The main threats to species are habitat destruction and the effects of non-native species (Van Dyke, Reference Van Dyke2003) but on oceanic islands, where species have evolved in isolation, the introduction of alien species is considered the principal threat to native species (Courchamp et al., Reference Courchamp, Chapuis and Pascal2003; Whittaker & Fernández-Palacios, Reference Whittaker and Fernández-Palacios2007). A wide variety of invasive plants and animals have been introduced to island ecosystems (Lever, Reference Lever1994; Williamson, Reference Williamson1996) but mammalian species, such as feral cats Felis silvestris catus, dogs Canis familiaris and rats Rattus spp., are the most problematic (Courchamp et al., Reference Courchamp, Chapuis and Pascal2003; Nogales et al., Reference Nogales, Rodríguez-Luengo and Marrero2006).

The Eurasian ferret Mustela furo has been introduced to New Zealand, Australia, UK, USA, Canada, Sicily, Sardinia and other countries (Long, Reference Long2003; GISD, 2005). Ferrets are kept as pets and used for hunting rabbits but escapees can form feral populations. On the Atlantic Ocean islands feral ferrets occur on the Azores (Mathias et al., Reference Mathias, Ramalhinho, Santos-Reis, Petrucci-Fonseca, Libois and Fons1998) and Madeira (Mathias, Reference Mathias1993). Introduced ferrets are considered an invasive species when they have detrimental effects on native species and cause economic damage (Clapperton, Reference Clapperton2001; Byrom, Reference Byrom2002; Courchamp et al., Reference Courchamp, Chapuis and Pascal2003). Ferrets consume a variety of native prey, including birds and their eggs, mammals, reptiles and invertebrates (Roser & Lavers, Reference Roser and Lavers1976; Smith et al., Reference Smith, Ragg, Moller and Waldrup1995; Clapperton, Reference Clapperton2001).

Ferrets were probably brought to the Canary Islands, where they are commonly used for hunting rabbits, in the 16th century. Occasionally ferrets are not recaptured after hunting but the species has not been included as a naturalized species in the official species list of the Canary Islands' Government (Izquierdo et al., Reference Izquierdo, Martín, Zurita and Arechavaleta2004) or in a list of the 13 invasive, non-native mammals in the Archipelago (Nogales et al., Reference Nogales, Rodríguez-Luengo and Marrero2006). Long (Reference Long2003) mentioned that ferrets were probably introduced to the Canary Islands but his original source (Encyclopaedia Britannica, 1976–1978) for this statement may be incorrect as there is not any other literature on this subject. In addition, this source incorrectly indicates that two species of native bat are naturalized.

The terrestrial animal, plant and fungal diversity of the Canary Islands has a high percentage of endemic species (27.5%; Martín et al., Reference Martín, Marrero, Zurita, Arechavaleta and Izquierdo2005), some of which are threatened by invasive species such as feral cats, rats Rattus rattus and Rattus norvegicus, hedgehogs Atelerix algirus, Barbary ground squirrels Atlantoxerus getulus, goats Capra hircus and Barbary sheep Ammotragus lervia (Nogales et al., Reference Nogales, Rodríguez-Luengo and Marrero2006). Recently, information about predation by ferrets on poultry in the north of La Palma Island, and some ferret road casualties, came to our attention. Here, we report evidence of a naturalized population of ferrets on La Palma Island and discuss the requirement for a control programme.

The 728 km² La Palma (Fig. 1) is a high (2,426 m altitude) and steep island. Its climate is influenced by the humid north-east trade winds, altitude and orientation (Afonso, Reference Afonso and Afonso1985), and this has resulted in heterogeneous environments (del Arco et al., Reference del Arco, Acebes, Pérez de Paz and Marrero1999). Details of the vegetation of La Palma are provided by Santos (Reference Santos1983), Pérez de Paz et al. (Reference Pérez de Paz, del Arco-Aguilar, Rodríguez-Delgado, Acebes-Ginovés, Marrero-Gómez and Wildpret de la Torre1994) and del Arco et al. (Reference del Arco, Acebes, Pérez de Paz and Marrero1999).

Fig. 1 (a) The location of the Canary Islands in the Atlantic Ocean. (b) The location of La Palma in the archipelago. (c) La Palma, showing the 28 localities where ferrets Mustela furo have been observed during 1998–2007 (see numbered locations in Table 1).

