Interactions between the invasive New Zealand mud snail, Potamopyrgus antipodarum, baetid mayflies, and fish predators
The nonindigenous gastropod Potamopyrgus antipodarum is quickly spreading through aquatic environments in the western United States, and populations often occur at very high densities. In previous studies, densities of baetid mayflies were lower in reaches with high densities of Potamopyrgus than in reaches with low densities of Potamopyrgus in Darlinton Spring Creek (Madison River drainage, Montana, USA), suggesting exploitation or interference competition. To determine when Potamopyrgus had the greatest effect on baetids, I quantified densities and biomasses of Potamopyrgus and baetids in low-snail and high-snail reaches over 18 months in Darlinton Spring Creek. Potamopyrgus had little effect on the densities or biomasses of baetids. Measurement of periphyton biomass at the same times as macroinvertebrate densities indicated that Potamopyrgus depressed periphyton biomass. Using an in situ experiment, I quantified intra- and interspecific competition between baetids and Potamopyrgus by measuring survivorship and body growth.Baetis and Diphetor survivorship decreased when combined with Potamopyrgus, but there was no effect of interspecific competition on body growth. Potamopyrgus survivorship decreased when combined with Diphetor or Baetis, and Potamopyrgus growth was negatively affected by intraspecific competition. To explore the effects of Potamopyrgus on secondary consumers, I compared the diets and growth of Salmo trutta and Cottus bairdi in an enclosure experiment in reaches with high and low-densities of Potamopyrgus. Only one Potamopyrgus was found in the stomachs of the fishes, indicating that Potamopyrgus was not an important food source. There were no differences in growth for either fish species between highsnail and low-snail reaches. Additionally, there was little evidence suggesting that these fishes shifted their diets to compensate for changes in food availability associated with high densities of Potamopyrgus. My results indicated that Potamopyrgus and baetids competed in the experiments; however, the effects of competition were not evident in the field surveys even though Potamopyrgus depressed periphyton biomass. Additionally, fishes were not negatively affected by Potamopyrgus. During my studies, Potamopyrgus densities were moderate. Thus, moderate densities of this invader may not have a large effect on communities. I caution against interpreting these results to mean that Potamopyrgus will not have effects in other aquatic systems.