Plant and animal species are inadvertently moved around the globe when ships take on ballast water in one port and discharge it in another; seeds of alien plants and either live animals or dormant stages are then available to colonize salt marshes. When the USA resumed trade with China, new invaders gained access to San Francisco Bay. Fred Nichols traced the arrival of a small clam, Potamocorbula amurensis, to 1876. Now it coats some benthos with thousands of clams/m2.
Other alien species have been intentionally introduced. In the 1950s, the US Army Corps of Engineers experimentally introduced S. alterniflora onto several dredge spoil islands to stabilize the material and provide wildlife habitat. A region-wide invasion of the Pacific Northwestern USA followed several decades of 'benign' behavior. Today, the species is dominant along the lower edge of salt marsh shorelines, where it displaces oysters and eliminates shorebird-feeding habitat.
Once a species has taken up residence, it might hybridize with native species and become more aggressive, either as the hybrid or subsequent genetic variants. Such is the case for S. alterniflora, which has been widely planted in Europe, China, Great Britain Australia, and New Zealand. In Great Britain, it hybridized with the native S. maritima to form S. townsendii, which then underwent chromosomal doubling to form S. anglica. S. anglica can grow at lower elevations than native species and vigorously colonizes mudflats. Dense clones of S. anglica reduce habitat for wading birds and displace native salt marsh plants.
Non-native strains of Phragmites australis were introduced to the USA 200 years ago, and they have since spread throughout much of North America. Today, the alien strain dominates the less-saline portions of salt marshes in the northeastern USA, where it displaces native plant species, alters soil conditions, and decreases waterfowl use. Disturbances such as ditching or dredging open salt marsh canopies and allow invasion of P. australis, while eutrophication, altered hydrologic regimes, and increased sedimentation favor its spread.
Invasive plant species have been linked to reduced diversity, shifts in trophic structure, habitat alteration, and changes in nutrient cycling. Invasive alien animals are equally problematic. In San Francisco Bay wetlands, alien mudsnails outcompete native ones and the Australasian isopod, Sphaeroma quoyanum, burrows into and destabilizes creek banks of tidal marshes, causing erosion. Marsh edge losses exceeding 100cmyr-1 have been reported in heavily infested areas. Another invader, the green crab, Carcinus maenus, has altered the food web of Bodega Bay, California, by reducing densities of a native crab, two native mussels, and other invertebrates. As the green crab moves north, it will likely reduce food availability for shorebirds.
In the southeastern USA, fur farmers introduced nutria (Myocastor coypus) from South America in the 1930s. These rodents feed on roots of salt marsh plants. When fur clothing went out of style, nutria populations expanded and began converting large areas of marsh to mudflat and open water.
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