In addition to our own observations and personal communications from other researchers, we compiled data from published and grey literature on the Canary Islands, and from electronic databases using the keywords Mustela furo, ferrets, Canary Islands, islands, exotic species and introduction. We found a total of 45 records of dead or living wild ferrets for 1998–2007 in 28 localities on La Palma (Table 1). The species appears to be widely distributed in the north (Fig. 1) and there is an apparently isolated population at El Paso (Fig. 1). Although most of the records are from 2003 and later the locations suggest that ferrets became established earlier than the first record in 1998 and/or originated from several escapees. The ferrets in El Paso could spread into the nearby La Caldera de Taburiente National Park. During 2007 a pair with three young was seen in Briestas (location 28 in Table 1 and Fig. 1; J.C. Barreto & W. Camacho, pers. comms) confirming that ferrets are breeding in the wild. The 45 records are in forests and in cultivated and rural areas, which is unsurprising given that ferrets are generalists (Clapperton, Reference Clapperton2001). In rural areas rabbits Oryctolagus cuniculus, the availability of which influences ferret distribution and abundance (Ragg & Moller, Reference Ragg and Moller2000), are available as prey (Cabrera-Rodríguez, Reference Cabrera-Rodríguez1997), and suitable food and shelter are available in other areas (Ragg & Moller, Reference Ragg and Moller2000; Morley, Reference Morley2002).

Table 1 Locations (see numbers on Fig. 1), dates and numbers of ferrets Mustela furo observed on La Palma Island during 1998–2007, with details of observers and any comments.

Ferreting is a long-held tradition in the Canary Islands and is regulated by law. The use of unmuzzled ferrets for hunting is forbidden but there is little or no regulatory compliance. If a ferret goes astray the majority of hunters dedicate a lot of time trying to recapture the individual. However, it is the few uncaptured individuals that have presumably established the feral population on La Palma. It has been suggested that ferrets cannot survive for long in the wild because of road deaths and because they may be captured on farms if they prey on poultry (Umbach, Reference Umbach1997). However, ferrets can survive and reproduce if they have suitable winter resting sites (Ragg & Moller, Reference Ragg and Moller2000; Baghli et al., Reference Baghli, Walzberg and Vehagen2005) and prey is available (Morley, Reference Morley2002).

Live-traps are commonly used to capture medium-size mammals (Courchamp et al., Reference Courchamp, Chapuis and Pascal2003) and have been used to control ferrets in New Zealand (Cross et al., Reference Cross, Smale, Bettany, Numata, Nelson, Keedwell and Ragg1998). A preliminary control programme was implemented on La Palma (J.C. Barreto & W. Camacho, pers. comms) during 2006. During January–April 36 live-capture traps (60 × 30 × 30 cm) were placed in lines along vegetation boundaries and paths at Las Tricias (location 14, Fig. 1) and eight ferrets were captured. In addition, two ferrets were captured in live traps at Santo Domingo (location 18) and Don Pedro (location 19) in August and November 2006, respectively. Further such control programmes need to be implemented to halt the spread of ferrets and to minimize damage to native biota. Other methods have been proposed for control of ferrets and other mustelids, such as anaesthetic immobilization (Fournier-Chambrillon et al., Reference Fournier-Chambrillon, Chusseau, Dupunch, Maizeret and Fournier2003) and toxic bait (Spurr et al., Reference Spurr, Ogilvie, Morse and Young2005), but these methods could have effects on non-target species (Courchamp et al., Reference Courchamp, Chapuis and Pascal2003).

Ferrets are generalist and opportunist feeders and are able to modify their diet in response to changes in prey abundance and availability (Clapperton, Reference Clapperton2001). The main negative impact of ferrets is expected to be on ground-nesting birds such as the white-tailed pigeon Columba junoniae and the woodcock Scolopax rusticola. Ferrets may also prey upon the endemic lizard subspecies Gallotia galloti palmae.

The ferret has the potential to become naturalized and spread to the other islands of the Archipelago, as appears to have occurred on La Graciosa Island (Fig. 1) where ferrets have occasionally been seen. A high number of ferrets are maintained in captivity in the Canary Islands (probably > 20,000) and 9,552 licenses were purchased for using ferrets for hunting (from a total of 25,019 for small game hunting permits) in the Archipelago during 2007. On La Palma, with only 531 ferret licences, the species is employed less for hunting than on other islands but it is unclear why the ferret has become established here rather than elsewhere. Public awareness of the problem and education campaigns, especially among hunters, are needed to reduce the threat of this invasive alien species to the Canary Islands biota. Law enforcement, controls and penalties are urgently needed to change the custom of releasing unmuzzled animals.