The influence of re-establishing Eurasian otter (Lutra lutra) on the activity of invasive American mink (Neovison vison) on the southwest coast of Norway

Abstract

Invasive species are one of the most impactful sources of biodiversity loss worldwide. Generalist predators may be especially damaging, as they can reduce survival and fecundity in native species and can compete with native predators for resources. In groups of related species, competition can be a major evolutionary driving force, leading to changes in distribution and behaviour of the competing species. One such pair of related competing species is the invasive American mink (Neovison vison), who was introduced to Norway whilst the competing native species Eurasian otter (Lutra lutra) was absent in many areas due to excessive hunting. In recent years, however, the otter has returned to large parts of its historical range but is still missing in the southernmost part of the country. This provides a natural control area and allows an insight into the difference in mink activity when otters are present and absent. I used camera traps placed in common marking sites to study sighting frequency and circadian activity patterns of otter and mink in an area without otters and an area that otters returned to a decade ago. I found that activity of mink in the presence of otter remained stable at a relatively low level over seven years, indicating that otters can suppress mink populations in the area, but are not able to completely remove mink, at least not in the short term. Furthermore, the activity patterns of mink differed in the presence of otters, although the impact was opposite of what was expected. The generally nocturnal mink was more diurnal in the absence of otters, but this could be due to diet choice. This study supports previous findings that otters suppress mink and can change their behaviour, although long term effects are uncertain. Still, the suppression of mink will likely positively impact its vulnerable native prey species.