Relative importance of propagule size and propagule number for establishment of non-indigenous species: a stochastic simulation study

Abstract

Propagule pressure is emerging as the most consistent predictor of establishment in non-indigenous species. Increasing propagule size (the number of individuals arriving in a novel environment at one time) is thought to increase probability of establishment by counteracting demographic stochasticity and Allee effects. Increasing propagule number (the number of introduction events) is thought to increase probability of establishment by counteracting environmental stochasticity. However, the relative importance of these effects and the conditions under which one effect may become predominant is largely unexplored. We first used stochastic population simulations, with a constant number of immigrants distributed over varying numbers of introduction events, to determine the relative importance of propagule size and number on the probability of establishment. Simulations were conducted under different environmental stochasticity and Allee effects regimes with parameters set to be representative of species with high risk of invasion. In absence of Allee effects, there was a slight increase in probability of establishment when going from very low to low propagule number. However, when Allee effects were included, probability of establishment consistently decreased with increasing propagule number. This pattern was consistent across a wide range of parameter values, and tended to be more pronounced when other model parameters resulted in low probability of establishment. This suggests that, within a range of conditions representative of non-indigenous species at high risk of invasion, large infrequent introductions are riskier than smaller more frequent ones. This is particularly true when Allee effects are likely to influence population dynamics at the early stages of the invasion process.