Propagule pressure, Allee effects and the probability of establishment of an invasive species (Bythotrephes longimanus)

Predicting establishment of exotic species is a central goal of invasion biology, and is dependent upon propagule pressure and population processes. We introduced invading spiny water fleas, Bythotrephes longimanus at different propagule pressures into 19 experimental enclosures, following populations over asexual generations, resting egg production, and emergence in the following year. We integrated experimental results with field data to generate a stochastic population model, predicting establishment in relation to propagule pressure and introduction date. Our results suggested that Allee effects are operational at higher densities or smaller volumes than previously predicted, that stochasticity plays an important role in establishment, and demographic stochasticity may be correlated between individuals. Further, our novel theoretical derivations suggest that organisms should modify their sex ratios to reduce Allee effects. The functional form using adaptive sex-ratios was consistent with both mesocosm and field data. Despite the occurrence of Allee effects and stochasticity, there was still no date during the growing season where we predict lakes to be entirely safe from Bythotrephes invasions. A single propagule had approximately a 0.15 establishment probability in our mesocosms, if introduced early in the season; propagule pressures of 10 had > 0.50 probability, regardless of introduction date.