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

A reassessment of the role of propagule pressure in influencing fates of passerine introductions to New Zealand,10.1007/s10531-010-9969-8,Biodiversity

A reassessment of the role of propagule pressure in influencing fates of passerine introductions to New Zealand   (Citations: 1)
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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.
Journal: Biodiversity and Conservation - BIODIVERS CONSERV , vol. 20, no. 3, pp. 607-623, 2011
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    • ...In a recent paper, however, Moulton et al. (2011) question the validity of the importance of propagule pressure on the probability of establishment of exotic species...
    • ...Veltman et al. 1996; Duncan 1997; Green 1997; Sol and Lefebvre 2000; Cassey 2001; Brook 2004; Duncan et al. 2006), and the interpretation of the results from these releases is at the heart of our differences with Moulton et al. (2011)...
    • ...Moulton et al. (2011) argue that the propagule size required for a species to establish successfully in New Zealand is, therefore, the ‘‘minimum number from any introduction event’’ that results in a viable population...
    • ...In contrast, for a species that fails to establish, Moulton et al. (2011) argue that the relevant propagule size is the ‘‘maximum number from all introduction events’’, because even a propagule of this size was insufficient for the species to establish...
    • ...For example, Moulton et al. (2011) present data on bird releases from four different acclimatization districts in New Zealand...
    • ...According to the logic of Moulton et al. (2011), 62 individual skylarks was enough for establishment to occur, whereas a release of 119 linnets was not sufficient for their successful introduction...
    • ...Moulton et al. (2011) applied this logic (i.e...
    • ...We accept the argument put forward by Moulton et al. (2011) that combining data from different Acclimatization Society districts has the potential to over-estimate the importance of propagule pressure on the establishment success of birds introduced to New Zealand...
    • ...In the circumstances, we find it odd that Moulton et al. (2011) do not analyse the New Zealand passerine bird introduction data at the district scale, but rather combine data from districts to analyse establishment success for the whole country, and for the two main islands (North and South) separately...
    • ...Analysis of the New Zealand passerine bird introduction data by district produces substantially different results to those of the analyses presented by Moulton et al. (2011)...
    • ...We used the data on propagule sizes for each passerine introduction into the four acclimatization districts presented by Moulton et al. (2011)...
    • ...Generalised linear mixed models were conducted in SAS v9.2 (Proc GLIMMIX) treating species identity as a random effect and testing for fixed effects of acclimatization district and propagule size (as detailed in Moulton et al. 2011) on establishment success...
    • ...Finally, we analysed propagule size binned into logarithmic classes (following Duncan et al. 2006; Blackburn et al. 2009a, b), as suggested by Moulton et al. (2011)...
    • ...It is unsurprising that the data used by Moulton et al. (2011) for passerine bird introductions to New Zealand support the importance of propagule size as a determinant of establishment success...
    • ...Why then do the analyses presented by Moulton et al. (2011) differ from all other analyses of New Zealand bird introduction data? There are at least three reasons...
    • ...Moulton et al. (2011) adopt this method despite arguing that different climates, geology and native vegetations between regions could all impact upon introduction fates...
    • ...In fact, the logic proposed by Moulton et al. (2011) dictates that a global-scale analysis of establishment success should reduce to one data point for each species—the smallest propagule size that results in a viable population for a species that establishes successfully at a location, and the largest propagule size for a species that fails to establish anywhere...
    • ...With the exception of the common myna (Acridotheres tristis), Moulton et al. (2011) do not include any mixed outcomes (i.e., species that were successfully introduced into one district by one or more Acclimatization Societies but failed in other districts) in their analysis...
    • ...In this case Moulton et al. (2011) summed all recorded introductions (100 individuals) to calculate a single propagule pressure for skylarks for the Otago region...
    • ...If it is known which of the multiple introductions were successful, and which were not, then presumably for a species that succeeds overall Moulton et al. (2011) would have advocated only the smallest of the successful propagules be included and the subsequent failed attempts discarded...
    • ...The starting population sizes were randomly sampled without replacement from the discrete distribution of bird propagule sizes for New Zealand given by Moulton et al. (2011)...
    • ...Moulton et al. (2011) argue that the historical record of bird introductions to New Zealand contains many uncertainties, which contribute to variation in the set of species included in analyses of establishment in this fauna, as well as variation between studies in the propagule sizes and/or pressures assigned to various introductions...
    • ...We agree with Moulton et al. (2010, 2011) that historical data on bird introductions are unlikely to be accurate...
    • ... of the effect of propagule size on house sparrow establishment success (Table 2). The comparison of skylark (N = 434, 100, 108, 62) and linnet (N = 119, 2, 22, 42) Table 3 Logistic regression statistics for the putative relationship between propagule size (log10 propagule) and establishment success across 100 iterations of a VORTEX model for two populations separately (P1 and P2) and their combined estimates [following the methods of Moulton ...
    • ...Nevertheless, as we showed above, the evidence from New Zealand bird introductions is itself robust, despite the concerns raised by Moulton et al. (2011)...

    Tim M. BlackburnThomaset al. Passerine introductions to New Zealand support a positive effect of pr...

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