American mink in the Cape Horn Biosphere Reserve, southern Chile: a population and community approach to understand a recent island invasion

Since 2001 exotic Mustela vison has been known to populate Navarino, an island located in the pristine Cape Horn Archipelago (54-55ºS). As an invasive predator, it could be affecting the biodiversity that evolved in absence of mammalian terrestrial predators. We report the abundance (using live-trapping and sign surveys) of mink and its seasonal impact (through bird sampling, diet analysis, and artificial nest predation) upon the wetland bird community. Mink signs were found in 79% of the surveys in all types of semi-aquatic habitats. Yet, relative population abundance (0.75 mink/km of coastline) was still below densities measured in other invaded or native areas. The habitat model accuracies indicated that mink were less specific in habitat use, probably due to the missing limitations normally imposed by predators or competitors. The selected models predicted that mink prefer to use shrubland instead of open habitat, coastal areas with heterogeneous shores instead of flat beaches, and interestingly, that mink avoid habitats strongly modified by beavers. Fifty-six bird species were registered with marked seasonal variations in richness and total number of birds. Near half (48%) of bird species were migratory, arriving to the area in summer. Seasonal differences in the diet of M. vison were observed (n=414 scats). Scat analysis showed fragments of birds in 36% of the samples collected during summer, but only 21% in winter scats. This agrees with the increase in the number of birds during their reproductive period. Species like Chloephaga picta and some passerine birds are at a greater risk of being predated by mink. Conversely, predation of mink on artificial nests was very low. Our results shows that mink have colonised the entire island and is opportunistically predating on birds. For the management of mink, we suggest the establishment of rocky coastal shores as priority sites deserving special conservation efforts.
Avifauna, Capture-mark-recapture, Castor canadensis, Diet, Invasive exotic species, Mustela vison, Seasonal variations, Sign surveys, Sub-Antarctic, Wetlands