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2004. Competition between Eurasian otter Lutra lutra and American mink Mustela vison probed by niche shift. Á/ Oikos 106: 19 Á/26.Interspecific competition is one of several constraints that might prevent an individual from maximising its energy intake. When an interspecific competitor is introduced, an individual is often forced to shift its diet according to the intensity of the competitive pressure. In this paper, we explore whether the introduced American mink (Mustela vison Schreber) shifts its diet when the density of its potential competitor, the Eurasian otter (Lutra lutra L.), is increased. We compared the diets of otter and mink at the same location but at two moments in time when the relative densities of these two species were different while controlling for the abundance of aquatic prey. Mink and otters are semi-aquatic mammals belonging to the same guild of mustelids and otters are expected to be the dominant competitor because they are larger and better at hunting underwater. The diets of otters and mink overlap to a great extent but while otters specialise mainly on aquatic prey, mink are able to exploit both aquatic and terrestrial prey. These observations prompted the hypothesis investigated in this work that at higher otter densities the diet of mink should change to include a higher proportion of terrestrial items. This hypothesis was supported by the data and at higher otter densities mink diet was observed to consist of a higher proportion of mammals and birds while fewer fish were present, although this pattern was present only in winter while no changes were observed in spring. Meanwhile the diet of otters remained basically unchanged. In the second part of the study, we investigated whether niche breadth and niche overlap between otter and mink changed at different otter densities. We found that niche overlap declined as the density of otters increased, in agreement with the prediction of habitat selection theory.

2004. Differential habitat use promotes sustainable coexistence between the specialist otter and the generalist mink. Á/ Oikos 106: 509 Á/519.Optimal foraging and habitat selection theories predict that heterogeneous environments should favour the coexistence of competitors, especially when the dominant competitor is a specialist and the sub-ordinate is a generalist. In this paper, we analysed differential habitat use as a potential mechanism for the coexistence of two competing riparian mammals, the specialist and dominant Eurasian otter (Lutra lutra ) and the generalist and sub-ordinate American mink (Mustela vison ). We tested three hypotheses: H1: mink coexist with otters for longer in areas with abundance of habitats hosting terrestrial prey because, by not relying on aquatic prey, mink can segregate from its competitor. H2: the characteristics of the habitat closer to the riverbank will affect the length of time the two species coexist, because mink are still tied to the water even in the presence of otters. H3: denser vegetative cover along the bank increases the duration of coexistence of mink and otters because it reduces the frequency of their encounters. The first hypothesis was supported by the data and we found that in areas where terrestrial prey was abundant mink coexisted for longer with otters. The second hypothesis was also supported by the data and the characteristics of the habitat closer to the riverbank were the most important in determining coexistence time. Finally, we did not find supporting evidence for the third hypothesis. This study provides strong evidence that habitat heterogeneity plays an important role in determining the likelihood of coexistence of American mink with Eurasian otters. This result is particularly important from a conservation standpoint. Mink are invasive and a threat to endangered species in parts of their range. The knowledge that mink have a higher chance to persist in the presence of otters when terrestrial prey is abundant should be used to target areas for preferential mink management.

American mink Mustela vison are an invasive species in many parts of the world (e.g. Europe, Iceland and South America) and in many of these areas they are a threat to the native wildlife. It is therefore important to establish a reliable and efficient method to estimate their relative abundance in order to manage their populations. In this study, the use of surveys of signs (scats and footprints) as a way to estimate the relative abundance of American mink was evaluated. Populations of mink in three areas of England were studied to evaluate the impact of three factors (surveyor, marking places and month) on the probability of deposition and detection of signs. Among these factors only month had a significant effect. In the second part of the study, sign surveys were compared with live trapping and it was found that the proportion of sections with mink signs was only loosely correlated to the abundance of mink as estimated from live trapping. In conclusion, this study suggests that, provided that surveyors are trained, that there is a sufficient number of marking places in each section, and that the surveys are carried out in comparable seasons, sign surveys are an appropriate way to monitor the relative abundance of mink. Sign surveys are better than trapping when it is necessary to monitor mink populations over large areas because they are less time consuming, require less manpower and are cheaper.

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