Interspecific interactions between the American mink (Neovison vison, Schreber 1777) and European otter (Lutra lutra, Linnaeus 1758)

Abstract

American mink (Neovison vison) have been declining in the UK after having increased for several decades. Mink is an invasive species to the UK, imported from North America to supply the fur farming industry, and it threatens the survival of an endangered native species, namely the water vole (Arvicola terrestris). Recent population recovery in the Eurasian otter (Lutra lutra) means the American mink are now coming into contact with a previously absent native mustelid, which could be a potential competitor.

This research examined the causes and consequences of the decline of the American mink in North Staffordshire. A range of approaches were: laboratory experiments, field experiments, field surveys and DNA analysis. Each of these approaches focused on a particular facet of the overall aims to determine resource overlap and ecological segregation between otters and mink; mink response to the odour of European otter and the impact of otter presence on water voles.

There are two mechanisms through which mink and otters could compete
(i) exploitation competition, by which one or both species depletes a common resource;
(ii) interference competition, by which one species precludes the other from accessing the resource through aggression and harassment. This study aimed to explore both of these mechanisms.

In support of interference competition, a temporal study of changes in mink and otter densities and distribution in the Staffordshire Moorlands was undertaken. A comparison of the results of sign surveys between 2002-03 and 2004-05 highlighted that the presence of otters can lead to a significant and rapid decline of mink density and distribution, with results demonstrating a 57% decrease in the number of mink scats, and mink being replaced by otters in 43% of sites which mink previously occupied. I further explored the interference hypothesis by undertaking olfactory tests observing the response of mink to otter odour. The results suggested significant aversion of mink to tunnels and rafts scented with otter odour compared with control odour.

Exploitation competition was addressed by analysing dietary changes of mink in response to increased densities of otters. The diet of mink at relative higher otter densities included a higher proportion of terrestrial prey items than at lower otter densities, indicating a lower niche overlap. These observations could be attributed to asymmetric exploitation competition, particularly when considering that otters are better adapted to an aquatic lifestyle and therefore are better at accessing aquatic prey than mink.

In conclusion, the evidence from this thesis provides a strong indication that competition between the European otter and the American mink is an important factor in limiting mink populations. However, the exact mechanism, or combination of mechanisms still requires further research. Understanding the causes of mink decline is highly relevant to UK biodiversity conservation because of its potential consequences for vulnerable water vole populations.