360 Introduced Species, Impacts and Distribution of Shading and Overgrowth South American water hyacinth (Eichhornia crassipes) blankets many nearshore areas of Lake Victoria, some lakes and rivers of the US Southeast, and many water bodies in other regions It blocks light and smothers beds of native submersed vegetation Decaying water hyacinth can deposit over 1000 metric tons (wet weight) of detritus per hectare per year, which in turn can heavily modify water chemistry, such as nutrient concentrations Dissolved oxygen drops, affecting many animals The entire biotic community changes (Hill et al., 2011) Terrestrial plants such as Asian kudzu (Pueraria montana) in the southeastern US can similarly cover existing vegetation and eliminate it by shading, as can marine plants such as the tropical algae Caulerpa taxifolia and C racemosa in the Mediterranean Modified Fire Regime In much of the American West and Hawaii, Old World grasses such as cheatgrass (Bromus tectorum) increase the frequency and intensity of fires, greatly harming native plants and the animals that use them (Mack, 2011) Similarly, the Australian paperbark tree (Melaleuca quinquenervia) has a spongy outer bark and highly flammable foliage, and it also produces a more abundant litter than native herbaceous communities in south Florida These features have produced a changed fire regime that has facilitated the invasion of 100,000 Both tree and ground fires are now more intense and frequent, to the detriment of the native plants (Serbesoff-King, 2003) An added threat to the region is the recent arrival in the area of Old World climbing fern (Lygodium microphyllum), which transmits fires to the top of the tree canopy (Volin, 2004) Modified Hydrology Mediterranean salt cedars (Tamarix spp.) have invaded the US Southwest They are deeply rooted and transpire rapidly; once established, they can survive on water deep in the soil, and their transpiration is a significant pathway of water loss in arid areas For example, at Eagle Borax Spring in Death Valley, California, within 25 years of invasion by salt cedar, the surface water of what had been a large marsh had disappeared completely, along with the majority of its associated biota (McDaniel et al., 2005) In Israel, Australian Eucalyptus trees have been used deliberately to drain swamps and bogs, thus eliminating the original vegetation and animals dependent on them (Biger and Liphschitz, 1995) In addition to changed evapotranspiration rates, plants can also affect hydrological regimes by changing soil elevation Off the southern coast of Great Britain and parts of the coast of the state of Washington in the US, the cordgrass Spartina anglica has elevated intertidal areas and transformed gently sloping, productive mudflats into poorly drained marshes with enormous impact on resident animal species Spartina anglica is a species formed in Great Britain in the late nineteenth century by the hybridization of introduced North American cordgrass (S alterniflora) with native European S maritima (Simberloff, 2011) Modified Nutrient Regime Many plant invaders fix nitrogen, and, in nitrogen-poor areas, this added increment can favor other invaders and be detrimental to native species, which have evolved to thrive in a low-nitrogen environment The Atlantic nitrogen-fixing shrub Morella faya has invaded young, nitrogen-deficient volcanic regions of Hawaii There are no native nitrogen-fixers, and the invader thus alters productivity, nutrient cycling, and ecological succession Because many nonindigenous plant species in Hawaii are more successful on more fertile sites, M faya may enhance the likelihood of other invasions (Vitousek, 1986) Competition Some introduced species use a particular resource so effectively that they deprive native species of it In addition, an introduced species can depress a native one not by eliminating some resource but by direct interference Resource Competition Shading and water depletion, already discussed, are means by which some introduced plants outcompete native species for a crucial resource Introduced animals have similar effects For instance, the house gecko, Hemidactylus frenatus, has invaded many Pacific islands, and it depresses the insect food base locally so that some native lizard populations decline (Petren and Case, 1998) In Great Britain, the greater foraging efficiency of the introduced American gray squirrel (Sciurus carolinensis) has led to the decline of the native red squirrel (S vulgaris) The impact of the invader is enhanced by the fact that it carries a pathogen to which it is resistant but that devastates the native squirrel (Gurnell et al., 2004) Interference Competition The South American fire ant, Solenopsis invicta, which has spread throughout much of the southeastern US, attacks individuals of many native ant species and is replacing some species, such as the ‘‘native’’ fire ants (perhaps themselves preColumbian introductions) S geminata and S xyloni in several habitats (Sanders, 2011) In a plant analog of aggression, the African crystalline ice plant, Mesembryanthemum crystallinum, accumulates salt (Vivrette and Muller, 1977), and the salt remains in the soil when the plant decomposes In California, this ice plant excludes native plants that cannot tolerate the salt (Vivrette and Muller, 1977) In both of these examples, the invader does not actually render a resource in short supply for native species; rather, it directly inhibits the native, preventing its access to the resource Predation Some predaceous invaders have devastated particular native species or groups of them The Nile perch (Lates nilotica), introduced to Lake Victoria, eliminated many species of endemic cichlid fishes by eating them (Pringle, 2011) Introduced rats (Rattus spp.) on over 80% of the world’s archipelagoes have destroyed at least 37 species and subspecies of island birds worldwide (Pascal, 2011) The brown tree snake and the rosy wolf snail, mentioned previously, are predators that have extinguished many native prey species These are all dramatic examples of how an introduced predator can affect a native fauna that has not coevolved with it However, even prosaic