Ecology, 90(5), 2009, pp. 1207–1216 Ó 2009 by the Ecological Society of America The impact of native competitors on an alien invasive: temporal niche shifts to avoid interspecific aggression? LAUREN A. HARRINGTON, 1,3 ANDREW L. HARRINGTON, 1 NOBUYUKI YAMAGUCHI, 1,4 MICHAEL D. THOM, 1,5 PABLO FERRERAS, 1,6 THOMAS R. WINDHAM, 2,7 AND DAVID W. MACDONALD 1 1 Wildlife Conservation Research Unit, Department of Zoology, University of Oxford, Tubney House, Abingdon Road, Tubney, Oxford OX13 5QL United Kingdom 2 Brasenose College, University of Oxford, Oxford OX1 4AJ United Kingdom Abstract. The American mink, Neovison vison, is an established, alien invasive species in the United Kingdom that originally colonized the country at a time when two native mustelids (otters, Lutra lutra, and polecats, Mustela putorius) were largely absent. Both of these species are now recovering their populations nationally. We compared the relative abundance and the behavior of mink in the 1990s and in the 2000s in an area of southern England where both otters and polecats were absent in the 1990s but reappeared in the intervening years. We found that mink were still abundant in the 2000s in the presence of otters and polecats, but that they appeared to have altered some aspects of their behavior. In accordance with previous studies, we found that mink consumed fewer fish in the presence of otters. We also found that mink were predominantly nocturnal in the 1990s (in the absence of competitors) but were predominantly diurnal in the 2000s (in the presence of competitors). We hypothesize that this temporal shift may be an avoidance mechanism allowing the coexistence of mink with the otter and the polecat, although we are unable to attribute the shift to one or the other species. We also found that mink in the presence of competitors weighed less but remained the same size, suggesting the possibility of a competitor-mediated decline in overall body condition. This is one of very few field studies demonstrating a complete temporal shift in apparent response to competitors. The implications of this study are that recovering otter populations may not lead to significant and long-term reductions in the number of invasive mink in the United Kingdom as has been suggested in the media, although we cannot exclude the possibility of a decline in mink in the longer-term. Key words: competition; Lutra lutra; Mustela putorius; mustelids; Neovison vison; resource partitioning; temporal segregation. INTRODUCTION Interspecific competition (usually interference compe- tition) is an important factor in the structuring of carnivore communities (Caro and Stoner 2003). Inter- specific interference competition is most often asymmet- rical, involving a dominant (superior or larger) and a subordinate (weaker or smaller) competitor, often involves direct aggression (e.g., Hersteinsson and Macdonald 1992) and, in extremis, can result in the death of the latter (Palomares and Caro 1999). Other negative impacts experienced by the subordinate species might include kleptoparasitism, reduced population growth rate, exclusion from certain habitats or regions, reduced densities and distribution, and local extinction (Linnell and Strand 2000). Nevertheless, guilds of similar species can exist in sympatry; among the mustelids, guilds of three to five coexisting species are not uncommon (Powell and Zielinski 1983). In theory, coexistence occurs through niche differentiation (Mac- Arthur 1958), which may involve resource partitioning (Schoener 1986), and/or be facilitated by avoidance mechanisms (either in space or in time; Berryman and Hawkins 2006). The American mink, Neovison vison (formerly Mus- tela vison; see Wilson and Reeder 2005), is an alien, invasive species in the United Kingdom (Macdonald and Harrington 2003; see Plate 1). Introduced to Britain early in the 20th century as an unintended result of fur farming, feral American mink became widely established at a time when two native mustelids, the Eurasian otter, Lutra lutra, and the European polecat, Mustela putorius, were absent over much of the country, due to pesticide poisoning, overhunting (Strachan and Jefferies 1996), Manuscript received 12 February 2008; revised 12 May 2008; accepted 21 July 2008. Corresponding Editor: B. J. Fox. 3 E-mail: lauren.harrington@zoo.ox.ac.uk 4 Present address: Department of Biological and Environ- mental Sciences, University of Qatar, P.O. Box 2713, Doha, Qatar. 5 Present address: Mammalian Behaviour and Evolution, University of Liverpool, Leahurst Field Station, Chester High Road, Neston CH64 7TE UK. 6 Present address: Instituto de Investigacio´n en Recursos Cinege´ticos, IREC (CSIC-UCLM-JCCM), Ronda de Toledo s/n, 13005 Ciudad Real, Spain. 7 Present address: The Hyde, Woolhope, Herefordshire HR1 4RE UK. 1207