The Invasibility of Native Ecosystem as a Necessary Condition of Spartina Alterniflora Invasion

Before the invasion of Spartina, there are few plant species distributed in the tidal zone for the severely physical condition. Thus, S. alterniflora, as a pioneer species of community succession, occupies an empty niche at the early stage of invasion (Chung et al., 2004). After successful establishment, the populations have a chance to rapid expansion. Such as in China, all of 90% foreshore of Jiangsu Province is silty, and the total area is 6,533 km2. Previous of Spartina introduced, plants only distributed in the zone above mean higher water level (MHWL), and the broad foreshore between MHWL and mean water level (MWL) and under MWL where Spartina could colonize and establish population was empty (Guan et al., 2003).

Contrasts to S. alterniflora, the native species are inferior competitors. The growing dominance of S. alterniflora restrained the growth of Scirpus mariqueter when the two species were competing. Consequently, S. alterniflora successfully invaded into S. marigueter community (Chen et al., 2004). The photosynthesis rate of Phragmites australis was significantly lower than that of S. alterniflora, and this disadvantage led to its loss when P. australis competed with S. alterniflora (Zhao et al., 2005; Zhao et al., 2008).

Spartina alterniflora, as a successful invader, can enhance the ecosystem carbon and nitrogen stocks and therefore facilitate its invasion. The species invasion altered ecophysiological processes, resulted in changes in NPP and litter decomposition, and ultimately led to enhanced ecosystem C and N stocks in invaded ecosystems (Liao et al., 2007, 2008). Furthermore, S. alterniflora accelerate sedimentation through its extensive rhizome and upright stems, at the same time, burial by sediment stimulate its growth and reproduction (Donnelly & Bertness, 2001; Morris et al., 2002).There is a positive feedback relationship between sedimentation and growth of S. alterniflora in coastal salt marshes occurs. A self-facilitation mechanism may be developed during the process of S. alterniflora invasion (Deng et al., 2008).

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Figure 2. The invasive mechanism of Spartina alterniflora (Deng et al., 2006)

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Figure 2. The invasive mechanism of Spartina alterniflora (Deng et al., 2006)

In a word, the invasive mechanism of S. alterniflora involves three factors: the invasiveness of invasive species, the invisibility of ecosystem and offering an opportune invasive passage (Fig 2). The invasiveness of S. alterniflora includes powerful ability of hybridization and introgression, mighty adaptability and superior reproductive ability, eurytopicity and competitive superiority. The ecosystems with empty niche, inferior native competitors and frequent disturbance, such as foreshore, are the necessary conditions of S. alterniflora invasion. The accident dispersal and purposed transplant of propagules of the species directly result in the invasion. Especially, in China, large-scale transplants accelerate the invasive rate and enlarge the explosive scope of S. alterniflora. The outbreak of the species represents a characteristic with point-resource dispersal and multi-loci expansion.

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