REPLACEMENT OF THE INDIGENOUS AMPHIPOD GAMMARUS-DUEBENI-CELTICUS BY THE INTRODUCED GAMMARUS-PULEX - DIFFERENTIAL CANNIBALISM AND MUTUAL PREDATION

1. Following introductions in the 1950s, Gammarus pulex has replaced the native Irish freshwater amphipod G. duebeni celticus in several river systems. We show that G. pulex has eliminated and replaced G. d. celticus in lower sections of the River Lagan system, N. Ireland, although G. d. celticus maintains pure populations upstream-from G. pulex. More rarely, the two species may be found together. 2. We investigate the potential roles of cannibalism and mutual predation in explaining these replacement patterns. G. d. celticus was significantly more cannibalistic on moulted conspecifics than G. pulex. Mutual predation by intermoult individuals on moulted congenerics was observed in all combinations of adult males and females, including those in the precopulatory mate-guarding phase, and juveniles. In general, predation between species exceeded cannibalism within species, particularly predation by G. pulex on G. d. celticus. Predation was differentially in favour of G. pulex in most combinations. These differences in predation frequencies were not dependent on body size. 3. A model is presented incorporating resource exploitation competition, cannibalism and mutual predation. The behaviour of the model agrees with our intuitive predictions and our conclusions based on empirical findings. Cannibalism in the absence of predation promotes coexistence, whereas mutual but differential predation in the absence of cannibalism may lead to elimination, depending on relative resource competitive ability. When cannibalism and predation occur together, predation of equal magnitude or greater than cannibalism leads to rapid elimination. Predation in favour of species 1 may overcome a competitive advantage for species 2, leading to elimination of species 2. 4. We conclude that the superior abilities of G. pulex to resist predatory attacks and to prey on moulted G. d. celticus may be major features of the elimination and replacement of the latter species by the former. 5. We encourage examination of replacements involving other crustaceans in terms of predatory behaviour and, more generally, urge adoption of approaches at the individual level to elucidate mechanisms underlying interspecific relationships.