The Ecology of Coastal Wetlands - Marine Ecology in a Changing World - page 149

in a considerable expansion throughout the British Isles and nearby
Europe over a relatively short period. S. anglica has also displaced native
Puccinellia maritima on Dutch coasts (Daehler and Strong 1996), and mudfl at,
seagrass, mangrove, and marsh habitats in southern Australia. The rapid
colonization of S. anglica over extensive fl ats can be viewed as an autogenic
control whereby plants may cause a rapid buildup of sediment, and thereby
signifi cantly affect coastal dynamics (Allen 2000).
The intertidal zone in the Yangtze Delta extended over about 1,550
km 2 , but an area of 750 km 2 has been reclaimed in the past half century,
and approximately one third remains as salt marsh (Gao and Zhang 2006).
In 1979, S. alternifl ora was transplanted into coastal China to stabilize tidal
fl ats. Since its introduction, this species has gradually invaded the former P.
australis communities, and the upper Scirpus mariqueter zone, aggressively
replacing native species (Cheng et al. 2006). At present, the plant zonation
of coastal wetlands in the Jiangsu Province typically includes S. alternifl ora
in the lower limit of vegetation, dominating the upper intertidal zone (Liu
et al. 2007).
Spartina foliosa has a narrow range of distribution, limited to the Pacifi c
coast of North America, from Humboldt Bay to Baja California. Similar to
S. alternifl ora, S. foliosa takes on a tall or “robust” form, which grows at lower
elevations, and a “dwarf” form at elevations closer to mean high water.
Further north, exotic species are invading including the hybrid S. alternifl ora
x S. foliosa in the San Francisco Estuary, where it has been reported to increase
in area a 100-fold since the 1970s, and Spartina densifl ora , a South American
native, ranking second in area covered, also appearing in Humboldt Bay.
Finally, S. patens and S. anglica , the latter a hybrid species of S. alternifl ora
and the European S. maritima , are also found, but have not yet dispersed
far beyond their introduction sites (Ayres et al. 2004).
In Willapa Bay, nearly one-third of the area originally covered by
mudfl ats is now infested with S. alternifl ora . This species was introduced
into Willapa Bay during the late 1800s but was not identifi ed until the 1940s.
During the fi rst 50 years, the population expanded slowly, but from 1945
to 1988 the plant spread rapidly throughout the bay, resulting in severe
habitat alteration as unvegetated mudfl ats were converted to salt marshes
(Simenstad and Thom 1995). Salt cedars, Tamarix spp., have also invaded
wetlands on the Pacifi c coast of North America (Whitcraft et al. 2007). The
trees are salt tolerant and notably disrupt water cycling in riparian wetlands
(Zedler and Kercher 2004). In salt marshes, Tamarix invasion converts
succulent plant communities less than 1 m in height into woody-dominated
communities with a 3-m canopy (Whitcraft et al. 2007).
Phragmites australis, a European reed, has increased in abundance in
North American marshes over the past century, outcompeting native plants
(Minchinton et al. 2006, Orson 1999, Saltonstall 2002). This large reed is a