«Stoat Mustela erminea Steve Csurhes and Anna Markula First published 2010 Updated 2016 © State of Queensland, 2016. The Queensland Government ...»
Department of Agriculture and Fisheries Invasive animal risk assessment
Steve Csurhes and Anna Markula
First published 2010
© State of Queensland, 2016.
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I n v a s i v e a n i m a l r i s k a s s e s s m e n t : Stoat (Mustela erminea) 2 Contents Summary 4 Identity and taxonomy 5 Description 6 Biology and ecology 6 Origin and distribution 9 Status in Australia and Queensland 10 History as a pest elsewhere 10 Risk of introduction 11 Pest potential in Queensland 11 Eradication 12 The ‘Bomford numerical risk assessment’ 12 Attachment 15 I n v a s i v e a n i m a l r i s k a s s e s s m e n t : Stoat (Mustela erminea) 3 Summary This study assessed the potential for stoats (Mustela erminea) to become an invasive pest in Queensland. It used an “evidence-based” approach to pest risk assessment where published information on a species’ biology, ecology and history as a pest elsewhere was used to make reasonable predictions of potential impact and probability of naturalisation in Queensland.
This study presents evidence that stoats could naturalise in cooler upland areas of southern Queensland, where climate is marginally suitable. They are not predicted to survive in other areas of the state, since such areas are too hot.
Potential impacts may be comparable to impacts experienced in New Zealand where stoats are a major predator of native birds.
The risk of introduction into the state is low.
Worldwide, there are few examples of successful eradication of introduced mammals (other than on small islands). Once established, the impacts of invasive species are generally irreversible. As such, preventing the entry, sale and possession of stoats in Queensland is wise.
M. erminea is very similar in appearance to Mustela frenata (long-tailed weasel). They can be distinguished by the lack of a black tip on the weasel’s tail (Wikipedia 2009).
There are a number of subspecies: M. e. aestiva, M. e. alascensis, M. e. algiricus, M. e. anguinae, M. e. angustidens, M. e. artica, M. e. audax, M. e. bangsi, M. e. celenda, M. e. cicognanni, M. e. cocognanii, M. e. erminea, M. e. fallenda, M. e. ferghanae, M. e. gulosa, M. e. haidarum, M. e. herminea, M. e. hibernica, M. e. imperii, M. e. initis, M. e. invicta, M. e. kadiacensis, M. e. kanei, M. e. labiata, M. e. leptus, M. e. lymani, M. e. microtis, M. e. minima, M. e. mortigena, M. e. muricus, M. e. nippon, M. e. olympica, M. e. orientalis, M. e. polaris, M. e. pusilla, M. e. richardsonii, M. e. rixosa, M. e. salva, M. e. seclusa, M. e. semplei, M. e. streatori, M. e. transbaikalica, M. e. vulgaris, M. e. whiteheadi (Wikipedia 2009; ZipcodeZoo 2009).
Stoats exhibit sexual dimorphism. Males are usually twice the size of females, weighing 67– 116 g and females 25–80 g (Animal Diversity Web 1999; Hellstedt & Henttonen 2006). Adult body length (from head to rump) can vary from 170–330 mm. The tail is 42–120 mm long, about 35% of the total body length (Animal Diversity Web 1999).
Their claws are sharp and non-retractile; ears are short, rounded, and set almost flat into the fur; eyes are round, black and slightly protruding; whiskers are very long; and the muzzle is black and dog-like. The body fur is short, normally a rich chestnut-brown on the head and back, and white or cream (sometimes shading to yellow or even to apricot) on the underside.
The tail has a black tip, which may be bristled out into a ‘bottlebrush’ when the animal is excited. The black tip is used as a decoy to predators (ISSG 2006; Wikipedia 2009).
During winter, the coat becomes thicker and the colour changes to clean white. An individual may be referred to as an ‘ermine’ when the fur is white and a ‘stoat’ when the fur is brown (Wikipedia 2009).
Stoats are very active for short periods of time. They “move rapidly, investigating every hole and crevice, and often stop to survey the surroundings by raising the head or standing upright on the hind legs. They may glide along with the body extended almost straight, taking many rapid steps with the short legs, or if alarmed, they gallop with great leaps, with the back arched” (King 1983).
