Invasive species

An invasive species is one that arrives (oftenwith human assistance) in a habitat it had not previously occupied, then establishes a population and spreads autonomously. Species invasions are one of the main conservation threats today and have caused many species extinctions. The great majority of such invasions are by species introduced from elsewhere, although some native species have become invasive in newly occupied habitats (see Box 7.1). In some areas of the world—especially islands (see Box 7.2)—introduced species comprise a large proportion of all species. For instance, for the Hawaiian islands, almost half the plant species, 25% of insects, 40% of birds, and most freshwater fishes are introduced, while the analogous figures for Florida are 27% of plant species, 8% of insects, 5% of birds, and 24% of freshwater fishes. Not all introduced species become invasive, however. Many plant species imported as ornamentals persist in