Inducible defences as key adaptations for the successful invasion ofDaphnia lumholtziin North America?

Abstract

The mechanisms underlying successful biological invasions often remain unclear. In the case of the tropical water fleaDaphnia lumholtzi, which invaded North America, it has been suggested that this species possesses a high thermal tolerance, which in the course of global climate change promotes its establishment and rapid spread. However,D. lumholtzihas an additional remarkable feature: it is the only water flea that forms rigid head spines in response to chemicals released in the presence of fishes. These morphologically (phenotypically) plastic traits serve as an inducible defence against these predators. Here, we show in controlled mesocosm experiments that the native North American speciesDaphnia pulicariais competitively superior toD. lumholtziin the absence of predators. However, in the presence of fish predation the invasive species formed its defences and became dominant. This observation of a predator-mediated switch in dominance suggests that the inducible defence against fish predation may represent a key adaptation for the invasion success ofD. lumholtzi.