Propagule pressure hypothesis.

Abstract

Abstract Propagule pressure is a composite measure of introduction effort consisting of: (i) the number of individuals introduced per introduction event (propagule size); and (ii) the frequency of introduction events (propagule frequency or number). The propagule pressure hypothesis posits that a high propagule pressure is a cause of invasion success; in other words, non-native species with a high propagule pressure have a higher invasion success than non-native species with a low propagule pressure. On the basis of a systematic review, we identified 92 relevant empirical studies testing the propagule pressure hypothesis. These studies have addressed different aspects - that is, suband sub-sub-hypotheses - of the overall hypothesis. Independently of the specific aspects considered by each study, the propagule pressure hypothesis is largely supported by currently available evidence. About 80% of the 92 studies reported supporting evidence. Similarly, the propagule pressure hypothesis is empirically supported across major taxonomic groups, habitats and methodological approaches. This hypothesis is among the most influential ones in the field and represents the recognition that in order to understand biological invasions, one must consider humans and their actions as key underlying drivers.