Differential microdistributions and interspecific interactions in coexisting Gammarus and Crangonyx amphipods

In freshwaters in Northern Ireland several amphipods occur, with the native Gammarus duebeni celticus and introduced Gammarus pulex and Crangonyx pseudogracilis often being found together in the same river and lake systems. We examined what may happen when the three amphipod species co-occur within the same patch of lake or pooled area of a river. We conducted a laboratory simulation of a lake/river habitat, and this showed that G. d. celticus and G. pulex exhibited very similar distribution patterns to one another, when presented with the same complex habitat template. Such microdistribution patterns contrasted markedly with the microhabitat usage by C. pseudogracilis. Cannibalism was low for all species in both single and mixed species treatments. Intraguild predation (IGP) was also very low on both G. pitier and G. d. celticus when all three species co-occurred: However, C. pseudogracilis suffered heavy IGP from both Gammarus species, with 33% of the tank population of C. pseudogracilis being eliminated in the presence of Gammarus within only 12 h. Such IGP may account for C. pseudogracilis occurring more frequently and in greater abundance in patches of natural lotic and lentic systems where predatory Gammarus are either absent or scarce.