Coexistence among native and introduced freshwater amphipods (Crustacea); habitat utilization patterns in littoral habitats

Studies of biological invasions predominantly stress threats to biodiversit y through the elimination and replacement of native species. However, we mu st realise that resident communities may often be capable of integrating in vaders, leading to patterns of coexistence. Within the past ninety years, t hree freshwater amphipod species have invaded Northern Ireland the North Am erican Gammarus tigrinus and Crangonyx pseudogracilis, plus the European G. pulex. These species have come into contact with the ubiquitous native spe cies, G. duebeni celticus. This study examined spatiotemporal patterns of s tability of single and mixed species assemblages in an invaded lake. Lough Beg and its associated rivers were surveyed in summer 1994 and winter 1995, and a selection of stations re-sampled in summer one and five years later. All possible combinations of the four amphipod species were found. Althoug h species presence/absence was stable between seasons at the scale of the w hole lough, it was extremely fluid at the scale of individual sites, 82% of which changed in species composition between seasons. Overall mean amphipo d abundance was similar across 5 distinguishable habitat types, but there w ere differences in species compositions among these habitats. In addition, although co-occurrences of Gammarus species did not differ from random, the re was a strong negative association between Gammarus spp. and C. pseudogra cilis. This latter pattern was at least in part generated by the better tol erance of C. pseudogracilis to lower water quality. A review of previous st udies indicates that the exclusion of C. pseudogracilis by Gammarus species from high water quality areas is likely to involve biotic interaction. Thu s, overall, co-existence of the four species, which is clearly dynamic and scale-dependent, appears promoted by spatial and temporal habitat heterogen eity. However, biotic interactions may also play a role in local exclusions . Since the three introduced species have not eliminated the native species , and each successive invasion has not replaced the previous invader, this study demonstrates that freshwater invaders may integrate with native commu nities leading to coexistence and increased species diversity.