Biology and ecology Life history Gestation period: 43 days Young per birth: 6-7 Birth interval: 12 months Weaning: 45 days Sexual maturity: 95 days females, 365 days males Sexual activity: unknown Life span: up to 12.5 years in captivity, 2−3 years in the wild (AnAge Database undated; ISSG 2006) Stoats are solitary animals and females generally only accept males when they are in oestrus.
A female may mate with more than one male and litters can be fathered by at least three different males. After mating, the female becomes aggressive towards the male and the male has no part in rearing the young (King et al. 2007).
Nests are made in hollow trees, rock piles or burrows (King 1983). Young stoats grow rapidly and can hunt with their mother by eight weeks of age (Nowak et al. 2005).
6 Stoats breed once a year, during September and November in New Zealand. The reproductive cycle is strictly controlled by the ratio of light-to-dark hours and is not dependent on prey availability. Research in New Zealand found that simulating the onset of summer by increasing day length significantly advanced seasonal reproduction in both male and female stoats (King et al. 2007; O’Connor et al. 2006).
Stoats have a unique aspect to their reproductive cycle; a period of embryonic diapause.
Two weeks after fertilisation the developing embryo stops growing for around eight to nine months, before continuing its development (O’Connor et al. 2006). When prey is abundant, females have higher reproductive success, whereas prey shortages cause an increase in embryo and nestling mortality (King et al. 2003).
The mating system is promiscuous. Female stoats exhibit extreme juvenile precocity, with female cubs mated in the nest before their eyes open. Up to 90% or more of females may be pregnant at any one time. In England, practically all female stoats are pregnant by the end of June (Hellstedt & Henttonen 2006; King et al. 2003; Wikipedia 2009).
Male cubs mature later than females, at 10–11 months. The average life span is less than 12 months in both sexes, due to high juvenile mortality of 55– 92%. Therefore, both sexes must survive to one year old to produce or father any young, and those that survive their first year have a good chance of living two to three years (ISSG 2006; King et al. 2003). Females tend to live longer than males, and generally experience two breeding seasons, whereas males only survive one (CentralPets.com undated).
For further information on reproduction in stoats, see King et al. (2007).
Social organisation Stoats are solitary animals and males and females only associate in the mating season.
They are territorial with defined home ranges. Home range size is dependent on the area, season and prey density. Male home ranges are usually twice the size of females, and territory boundaries are marked with scent. Dominant animals scent mark more frequently than subordinates and scent marking can resolve conflicts between individuals (Erlinge et al. 1982). Home range size for males can vary from 4–200 hectares and are usually 10–40 hectares (Reid & Helgen 2008). When prey is abundant, home ranges are smaller and travelling distance reduced. Stoats regularly visit all parts of their range, scent-marking and hunting. They can travel one to eight kilometres in a single hunt (King 1983).
During spring and summer, young and adult males become more active, establishing or extending home ranges, or dispersing in search of breeding territories. They have been recorded travelling as far as 35 km (King 1983). Stoats are also capable of long distance dispersal on land and can cover up to 60 km in a few weeks (ISSG 2006).
In the Northern Hemisphere, stoats are usually diurnal in summer, their behaviour changes in autumn, and by winter they are nocturnal (Hellstedt & Henttonen 2006).
The density and structure of stoat populations are unstable, due to their short life span and high reproductive capacity. Population size fluctuates markedly and depends on prey abundance. Stoats compete intraspecifically for resources and interspecifically with weasels (Mustela nivalis). In areas without weasels, stoats are smaller; about 70 g (Hellstedt & Henttonen 2006; Reid & Helgen 2008).
I n v a s i v e a n i m a l r i s k a s s e s s m e n t : Stoat (Mustela erminea) 7 Diet Stoats are specialist predators of small, warm-blooded vertebrates, generally mammals the size of rabbits or water voles and smaller. They evolved to hunt unstable populations of small rodents in the boreal regions of the northern hemisphere (King et al. 2003). Despite this specialisation, they have a broad, opportunistic diet, tending to eat whatever is seasonally available.
