Mapping mustelids: exploring trends in the distribution and density of records of native Eurasian otters and invasive American mink in Britain

Invasive species are a major threat to global biodiversity, often displacing native species through competition, predation, and habitat alteration. Biological controls, including the use of native species to manage invasives, present a promising alternative to intensive manual eradication. In the UK, the highly adaptable American mink (Neogale vison) has become a significant invasive species, threatening native fauna such as water voles and seabirds. This study investigated the potential role of the native Eurasian otter (Lutra lutra) in influencing mink populations by examining demographic trends and spatial dynamics of both species across England and Scotland from 1970 to 2010. Otter distribution, density and record frequency increased across the UK over time, coinciding with an apparent decline in mink distribution and density. However, statistical analyses revealed otter frequency weakly predicted mink frequency. These findings suggest that while interspecific interactions such as competition and aggression may play a role in shaping species distributions, other factors such as behavioural shifts and prey availability are likely contributing to the observed trends. This highlights the complexity of interpreting demographic data for cryptic species and emphasises the importance of comprehensive, long-term monitoring to better understand these dynamics. This study concluded that while an expansion of otter populations and decline of mink populations were observed, the current evidence is insufficient to confirm that otter are significantly influencing mink populations. Further research, including GPS tracking of mink and detailed dietary studies, is needed to clarify the mechanisms driving these population trends and to guide effective management strategies.