Passerine introductions to New Zealand support a positive effect of propagule pressure on establishment success

There is growing consensus in the literature on biological invasions that propagule pressure (or a component thereof) is the primary determinant of establishment success in introduced species. However, a recent paper (Moulton et al. Biodiver Conserv 20:607-623, 2011) questions whether this consensus is justified. It argues that the effect of propagule pressure is not general because most of the evidence for it comes from analyses of historical bird data to New Zealand, and, moreover, that both the analyses and the data on which they are based are faulty. Moulton et al. (Biodiver Conserv 20:607-623, 2011) present a re-analysis that fails to find a relationship between establishment success and propagule pressure in New Zealand bird introductions. Here, we show why these criticisms are unjustified. A robust analysis of New Zealand bird data reveals that propagule pressure is indeed positively related to establishment success, and we present a simple population viability analysis to demonstrate why the method adopted by Moulton et al. (Biodiver Conserv 20:607-623, 2011) fails to demonstrate this result. We further show that there is abundant evidence for a relationship between establishment success and propagule pressure in biological invasions outside of historical bird introductions to New Zealand. We conclude that propagule pressure is indeed a primary determinant of establishment success in introduced species.