In New Zealand, stoats have been recorded to consume birds, feral house mice, rabbits, rats, possums, insects, lizards, fish, crayfish, carrion and rubbish (ISSG 2006), with a preference for birds and rabbits (Murphy & Dowding 1994). In their native range they usually eat small mammals such as mice, rabbits, pikes, rats and squirrels, as well as porcupines, frogs, eggs, insects and fish (CentralPets.com, undated). In Great Britain, their diet consists of lagomorphs (65%), small rodents (16%) and birds and bird eggs (17%) including domestic poultry and game birds (McDonald et al. 2000).
Stoats can utilise alternative food resources when their usual prey is absent. A study in the alpine regions of Italy found that fruit such as juniper berries and bilberries were more common, or made up the majority of the stoat’s diet, when rodent prey was scarce (Martinoli et al. 2001). In very cold climates, stoats can hunt under snow for small rodents and lemmings (Animal Diversity Web 1999).
Stoats can kill animals much larger than themselves. When they have identified potential prey, they approach as close as possible before quickly grasping the back of the head and neck. The stoat wraps its body and feet around the prey, and kills it by repeated bites to the base of the skull (Animal Diversity Web 1999).
Stoats often ‘surplus kill’, meaning they will kill a higher number of prey items than they can eat at one time, storing the extra food for later. Due to their high energy expenditure, they require a daily quantity of food equivalent to 19−32% of their body weight (SUNY-ESF 2009).
Figure 1. Stoat standing upright on rear legs Photo: Steve Hillebrand, USFWS.
Image from Wikimedia Commons in the Public Domain.
8 Preferred habitat Stoats are habitat generalists found wherever suitable prey is available. Habitats include successional or forest-edge habitats, scrub, alpine meadows, riparian woodlands, hedgerows, riverbanks, tundra, agricultural areas, coastland, wetlands, grasslands, disturbed areas, and human settlements such as villages and suburban gardens. Only deserts and dense forests are avoided (ISSG 2006; Reid & Helgen 2008).
Stoats can readily colonise offshore islands as they can swim up to 1.5 km across sea (Wikipedia 2009). In open habitats, they use vegetation and other cover to avoid predation, and in alpine areas spend much of their time in runs and burrows under the snow (ISSG 2006). They are skilful tree climbers and can descend trunks head-first (Wikipedia 2009).
Stoats are found from sea level to elevations of 3000 m (King 1983). In New Zealand they inhabit high altitude alpine grasslands, despite the lack of diverse small mammal communities that they would normally rely on in their native range (Smith et al. 2007).
Predators and disease Stoats are preyed upon by almost any carnivore that is large enough to eat them. For example, wolves, foxes, wolverines, coyotes, domestic cats, badgers, and some birds of prey (Wikipedia 2009).
A nematode (Skrjabingylus nasicola) is a major cause of death. This parasite causes skull deformity by eroding bones of the nasal sinuses, which is thought to lead to pressure on the brain. Infection rates may be as high as 100% in certain parts of the stoat’s native range.
Stoats are also susceptible to fleas, ticks, lice and mites (McDonald & Larivière 2001).
Diseases include canine distemper virus, rabies, Aujeszky’s disease, Lyme disease, plague, and leptospirosis.
Stoats act as a reservoir for Mycobacterium bovis, the bacteria that causes bovine tuberculosis (McDonald & Larivière 2001).
Origin and distribution Stoats are found almost throughout northern temperate, subarctic, and Arctic regions of Europe, Asia and North America, from Greenland and the Canadian and Siberian Arctic islands south to about 35º N (Reid & Helgen 2008; Wikipedia 2009). They are considered native to Afghanistan; Albania; Andorra; Austria; Azerbaijan; Belarus; Belgium; Bosnia and Herzegovina; Bulgaria; Canada; China; Croatia; Czech Republic; Denmark; Estonia;
Finland; France; Georgia; Germany; Greece; Hungary; India; Ireland; Italy; Japan; Kazakhstan;
Kyrgyzstan; Latvia; Liechtenstein; Lithuania; Luxembourg; Macedonia, the former Republic of Yugoslav; Moldova; Mongolia; Montenegro; Netherlands; Norway; Pakistan; Poland;
Portugal; Romania; Russian Federation; Serbia; Slovakia; Slovenia; Spain; Sweden;
Switzerland; Tajikistan; Turkey; Ukraine; United Kingdom; United States; and Uzbekistan.
They have naturalised in New Zealand (Reid & Helgen 2008).
Stoats are widespread and abundant with the total adult population size exceeding 100,000.