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During the past few centuries a large number of species has been introduced into new regions. Some of these alien species have become naturalized and, in many cases, also invasive (for terminology see Richardson et al. 2000a). Biological invasions homogenize the Earth's biota (Wilson 1975) and constitute a threat to agriculture, natural ecosystems and biodiversity (Drake et al. 1989; Vitousek et al. 1997). Moreover, biological invasions cause tremendous economic costs (Pimentel et al. 2000). The control of invasive organisms is expensive, labour intensive and usually has limited success (Myers et al. 2000; Hulme 2006). Therefore it is important to prevent new introductions of potentially invasive species. Unfortunately, while highly relevant in this context, we know little about species’ characteristics associated with their successful establishment outside the native range (Kolar & Lodge 2001). This information is required to develop screening procedures for potential invasiveness of species considered for introduction.
In the field of invasion ecology, there has been considerable confusion about correct terminology (Richardson et al. 2000a). Generally, we adhere to the definitions used in the proposed framework of Richardson et al. (2000a). In this framework, a naturalized plant species is an alien species that manages to reproduce consistently and sustains populations over many life cycles without direct intervention by humans. An invasive plant species, on the other hand, is a naturalized species that shows pronounced spread outside its native range. However, species that are considered invasive by some biologists are considered to be only naturalized by others. Because of these ambiguities, and because both naturalized and invasive plants have successfully established outside their native range, we will not distinguish between both classes and refer to them jointly as naturalized plant species.
Previous work has shown that naturalization is affected by the frequency of and time since introduction (Scott & Panetta 1993; Kowarik 1995; Wu, Chaw & Rejmánek 2003; Pyšek & Jarošik 2005), successful reproduction in small populations (Liebhold & Bascompte 2003; van Kleunen & Johnson 2005), hybridization with, and introgression of genes from, other species (Bleeker & Hurka 2001; Hurka, Bleeker & Neuffer 2003), global change (Dukes & Mooney 1999), anthropogenic disturbances (Byers 2002) and interactions of species with their new environments (Richardson et al. 2000b; Heger & Trepl 2003). Most of these factors, however, cannot be used as predictors of naturalization before introduction of a species because either the required information is lacking or it can only be assessed after the species has been introduced outside its native range. Therefore one of the major challenges in invasion biology remains identifying determinants of naturalization that can be assessed in the native range of a species before its introduction elsewhere.
We use the Iridaceae (iris family) from southern Africa to test for determinants of naturalization. Of the c. 1800 species of this cosmopolitan family, more than half are native to southern Africa (Goldblatt, Manning & Archer 2003). Because most species of the Iridaceae have attractive colourful flowers, many have been introduced outside their native range for horticultural purposes (Manning, Goldblatt & Snijman 2002; Burbank 2004). Some species of Iridaceae are among the world's most invasive species (Weber 2003). Data on intraspecific taxonomic diversity, and on biogeographical and biological species characteristics, which may be important potential predictors of naturalization, are readily available for these species. Therefore, this study system is ideal for testing for characteristics associated with horticultural introduction and naturalization.
We compiled and analysed a data set comprising all 1036 species of the Iridaceae native to southern Africa. Our specific objectives were to test whether the likelihood of (i) introduction for horticultural purposes is associated with taxonomic affinity and simple biogeographical and biological species characteristics, and (ii) naturalization is associated with global horticultural usage, taxonomic affinity and simple biogeographical and biological species characteristics.