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cies extinctions because the degree of certainty generally associated with listed causes of plant extinctions in the International Union for Conservation of Nature database seemed too speculative. Still, if predation is the true cause of most species interaction-based extinctions then perhaps this can help to explain why competition from invasive plants has led to so few plant extinctions—at least so far.

INVASIONS AND EXTINCTIONS ON ISLANDS

Patterns of species invasions and extinctions have been well documented across a wide variety of islands and for a number of taxonomic groups (Chown et al., 1998; Sax et al., 2002; Sax and Gaines, 2003; Blackburn et al., 2004). In general, many species of plants, vertebrates, and invertebrates have been introduced to islands (Eldredge and Miller, 1995; Chown et al., 1998; Sax et al., 2002). Many of these introduced species have become naturalized, i.e., they have formed self-supporting populations capable of perpetuating themselves. Islands have also lost many native species; among vertebrates, extinctions have been greatest for bird species, largely because most other vertebrate groups are relatively depauperate on islands (Lomolino et al., 2006). In contrast to birds, plants have generally suffered few extinctions on islands (James, 1995; Sax et al., 2002). For example, in New Zealand, 38 of 91 native land bird species have become extinct, whereas only 3 of >2,000 native plant species have become extinct (Sax et al., 2002). Overall, these patterns of extinction and naturalization have led to large changes in net species richness on islands around the world. Bird richness on most oceanic islands has remained largely unchanged, because the number of extinctions has been largely matched by the number of exotic birds that have become naturalized (Sax et al., 2002). This relative consistency in net bird richness may be important in understanding and predicting future extinctions, but is not a “good” thing from a conservation perspective, because it means that many unique endemic species have been lost and replaced by more cosmopolitan species from mainlands (McKinney and Lockwood, 1999). In contrast to birds, mammal richness has increased dramatically, particularly on oceanic islands, which have few native mammal species (Blackburn et al., 2004). Freshwater fish richness has also increased, because few native fishes have gone extinct (at least so far), whereas many exotic species have become naturalized (Sax and Gaines, 2003). Invertebrate richness may also have increased, because many invertebrates have become naturalized on islands, e.g., >2,500 species on Hawaii alone (Eldredge and Miller, 1995), but records of extinction are less certain, so it is difficult to know how net richness has changed without additional investigation. Finally, vascular plants have seen dramatic increases in richness across both continental and oceanic



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