Post-invasion amphipod communities of Lough Neagh, Northern Ireland: Influences of habitat selection and mutual predation

1. Lough Neagh, Northern Ireland, has been invaded by three non-indigenous amphipod species this century; the North American Gammarus tigrinus Crangonyx pseudogracilis, plus the European G. pulex. These species have come into contact with the sole native Lough species, G. duebeni celticus. 2. The Lough and its associated rivers were surveyed in 1993-94 and a selection of sites re-sampled the following year. The distributions, relative abundances and habitat utilization patterns of the four amphipod species were determined. All single and mixed species combinations of the four species were found. Grammarus pulex and G. d. celticus are ubiquitous in their habitat selection; however, the former species is replacing the latter. Gammarus tigrinus occurs in deep, mid-lough areas and, to a limited extent, colonizes sheltered, vegetated shores. Crangonyx pseudogracilis has a fragmented distribution, but utilizes a variety of habitats. 3. Considering those habitat types utilized by all four species, evidence was found for species interactions from field data. First, although both North American species have much greater reproductive outputs, their mean densities were significantly lower than the mean densities of both G. d. celticus and G. pulex. Secondly, the mean density of total Gammarus was significantly higher where C. pseudogracilis was absent compared to where it was present. Thirdly, re-sampling at selected sites in 1994-95 revealed that G. tigrinus and C. pseudogracilis had disappeared from areas dominated by the other two Gammarus species. 4. Since differential mutual predation has been linked to the replacement of G. d celticus by G. pulex in Irish rivers, predatory interactions among all other combinations of the four species of Lough Neagh were investigated. Male G. d. celticus and G. pulex preyed significantly more frequently on moulted female G. tigrinus than did male G. tigrinus in reciprocal interactions. Predatory/survival abilities may be influenced by body size (G. d. celticus > G. pulex > G. tigrinus). However, G. tigrinus preyed on female G. d. celticus significantly more frequently than on female G. pulex. 5. All three Gammarus species preyed heavily on the smaller C. pseudogracilis, with G. pulex the most severe predator, followed by G. d. celticus and then G. tigrinus. Such predation occurred regardless of the moult state of victims, and frequencies of predation were particularly high on reproductive females. Thus, high rates of Gammarus predation may explain the limited invasion success of C. pseudogracilis. 6. The dynamic nature of Lough Neagh's amphipod communities appears to arise from a combination of overlapping habitat utilization, differential predation and episodes of local immigration/exclusion. Species diversity, and probably biomass, have increased following the introductions, which are likely to have significant effects on other species and at other trophic levels.