Invasive Species - Ecological Systems - page 167

nonnative species can profoundly affect ecosystems by altering community
composition, resident species interactions, physical habitat structure, hydrology,
nutrient cycling, contaminant cycling, primary production, and natural disturbance
(fire, flood, erosion) regimes [ 17 , 62 - 64 ]. They can disrupt food webs [ 65 , 66 ] and
plant-animal mutualisms that are crucial for pollination and seed dispersal [ 67 , 68 ].
Even where environmental stressors such as habitat degradation have already
caused population declines of native species, invasions can accelerate these
declines [ 69 ]. They are a major cause of animal extinctions [ 70 , 71 ], particularly
in insular habitats, such as lakes, river basins, and islands [ 72 , 73 ]. The invasion-
mediated loss of genetically distinct native populations in continental regions has
likely been grossly underestimated. There are examples of once widely distributed
species being reduced to near extinction as a result of introduced pathogens [ 17 ].
Some of the greatest impacts on biodiversity are caused by nonnative predators, and
the most conspicuous examples involve introductions to oceanic islands [ 74 , 75 ]
and freshwater ecosystems [ 76 ]. Large mammalian herbivores have also had
devastating effects on island biodiversity [ 77 , 78 ]. Other factors contributing to
species loss at local to global scales include hybridization [ 79 , 80 ], competition
[ 69 ], disease transfer [ 81 ], food web alteration [ 65 , 66 , 68 ], and physical habitat
alteration [ 17 ].
Entire ecosystems may be transformed by invaders that alter resource availabil-
ity, disturbance regimes, or habitat structure. Some invaders alter the disturbance
regime of habitats through fire suppression (e.g., the shrub Mimosa pigra in
Australian flood plains), fire enhancement (e.g., Eurasian cheatgrass Bromus
tectorum in the Western United States), increased erosion (e.g., the Australian
shrub Acacia mearnsii in South Africa), reduced erosion (e.g., exotic plants with
extensive root systems that stabilize hills, stream banks, or sand dunes), and
increased soil disturbance (e.g., the rooting activities of feral European pigs Sus
scrofa can destroy the herbaceous understory of a forest, causing soil mineral
depletion, rapid organic decomposition, and loss of habitat). Through its
filter-feeding activities, the zebra mussel ( Dreissena polymorpha ) has dramatically
increased water transparency in North American and European lakes, thus
stimulating the growth of benthic algae and macrophytes and altering physical
habitat for invertebrates and fishes [ 82 ]. In Hawaii, a nitrogen-fixing tree, Myrica
faya , significantly enriched nutrient-poor volcanic soils at a rate 90-times greater
than native plants and thus has a dominant influence on ecosystem properties
including soil chemistry and productivity [ 83 ]; Myrica has also added habitat
structure, shading, and high-quality leaf litter that has promoted enhanced

populations of nonnative earthworms [ 84 ].

Socioeconomic Impacts

The economic value of cultivated nonnative species (such as crop plants) is widely
appreciated, but the same cannot be said for the enormous costs incurred by
invasions in general. In several countries, nonnative species comprise more than