few dozen (34–37) subspecies (King 1983, Wozencraft 2005), but actual variability is probably much lower. The phylogeography of the Stoat from the Palaearctic and Nearctic regions was investigated based on mitochondrial DNA (Kurose et al. 2005). M. erminea exhibited a very low level of genetic variation, and geographic structure among Palaearctic populations was unclear. Previously the Stoat was united with some other small Mustela species in one group or subgenus. However, M. erminea essentially differs from other species of the genus in cytogenetic and biochemical characters, biology (long delayed implantation), and some morphological characters (Abramov 2000). Molecular data suggest that M. erminea was the first to split from the other Mustela species (Masuda and Yoshida 1994, Kurose et al. 2000).

European regional assessment: Least Concern (LC)
EU 27 regional assessment: Least Concern (LC)

The Stoat (Mustela erminea) is a widespread and abundant species with no significant major threats, hence it is listed as Least Concern for both Europe and for the EU 27 Member States.

The Stoat occurs throughout Europe except southern Europe and the Mediterranean. It is absent from Iceland, Svalbard, some small North Atlantic islands, and the Mediterranean islands. Its vertical range is from sea level to 3,000 m in Europe (Kotia et al. 2011).

The Stoat has a wide Holarctic distribution, covering most of Europe, northern Asia and North America. Outside of Europe, the Stoat occurs from Arctic Siberia south to northern Mongolia and north-eastern China. It is absent in vast arid plains and deserts of Middle and Central Asia. The Stoat has a restricted distribution in the Himalayas, where it is confined to the west in Ladakh (India), Pakistan and Afghanistan. The Stoat is found on many Arctic Islands and some Pacific islands (some Kuril Islands, Sakhalin, Hokkaido and northern Honshu). In North America, it occurs from Alaska, Greenland, and the Canadian Arctic south through most of the northern United States to the Great Lakes region and the southern part of North American Cordillera. The Stoat has been introduced to New Zealand.

In much of its range, the Stoat is widespread and common in suitable habitats. The density and structure of populations of this species are unstable, due to short life spans and high reproductive capacity; populations are greatly influenced by fluctuations in prey supply, especially small mammals (King 1983). Population fluctuations of the Stoat and its prey tend to be more marked at more northerly latitudes (Pulliainen 1999), although fluctuations have also been recorded in Spain (Blanco 1998, Palomo and Gisbert 2002). In Spain, it had been speculated that the population may be declining as a result of declines in the Southern Water Vole Arvicola sapidus (Palomo and Gisbert 2002), but the population trend has not been quantified in Spain or Portugal (Palomo and Gisbert 2002, Cabral et al. 2005). In France, it was declining, but now has stabilised as a result of full protection (EMA Workshop 2006), and the species is not threatened in France (UICN France, MNHN, SFEPM and ONCFS 2017).

In the Russian Federation (whole territory) the number of Stoats slightly declined from 686,400 in 2008 to 545,200 individuals in 2013 (Volodina 2021).

The Stoat is a specialist predator of small mammals (especially rodents and small lagomorphs), however it also occasionally feed on birds’ eggs, fish, lizards, amphibians, invertebrates, berries and carrion (King 1983). It sometimes attacks animals considerably larger than itself, such as adult hares. Food may be stored underground for the winter. The Stoat is found in many habitats, from open tundra to deep forest, but seems to prefer areas with vegetative or rocky cover. The local distribution of Stoats is broadly related to that of small rodents and lagomorphs. They usually avoid dense forest and deserts, and settle in forest edge habitats, scrub, marshes, riparian woodlands, hedgerows, and alpine meadows.

It is a solitary animal – there are no pair bonds between adults. Mating occurs once a year, from March to September – depending on the geographic region. The Stoat has delayed implantation, varying from 240 to 390 days. Litter size averages between six and eight cubs (range 2–18). It is active both day and night. The home range is around 1–200 ha, and varies with sex and region (King and Powell 2007). Population density fluctuates with prey abundance. Under good conditions there may be an individual for every 10 ha. The home ranges of the males includes portions of those of the females. Resident animals of both sexes maintain exclusive territories. Boundaries of these territories are regularly patrolled and scent–marked, and neighbours usually avoid one another. Dens are in hollow trees, burrows, rock crevices, sometimes in nests of prey. It maintains several nests within its range, which are lined with dry vegetation or the fur and feathers of its prey. The Stoat is primarily terrestrial but swims well and easily climbs trees to great heights, where they visit the nests of birds and squirrels. It generally hunts in a zigzag pattern, progressing by series of leaps of up to 50 cm each. It can easily run over the snow, and if pursued, it may move under the snow. It may travel 10–15 km in a night, though the average hunt covers 1.3 km. Its slender body allows it to enter and move quickly through the burrows and tunnels of its prey.

In the Iberian Peninsula, the species is dependent on two Arvicola species, and these are declining, so loss of prey base may be a threat (Palomo and Gisbert 2002). Habitat loss (e.g., as a result of urbanisation: Pulliainen 1999) is also a problem in parts of the range. The species is legally hunted for its fur in Russia. According to the State Hunting Register of Russia (Volodina 2021), in 2017–2021 the official harvest of the Stoat in the whole of Russia was 1,000–1,400 pelts per year. In western and central Europe, the Stoat was frequently hunted for its white winter fur up until at least the 1930s (with c. 30,000 pelts sold in Finland alone during that decade; Pulliainen 1999). Availability of prey is the principal factor controlling population density (King 1983, Pulliainen 1999), but disease, parasites and other pressures can also contribute.

The species is legally hunted for its fur in Russia. In western and central Europe, the Stoat was frequently hunted for its white winter fur up until at least the 1930s.

The Stoat is listed on Appendix III of the Bern Convention. It occurs in many protected areas across its range. Monitoring of exploitation is required by the Bern Convention (R. McDonald pers. comm. 2006). The species is protected under national legislation in some range states (e.g. Spain), although this is not necessarily enforced (Palomo and Gisbert 2002). However, in many parts of its global range the species is not protected and trapping is legal.

Population monitoring and further research into the threats faced is recommended for this species.