Factors influencing the distribution of native and introduced Gammarus spp. in Irish river systems

Populations of Gammarus duebeni celticus, previously the only amphipod spec ies resident in the rivers of the Lough Neagh catchment, N. Ireland, have b een subjected to invasion by G. pulex from the British mainland. Numerous p revious studies have investigated the potential behavioural mechanisms, pri ncipally differential mutual predation, underlying the replacement of G. d. celticus by G. pulex in Irish waters, and the mutually exclusive distribut ions of these species in Britain and mainland Europe. However, the relative degree of influence of abiotic versus biotic factors in structuring these amphipod communities remains unresolved. This study used principal componen t analysis (PCA) to distinguish physico-chemical parameters that have signi ficant roles in determining the current distribution of G. pulex relative t o G. d. celticus in L. Neagh rivers. We show that the original domination o f rivers by the native G. d, celticus has changed radically, with many site s in several rivers containing either both species or only G. pulex. G. pul ex was more abundant than the G. d. celticus in sites with low dissolved ox ygen levels. This was reflected in the macroinvertebrate assemblages associ ated with G. pulex in these sites, which tended to be those tolerant of low biological water quality. The present study thus emphasizes the importance of the habitat template, particularly water quality, for Gammarus spp. int eractions. If rivers become increasingly stressed by organic pollution, it is probable the range expansion of G. pulex will continue. Because these tw o species are not ecological equivalents, the outcomes of G. pulex incursio ns into G. d. celticus sites may ultimately depend on the prevailing physic o-chemical regimes in each site.