A precipitous decline in an invasive snail population cannot be explained by a native predator

Population dynamics of invasive species can exhibit rapid population growth followed by rapid decline. Populations of Potamopyrgus antipodarum, an invasive gastropod native to New Zealand, can follow this pattern, but potential mechanisms are unclear. We assessed the biomass of P. antipodarum and native macroinvertebrates over 16 years in Polecat Creek Wyoming (USA), a spring-fed tributary of the Snake River. We documented a 15-fold drop in the biomass of P. antipodarum. Biomass of collector–filterers fluctuated in the opposite direction to that of P. antipodarum biomass, suggesting the invasive snails may suppress taxa in this functional feeding group. Because the biomass of planarians (turbellarian flatworms) fell sharply with the biomass of P. antipodarum, and because planarians (Dugesia spp.) prey on P. antipodarum, we also evaluated the functional response of Dugesia to the invasive snails and estimated predation rate. To assess whether predation by Dugesia could have caused the large drop in biomass of P. antipodarum, we used monthly matrix projection models and their periodic matrix products to estimate the annual population growth rate (λ) of P. antipodarum with and without predation. When we added predation by planarians to the matrix population models, we observed that the proportion of models with λ< 1 increased by ≈8% at most. Thus, Dugesia may shift the population from growing to declining, but predation alone probably did not explain the large decline of P. antipodarum in Polecat Creek.