Many species have been deliberately or accidentally introduced to areas outside of where they naturally occur (Long 1981; Lever 1994). Some species have established populations successfully and spread, but others either failed to establish or established but have not spread. Although these introductions have often had disastrous consequences for indigenous biota (e.g. Savidge 1987), they also provide opportunities to improve our understanding of biological invasions and other ecological processes.
The historical record of bird introductions to New Zealand has been used extensively to test hypotheses about the factors influencing introduction success and the subsequent spread of introduced birds (Veltman, Nee & Crawley 1996; Duncan 1997; Green 1997; Sorci, Møller & Clobert 1998; Duncan, Blackburn & Veltman 1999; Legendre et al. 1999). A similarly detailed record of bird introductions to Australia exists: at least 55 species have been introduced to mainland Australia, of which 19 have successfully established (Long 1981; Newsome & Noble 1986). Our aim in this study is to test hypotheses about factors thought to influence introduction success and geographical range size using this historical record. The hypotheses that we test can be divided into three groups:
- 1Availability of climatically suitable habitat. One of the most frequently stated hypotheses in the biological invasion literature is that species should have a greater chance of success if they are introduced to an area with a climate that closely matches that in their original range (Brown 1989; Mack 1996; Williamson 1996). This hypothesis has never been tested using a set of successful and failed introductions. Here we test the hypothesis that introduced species with a larger area of climatically suitable habitat in Australia should have both a higher probability of successful introduction and, if successful, should have a larger geographical range in Australia.
- 2Life history traits. Certain life history traits are hypothesized to increase the probability of successful introduction and to be associated with larger geographical range sizes. In particular, species with a faster population growth rate should have higher introduction success because they will be less vulnerable to local extinction when their population is small and, following successful introduction, should be able to colonize more sites and thereby attain a larger range size (Gaston 1988; Hanski 1991; Hanski, Kouki & Halkka 1993; Holt et al. 1997). We test this hypothesis by searching for significant correlations between introduction success, geographical range size and biological traits that are probable correlates of population growth rate, including body size, clutch size, number of broods per season and incubation time. In addition, species with sexually dichromatic plumage, migratory species, solitary species and species with a narrow diet are hypothesized, or have been shown, to have a lower probability of successful introduction (Mayr 1965; Veltman et al. 1996; Sorci et al. 1998). Birds that are partial migrants in their natural range have also been shown to have a larger geographical range size following introduction to New Zealand (Duncan et al. 1999).
- 3Historical factors. Greater introduction effort (number of introduction attempts, number of locations at which species are introduced or the total number of individuals introduced) is the strongest predictor of success for birds introduced to New Zealand (Veltman et al. 1996), and predicts the success of bird introductions to Australia (Newsome & Noble 1986). For introduced Australian birds, we extend this result by testing if greater introduction effort is associated with a larger geographical range, as is the case for birds introduced successfully to New Zealand (Duncan et al. 1999). We also test if successful introduction elsewhere and the overseas range size of a species predict introduction success in Australia, as has been found elsewhere (Moulton & Pimm 1986; Rejmánek 1996; Reichard & Hamilton 1997).