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

Abstract

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.

Experimental evolution in fluctuating environments: tolerance measurements at constant temperatures incorrectly predict the ability to tolerate fluctuating temperatures

“…Usually the invasion success becomes the higher, the larger the propagule pressure is, and this has been a common finding also in experimental studies (Memmott et al 2005, Colautti et al 2006, Davis 2009, Lockwood et al 2009, Simberloff 2009, Blackburn et al 2011, Wittmann et al 2014, but see Gilbert andLechowicz 2005, Chytrý et al 2008), during every stage of invasion from dispersal to spread (Blackburn et al 2015). The genetic variance of the invading population is usually positively correlated to the magnitude of the propagule pressure, and for that reason it can be difficult to disentangle the relative impacts of genetic variability vs. propagule pressure on invasion success, without a manipulative approach (Bock et al 2015).…”

Inaccuracies in the history of a well-known introduction: a case study of the Australian House Sparrow (Passer domesticus)

“…This is nicely exemplified by the work that has examined the effects of propagule pressure on invasion success of passerine birds introduced to Australia, New Zealand and the United States (Blackburn et al 2011a(Blackburn et al , 2013Moulton et al 2011Moulton et al , 2012aMoulton and Cropper 2014). In their study, Moulton et al (2012b) highlighted the discrepancies between four different key sources with respect to propagule size and used the four different estimates to model the effect of propagule pressure on the success of introductions.…”

How the Yellowhammer became a Kiwi: the history of an alien bird invasion revealed

“…Dawson 1984;Veltman et al 1996;Duncan 1997;Green 1997;Sol and Lefebvre 2000;Cassey 2001;Duncan and Blackburn 2002;Brook 2004;Møller and Cassey 2004;Duncan et al 2006;Blackburn et al 2011a;Moulton et al 2011). Bird introductions there started in the middle of the nineteenth century, under the auspices of a range of Acclimatisation Societies ( Figure 1) founded with the express aim of naturalising alien species (McDowall 1994).…”

“…Veltman et al 1996;Blackburn et al 2011a) or indirectly (e.g. Green 1997) through secondary sources (Long 1981;Lever 2005), on an influential book by the New Zealand naturalist George Malcolm Thomson.…”

“…Not surprisingly, given these complications, there is variation in the numbers of introduced individuals reported by individual authors (see Moulton et al 2011); some works apparently underestimate the numbers (e.g. Thomson 1922;Lever 2005;Blackburn et al 2011a), whilst others overestimate them for certain regions (Veltman et al 1996;Duncan 1997). However, this of course raises the question of what was the actual number of birds introduced, and how much more accurate an estimate of this number can we get by mining historical sources?…”