Symmetry in propagule pressure reduces establishment success of engineered bacteria with an Allee effect

The global threat of invasive species is a major ecological concern for the preservation of healthy, native ecosystems. However, the factors that favor establishment and subsequent invasiveness remain poorly studied. One candidate that has been proposed to predict establishment success is propagule pressure, or the number of individuals introduced at a given time. While promising, several recent studies have shown no link between propagule pressure and establishment success. As such, the role of propagule pressure in establishment success remains unclear, likely owing to the complexity of studying establishment dynamics in natural systems. To better understand how and when propagule pressure becomes important during an invasion, we used an engineered strain of Escherichia coli. This strain of bacteria has been engineered to have an Allee effect, a density-dependent population growth dynamic that is often observed in invasive species. Using this strain of bacteria as a surrogate for invasions in a natural systems, we measured how the timing between successive introduction events and how differences in the size of the introduced population determines establishment success. We observed that, for populations at the Allee threshold, a second introduction event of bacteria can lead to successful establishment, even when a single introduction event fails to allow growth. Moreover, we found that symmetry in the density of bacteria introduced in successive introduction events limits establishment success by reducing the timing between introduction events that allows for successful establishment. Overall, our results help to identify conditions where propagule pressure has a positive impact on establishment success, and may have implications in preventing and controlling establishment and spread of invasive species