DOI: 10.1111/j.1472-4642.2008.00529.x © 2008 The Authors 232 Journal compilation © 2008 Blackwell Publishing Ltd www.blackwellpublishing.com/ddi Diversity and Distributions, (Diversity Distrib.) (2009) 15, 232–241 BIODIVERSITY RESEARCH ABSTRACT Aim Using data from field introduction experiments with Gammarus spp. con- ducted in the rivers of a small island, commencing in 1949, with resampling in the 60s, 70s, 80s, 90s and finally in 2005, we aimed to examine the long-term interaction of the native freshwater amphipod Gammarus duebeni celticus with the introduced G. pulex. Using physico-chemical data from a 2005 island-wide survey, we also aimed to find what environmental factors could influence the distribution of the two species. Location The Isle of Man, British Isles. Methods We used species distribution data from 10 river sites from a field introduction study in 1949–69, resampling of these sites in 1974/5, 1988, 1995 and 2005. We sampled a further 82 sites in 2005, where physico-chemical characteristics of each site were obtained. ANOVA was used to ascertain significant differences in environmental variables between native and introduced amphipod assemblages. Multiple discriminant analysis was used to relate Gammarus distribution to environmental gradients in rivers. Results Field introductions of G. pulex into previously G. d. celticus dominated sites failed, despite the large propagule pressure in terms of numbers of individuals released and repeated introductions. The island-wide survey revealed that G. d. celt- icus was found in areas of higher water quality than G. pulex. Sites where the two species co-occurred also tended to be of lower water quality than native only sites. Continuing metal pollution from past mining activity may be a major factor exclud- ing all amphipods from many rivers. The North American amphipod Crangonyx pseudogracilis, which was first discovered on the Isle of Man in one river site in 1995, was found in a further five sites. Main conclusions This ecological experiment, through long-term monitoring, thus continues to elucidate factors influencing the distributions of introduced and native species. Under some physicochemical regimes, G. d. celticus is able to with- stand its usual displacement by G. pulex, whereas increased river pollution could promote replacement of the native by the introduced species. Keywords Amphipods, biological invasions, Gammarus, introductions, Isle of Man, metal pollution, mining, propagule pressure. INTRODUCTION Amphipod crustaceans have featured in many freshwater introductions worldwide, via coincidental transport with humans and deliberate introductions for fish farming, angling interests and even ecological experiments (Dick et al., 1997; MacNeil et al., 1999). The ecological impacts of such introductions range from benign to drastic (Pinkster et al., 1992; Dick et al., 2002; Grabowski et al., 2006; Kelly et al., 2006; Van Riel et al., 2006). Understanding reasons for the success and failure of 1 The Government Laboratory, Ballakermeen Road, Douglas, Isle of Man IM1 4BR, 2 School of Biological Sciences, Queen’s University Belfast, 97 Lisburn Road, Belfast, N. Ireland BT9 7BL, 3 Wildlife and Conservation Division, Department of Agriculture, Fisheries and Forestry, Knockaloe Farm, Peel, Isle of Man IM5 3AJ, 4 127 Iroquois Place, Waterloo, Ontario, Canada N2L 2S6 *Correspondence: Calum MacNeil, The Government Laboratory, Ballakermeen Road, Douglas, Isle of Man IM1 4BR. E-mail: calummanx@hotmail.com or calum.macneil@lab.dlge.gov.im Blackwell Publishing Ltd A long-term study (1949–2005) of experimental introductions to an island: freshwater amphipods (Crustacea) in the Isle of Man (British Isles) Calum MacNeil 1 *, Jaimie T. A. Dick 2 , Fiona R. Gell 3 , Richard Selman 3 , Paul Lenartowicz 1 and H. B. Noel Hynes 4