INTRAGUILD PREDATION AND SPECIES EXCLUSIONS IN AMPHIPODS - THE INTERACTION OF BEHAVIOR, PHYSIOLOGY AND ENVIRONMENT

1. Data from field surveys, laboratory experiments and computer simula tions of community dynamics revealed that a novel interaction among in traguild predation, physiological adaptation and environment may expla in the complex distributions of two putatively competing aquatic amphi pods. 2. Gammarus pulex and G. tigrinus both thrive in fresh and oligo haline waters in western Europe. However, the native European G. pulex excludes the invading North American G. tigrinus from freshwaters of relatively low conductivity, whereas the reverse occurs at higher cond uctivities. Additionally, there is much spatio-temporal fluctuation in the patterns of coexistence of these species.3. Laboratory experiment s in The Netherlands and Ireland revealed that mutual predation of mou lting individuals occurred frequently between these species. However, predation frequencies were differentially in favour of G. pulex under the ionic conditions to which this species is physiologically adapted (freshwater). On the other hand, predation was not differential under the ionic conditions to which G. tigrinus is physiologically adapted ( oligohaline water). 4. A mathematical model, which extends the logisti c equation to include mutual intraguild predation, simulated interacti ons over a range of values of relevant population parameters. This ind icated that G. pulex would be excluded when balanced instantaneous rat es of mutual predation were combined with the known greater reproducti ve output of G. tigrinus. However, this reproductive advantage is over come by any relatively small bias in the instantaneous rate of predati on favouring G. pulex, leading to the exclusion of G. tigrinus. This o ccurs even when the reproductive advantage to G. tigrinus is relativel y large. Moreover, the model generated 'switches' in species dominance that are determined by the relative values of reproductive rate and m utual predation. The time taken to 'switch' may explain the transient periods of apparent coexistence of these species observed in the field . 5. The complex community dynamics of such species may thus be unders tood in terms of variation in the intensity of species interactions me diated by behavioural, physiological and environmental factors.