All of invasive species are introduced by chance or deliberately into a region or country by human being. However, the fates of invasive species vary with different invasive pattern and passages. In the last two hundred years, S. alterniflora was introduced many countries in the world, and its distribution has expanded from Atlantic coast of America to Europe, western coast of North America, New Zealand and coast of China (Baumel et al., 2001).
In England, S. alterniflora was introduced by chance in the Southampton area from east coast of North America in 1816, and then the species dispersed to British and Ireland where it hybridized with the native Spartina maritime Fernald, resulting in a sterile perennial hybrid named Spartina townsendii H. & J. Groves. Spartina townsendii still remains localized in the locality of Hythe (Raybould et al. 2000). Chromosome doubling in the hybrid gave rise to a new fertile and robust rhizomatous allopolyploid species, Spartina anglica C.E. Hubbard, which has been actively colonizing British and French salt marshes and estuaries since its formation around 1890 (Baumel et al. 2001). In France, S. alterniflora was also introduced during the 19th century in Brittany near Brest (Elom Estuary), and in the Basque region near Hendaye (Bidassoa Estuary). In Hendaye, S. alterniflora hybridized with native Spartina species, S. maritime, and produced hybrid S. x neyrautii. In England and France, the S. anglica was rapidly expanding, and the distribution of S. alterniflora was significantly reduced. The species only remained in local sites, such as the sole remaining site of S. alterniflora in Britain was at Marchwood (Raybould et al., 1991).
S. alterniflora plant parts or seeds probably contaminated barrels used to pack oyster spat and young adults for shipment to Willapa Bay, Washington in the 1800's and early 1900's. 9% of the total intertidal habitat in Willapa Bay was infested by the species. Experimental studies for control of this weed by State of Washington and federal agencies were in the late 1980s. The expansion of S. alterniflora populations introduced accidently was at a lower level and local distribution for Allee effect (Davis et al., 2004a, 2004b; Taylor et al., 2004).
The invasion of S. alterniflora resulted from purposive introduction rather than involuntary infection. A population of S. alterniflora had been identified in the late 1930s, and the Oregon Department of Agriculture began control efforts in 1990. The infestation was finally eradicated in 1997 by digging. S. alterniflora was deliberately planted in San Francisco Bay in 1973 as part of a marsh reclamation project near the town of Fremont. Five years later, almost ten thousand plants of S. alterniflora were transplanted from Fremont into the shoreline of Alameda Island, and later to San Bruno. During the 1980s, restoration projects throughout the Bay may have further spread the species. Seeds and individuals of three ecotypes of S. alterniflora were introduced from North Carolina, Florida and Georgian on the east coast of America to China respectively. Initial plant materials were planted in Luoyuan County of Fujian Province, China. In the after years, the seeds or / and vegetative individuals were transplanted from Luoyuan County to coastal provinces of China. The area of the S. alterniflora was explosively expanding from 2.6 km2 in 1985 to 1,120 km2 in 2002. Thus, S. alterniflora as one of sixteen species was placed on the first invasive species list by State Environmental Protection Administration of China (SEPAC) (Wang et al., 2006).
From the above discussion, one may concluded that purposely planting is the sufficient condition of S. alterniflora invasion. In China, after many times and distributed introduced, the expansion of S. alterniflora population exhibits a multi-point outbreak pattern.
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