Asymmetry in mutual predation: possible reason for the replacement of native gammarids by invasives

Abstract

Mutual predation among two invasive gammarids (Dikerogammarus villosus and Echinogammarus ischnus) and two natives (Gammarus fossarum and Gammarus roeseli) was studied in laboratory experiments. Both invasives were much stronger predators than the two natives. Between 56-86% of the replicates resulted in the death of natives when the invasive species were the predators, while only 10-18% resulted in the death of invasives in reverse combinations. Predation by invasives on natives generally exceeded cannibalism within species by a factor of 2 to 4. Both invasive species preyed less heavily on each other than on natives. The results suggest that the observed asymmetry in strength of mutual predation can be a key factor in the displacement of native gammarids by invasives in parts of Central European rivers and that low predation between invasives may permit their coexistence. Thus, the results of this study support the opinion that intraguild predation can be an important reason for species replacement in the field.