The ability of invasive species to successfully spread and establish in an area depends on their spatial and temporal responses to environmental features[36]. Occupancy models offer a great opportunity in estimating the factors that determine an invasive species’ presence in space and time, and their outputs can be utilized in designing and guiding management actions[17],[37]–[39].
Floating rafts comprise one of the most widespread methods in surveying American mink presence in riparian and coastal areas[40]–[44]. According to Reynolds et al.[30] and Reynolds et al.[45], floating rafts are much more efficient in tracking American mink compared to alternative methods, however detection may vary by season. Indeed, the probability of detecting mink in our study area varied by month, with September and October illustrating the highest probabilities. During this period, juvenile individuals disperse and females establish their territories[46], whereas in late October populations are generally stable[47] which might explain the higher detection estimates. Similarly, Roy et al.[48] reported a higher rate of trapped mink during this time. Conversely, individuals were less likely to be detected during the months of April and May. These months correspond to the gestation, parturition and weaning period, during which mink activity is at its lowest[49],[50]. When accounting for imperfect detection, mink occupancy within our study area was estimated at 67.59%.
American mink initial occupancy within the study area was found to be mainly driven by features of its immediate habitat. This may reflect the species’ opportunistic behaviour or may be due to its small ranging ability[49],[51]. Based on our results, mink were more likely to occupy river banks and lake shores with more shrubs and rocks. These patterns have been previously described in several other studies[18],[19],[21]–[23],[51],[52], and may reflect the species’ requirements in nesting and in avoiding predation or competition[20],[21],[53]–[55]. Yamaguchi et al.[21] and Schüttler et al.[22] found that the species avoids exposed areas and mainly uses areas with dense vegetation, which provide more cover and nesting opportunities[46],[53]. In addition, the species may favor such areas because they offer more feeding opportunities[23]. According to Torre and Diaz[56] and Torre et al.[57], in Mediterranean ecosystems shrubs support a greater diversity and density of small mammals, many of which may serve as prey for the mink. Similarly, rocky areas can be used for denning when the availability of other nesting types is limited[52], or to avoid predation[52],[58]. According to Brainerd et al.[59], Zalewski[60], Achterberg et al.[61] and Stier[62], mustelids usually favor tree cavities for nesting. However, when availability is low the species may seek alternative sites, such as areas with rocks or boulders[52]. Elevated rock cavities resemble trees, as both are inaccessible to terrestrial predators[58], whereas dens located on the ground, in between rocks, provide protection against predators with digging abilities[52]. Hence, predation risk is likely to be a significant factor influencing the choice of cover and den sites for the species[21],[46],[63],[64]. The fact that many of the mink’s natural predators and competitors can be found all across our study area, may force individuals in selecting densely vegetated or rocky riparian areas[6],[21],[46],[63].
In addition to the above, the specie’s probability of initial occupancy was found to be positively associated by the density of medium sized rivers at the landscape scale. Zabala et al.[65], mustelids show a preference towards secondary rivers with sufficient riparian vegetation cover, while they avoid areas with polluted waters or modified river beds. Moreover, Sidorovich and Macdonald[66] found that mustelids avoid large, fast flowing rivers, while Garcia et al.[23] concluded that because secondary rivers are characterized by the existence of islets within the watershed, these may serve as resting sites for the species[21],[63]. Our study area does not include large rivers (Stahler order > 6), and therefore the positive association between mink occupancy and medium sized rivers may be due to the fact that this river category is characterized by continuous water flow throughout the year, in contrast to smaller rivers in the area which are characterized by seasonal flow and shallow water depth.
The probability of a vacant site being colonized between surveys was found to be positively associated with the percent cover of shrubs and reeds. However, none of these variables were informative. Previous studies have shown that colonization may be affected by stream size, water depth and flow regime[43],[67],[68]. In addition, colonization may be closely related to habitat features that increase denning and resting site availability[68], or may be closely related to prey distribution and urbanization[69]. Conversely, distance to the nearest fur farm within our study area was very informative in predicting American mink probability of local extinction. Not only that, but almost 70% of all mink occurrence sites were found within a 5 km radius from nearby fur farms. According to Bonesi and Palazón[7], fur farms are considered as the main source of American mink reintroduction. With that in mind and given that mink are very elusive species[70], eradication campaigns are often unsuccessful. The number of mink escaping from fur farms located within the study area is unknown, however reducing the number of introduced individuals into the wild may help control American mink successful establishment and spread in the wider region[71]. None the less, both wild and introduced individuals show similar feeding habits and illustrate the same ability to hunt their prey[72]. In addition, individuals introduced from fur farms may act as a source of increasing genetic diversity and adaptation for wild populations[73].