Erik Jeppesen

Biological invasions constitute major threat to native fauna and ecosystem functioning. Not least fish introductions are widespread and have strong ecosystem effects, in Tasmania, nine freshwater fish species have been introduced over the last 150 years with negative effects on the native biota. We studied 14 Tasmanian lakes, along a gradient of exotic predatory fish density, using a stable isotope (δ15N and δ13C) approach combined with stomach content analyses. The niche width of galaxiids was reduced in lakes with exotic predators, particularly in clear-water lakes. The proportion of galaxiids in the diet of the exotic predators was higher in turbid lakes, probably because galaxiids used more the open waters in these lakes, making them more vulnerable to predators. Zooplankton biomass varied substantially between the lakes, and the presence of exotic predators apparently increased the calanoids maximum body size, but not of cladocerans. The zooplankton community food web was wider in lakes with lower pelagic contribution to the fish diet. Our results suggest a negative effect by exotic predators on the niche width of galaxiids, but weak cascading effects on phytoplankton biomass, and a negative effect of introduced trout on the conservation status of Tasmanian lake ecosystems.