Decomposing propagule pressure: the effects of propagule size and propagule frequency on invasion success

Abstract

Propagule pressure quantifies the inflow of individuals to a location and appears to be a key driver of invasion success. It is often defined as the average number of individuals introduced per time unit, or equivalently as the product of the average number of individuals introduced per introduction event (propagule size) and the frequency of introduction events (propagule frequency). Here we study how the influence of propagule size, frequency, and their product depends on the underlying ecological conditions. While previous studies have focused on introductions under environmental heterogeneity or a strong Allee effect, we examine a range of ecological scenarios that differ in the type of density dependence and in the sign of per capita growth rate. Our results indicate that the relative influence of propagule size and frequency depends mainly on the sign of per capita growth rate. Given a certain average number of individuals introduced per time unit, a high propagule frequency accelerates invasions under ecological scenarios with positive average per capita growth rate throughout the invasion process (‘easy’ scenarios). If per capita growth rate is negative throughout the invasion process (‘difficult’ scenarios) or if there is both an easy and a difficult stage (‘mixed scenarios’), a high propagule size leads to a faster invasion than a high propagule frequency. To explain this finding, we argue that for a fixed value of the product of propagule size and frequency, an increase in propagule size leads to an increase in demographic variance, which promotes invasion success in difficult and mixed but not in easy scenarios. However, we also show that in many of these cases, the product of propagule size and frequency still correlates more strongly with invasion success than either of the single components. Finally, we illustrate our approach with empirical examples from the literature.