A reassessment of the role of propagule pressure in influencing fates of passerine introductions to New Zealand

Abstract

Several studies have argued that principal factor in determining the fate of bird introductions is introduction effort. In large part, these studies have emerged from analyses of historical records from a single place-New Zealand. Here we raise two concerns about these conclusions. First, we argue that although many bird species were introduced repeatedly to New Zealand, in many cases the introductions apparently occurred only after the species were already successfully naturalized. The inclusion of such seemingly superfluous introductions may exaggerate the importance of propagule pressure. And second, we question the reliability of the records themselves. In many cases these records are equivocal, as inconsistencies appear in separate studies of the same records. Our analysis indicates that species were successful not because they were introduced frequently and in high numbers, but rather it is likely that they were introduced frequently and in high numbers because the initial releases were successful.

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

“…The quality of New Zealand historical data on bird introductions has recently been criticised by Moulton et al (2011Moulton et al ( , 2012a. They go so far as to state that "these historical records are in many cases mere rumors and impressions, and not actual "data" in any sense (Moulton et al 2012a).…”

“…Wonham et al 2013), analyses of bird invasions are often based on direct data on numbers derived from historical records of introductions ). Nevertheless, the quality of data on bird propagule pressure is increasingly being questioned in the literature (Moulton et al 2011(Moulton et al , 2012aSantos 2012), and this criticism has even been used to question the importance of propagule pressure as a key driver of establishment success.…”

“…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).…”

“…Auckland Acclimatisation Society 1868; Daily Southern Cross 4 April 1871, Page 3), while others were re-transported to Australia (Press 27 March 1880, Page 3). 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).…”

“…We aim to demonstrate how imprecise information can be propagated from one source to another, but that by going back to the primary sources of information (and avoiding "second-hand" resources), many uncertainties and inconsistencies that stem from using incomplete compilations can be eliminated (see also Moulton et al 2011Moulton et al , 2012aHaemig 2014). Finally, by correcting some erroneous data and pointing to misinterpretations in other treatments, we provide the most accurate information to date on yellowhammer introductions to New Zealand.…”