THE BEHAVIORAL BASIS OF A SPECIES REPLACEMENT - DIFFERENTIAL AGGRESSION AND PREDATION BETWEEN THE INTRODUCED GAMMARUS-PULEX AND THE NATIVE G-DUEBENI CELTICUS (AMPHIPODA)

Previous studies have shown that differential predation by males on mo ulted female congenerics may be largely responsible for the eliminatio n and replacement of the native Irish freshwater amphipod Gammarus due beni celticus by the introduced G. pulex. Predation of moulted females occurs both shortly after their release from precopulatory mate-guard ing and whilst they are being guarded by their mates. In the present s tudy, two hypotheses concerning the underlying cause(s) of the differe ntial predation pattern are tested. Firstly, female G. d. celticus may be more vulnerable to predation than female G. pulex due to the forme r being released from precopula guarding with the new exoskeleton in a less hardened state. Secondly, G. pulex may be an inherently more agg ressive species than G. d. celticus during predatory interactions over guarded females. The first experiment indicated that differential pre dation was not mediated by species differences in the state of the fem ale exoskeleton at the time of release from precopula by guarding male s. The second experiment, however, showed that male G. pulex were sign ificantly more aggressive than male G. d. celticus in attacking both g uarding male congenerics and guarded moulted female congenerics. In ad dition, in defence against predatory attacks, paired male and female G . pulex were significantly more aggressive than paired male and female G. d. celticus. These differences in aggressive behaviour led to a si gnificantly higher frequency of predation on G. d. celticus females th an on G. pulex females, and also explains this finding in previous stu dies. It is concluded that differential predation due to differences i n aggressive behaviour may explain the pattern of replacement between these species.