Intraguild predation may explain an amphipod replacement: evidence from laboratory populations

In a laboratory experiment that permitted both observations of the behaviou r of individuals and the monitoring of small populations, the role of 'intr aguild predation' in the elimination of the freshwater amphipod Gammarus du ebeni celticus by the introduced G. pulex was examined. Over 18 weeks, deat hs in single and mixed species replicates were monitored. Rates of 'mortali ty' (deaths not due to cannibalism or predation) did not differ between the species. Gammarus cl. celticus, however, was more cannibalistic than G. pu lex and, in both species, males were more often cannibalized than females. In mixed species replicates, the mean proportions of animals preyed upon di d not differ among replicates with differing starting proportions of the tw o species, nor was there a difference between the sexes in numbers preyed u pon. G. pulex, however, preyed more frequently on G. d celticus than vice v ersa, and this became more pronounced over time. In 87% of mixed species re plicates, G. pulex eliminated G. d. celticus. The results support the propo sition that intraguild predation may be the primary mechanism whereby G. pu lex rapidly replaces G. d. celticus in freshwater. Integrating behavioural observations with population level monitoring may thus link pattern and pro cess in behaviour and ecology.