358 Introduced Species, Impacts and Distribution of The Geography and Magnitude of Invasion by Introduced Species How Many Are There, Where Do They Come From, and Where Do They Go? No comprehensive list of introduced species exists for most taxa in most locations; indeed, there is often no list of native species However, for well-studied groups, some figures are impressive In Florida, for example, 27% of established plant species, 8% of insects, 29% of land snails, 24% of freshwater fishes, 22% of reptiles, 5% of birds, and 24% of land mammals are introduced (Simberloff et al., 1997) For the Hawaiian Islands, as for many islands, some of the analogous figures are even greater: almost half the plants, 25% of insects, most freshwater fishes, 40% of birds (Staples and Cowie, 2001) In some areas (e.g., Alaska) there are far fewer introduced species, but almost no regions are immune Although the absence of adequate quantitative data makes it difficult to describe this pattern fully, it is widely believed that Eurasian species are more likely to invade other regions than vice-versa and more likely to have large impacts For instance, most major human pathogens and most plant pathogens that have had global impacts originated in Eurasia Similarly, more Eurasian insects, vertebrates, and plants have invaded other regions than vice-versa The reasons for this imbalance are obscure Some authors have argued that Eurasian species have an innate superiority, generated either by the larger numbers of species evolving greater competitive ability on the larger landmass, or by the happenstance that Eurasian species were highly coevolved – plants, pathogens, and animals (including especially grazers and humans) – and overwhelmed native species by their joint action For example, the structure and behavior of Eurasian hooved livestock was devastating to native tussock grasses in the North American prairie but favorable for Eurasian turfgrasses, which now dominate vast regions (Mack, 1986) However, even if Eurasian species, singly or in groups, were not innately superior, one might have expected a preponderance of them among introductions, because the opportunities for such species to reach other regions were greater throughout most of recent history For instance, the majority of introduced insects in the US through the eighteenth century came from Europe in soil ballast, loaded in Europe and exchanged in North America for various raw materials (Lindroth, 1957) Similarly, as Europeans colonized other regions, they formed acclimatization societies to introduce the birds of their homelands (Lever, 1992) Except for gamebirds, there was little analogous movement of species in the opposite direction Nowadays, as travel and trade are burgeoning worldwide, opportunities for introduction from any region to any other one are greatly enhanced, and one might expect habitat and climatic matching to become more important as limiting factors When Did They Get There? The timing of introductions has depended heavily on available means of transport and patterns of travel and trade, and it has tended to increase strongly from the late eighteenth century through the present For example, introductions of aquatic plants and animals into the Great Lakes rose steadily from one species between 1810 and 1839 to over 40 between 1960 and 1990, a surge associated with the opening of the Saint Lawrence Seaway (Mills et al., 1993) The advent of rapid steamship transport across oceans coincided with a dramatic increase in introductions of many taxa, as hitchhikers that could not have stayed alive over a month or more in transit were able to survive a voyage of two weeks across the Atlantic, for example Air travel decreased the need to survive a long transit period still further Overlaying this dominant pattern of increasing rates of introduction with increased transport volume and decreased transit time are idiosyncrasies associated with particular taxa and regions For example, beginning around 1920, there was a decrease in the rate at which foreign insect species, especially herbivorous species, were introduced to the US; this downturn coincided with the enforcement of plant quarantine laws At the same time, a dramatic increase of introduction of wasp species reflected greatly increased biological control efforts, especially the use of parasitic wasps (Simberloff, 1986) Distribution of Introduced Species among Habitats Habitats modified or routinely disturbed by humans generally house more introduced species and larger populations of introduced species than pristine habitats, though even the latter are occasionally invaded The reason for this pattern has been hotly debated In general, the very fact of human disturbance renders a habitat less suitable to the native species that had evolved in the original habitat However, some species, often associated with humans, are superbly adapted to habitats that humans create Thus, certain introduced plants are routinely found on lists of serious weeds from many parts of the world A common claim that disturbed habitats are invasible because they are species-poor is incorrect Many habitats relatively undisturbed by humans, such as saltmarshes and mountaintops, have few native species, but they also have few introduced species, and for the same reason – they are biologically difficult environments and few species of any kind have physiological adaptations that permit them to thrive there Conversely, enormously diverse tropical communities often have as many invaders (with as large impacts) as less species-rich temperate analogs Nor does periodic disturbance per se automatically conduce to invasiveness For example, the intact upland pine forests of north Florida are far less stricken by invaders than many other habitats in the state, yet they frequently burn naturally because of lightning strikes If anything, this disturbance favors the many native species that are adapted to a fire regime over a plethora of potential invaders that are not However, the increasing invasion of this region by fire-adapted cogon grass (Imperata cylindrica) poses a threat to the entire forest ecosystem (Schmitz et al., 1997) Most well known examples of introduced species with enormous impacts come from either terrestrial or freshwater habitats Marine habitats are less represented in the invasion literature Freshwater habitats – lakes and rivers – can often be seen as habitat islands, surrounded by land, and thus their communities are believed to be inherently invasible for the same reasons that islands appear to be particularly invasible,